, DROP FORGETMPACT NOISE: TEMPORARY AND _ z
i? PERMANENT EFFECTS" ON HEARINGTHRESHOLDS ’ ‘

’ Thesis for the Degree of Ph. D. ‘ ‘
MICHIGAN STATE UNIVERSITY

'9 ROBERT F. LINDBERG

1971

 

 

ANNE": v . "3 '3’.) "fag," 1.: )1 [31¢an ”n .L..-,~' 4 - :l“...."'fifl\1.“:"‘l‘_:',T:“I T... .T WV. , T T ..‘~
444%.; “5.5%?“- , ‘fa: 1-;-: ' s .- -; , - . ,. ‘ . ‘ 4 .
.‘t..‘u.,‘.'.Linu‘iu'pnfl‘ .Auduv. I'll-’V‘ um aw - mm“. T‘ TEAM...“ -.‘ ' ' " T “"' " """ ""“" "
.M.‘ .v,:. a, . .' .E. . ,E T
\

 

ABSTRACT

DROP FORGE IMPACT NOISE: TEMPORARY AND
PERMANENT EFFECTS ON HEARING THRESHOLDS

BY
Robert F. Lindberg

The purpose of this study was to obtain basic
information concerning the nature of drop forge noise in
the industrial setting and its effects upon the hearing
thresholds of men exposed to such noise. Since a review
of the literatdre revealed a dearth of information, a
three phase study was completed to study the nature of
the impact noise and its temporary and permanent effects
on hearing thresholds.

The nature of drop forge impact noise was inves-
tigated in Phase I. At a Lansing drop forge, peak SPL,
rise time, duration, octave band spectrum,number of
impacts and repetition rate measurements were made on
impacts produced by drop hammers. Also, continuous noise
measurements from fans,furnaces and other forge noises
were obtained including linear, dB A and octave band
readings. For all parameters, several readings were
made and the means and standard deviations were computed.

Results revealed that peak levels from different hammers

Robert F. Lindberg

hour recovery period. No accumulated TTS was found over a
five day work period when ear protection was worn.

Phase III was designed to study the effects of
drop forge noise on permanent threshold shifts. A
retrospective study was completed of the audiometric
records of 71 drop forge workers who were screened with
strict criteria regarding work,ear protection, and hear-
ing loss histories. Results indicated considerable
variability in amount of hearing loss among drop forge
workers. Typically,the greatest impairment occurred
in the frequencies above 1000 Hz during the first 15 years
of exposure. whereas the low frequencies showed continued
reduction in thresholds throughout the work history.
Effect of aging upon thresholds was substantially less
than the loss resulting from noise exposure even for
men who were near retirement (age 65). Differences in
hearing thresholds were not marked between forge companies.
Small differences were found related to job and ear
protection, but these differences were partially related
to years of exposure and age.

Three primary conclusions from this study are
warranted. First, both impact and continuous (steady
state) noise levels measured in a drop forge were found
to be potentially hazardous to the workers‘auditory

systems according to current DRC. Secondly, permanent

Robert F. Lindberg

varied by stroke of hammer and position of employee, but
were consistent for the same strokes on different series.
Rise times and durations were longer than other types
of impulsive noise (e.gu,gunfire) and decreased in time
from the first stroke of a series to the last. Number
of impulses and repetition rates suggest that hammer
shop workers receive excessive exposure to high level
noise. Overall shop noise and steady state background
noise were found to exceed the current Walsh-Healey
damage-risk criteria (DRC) for continuous type noise.

In Phase II, the temporary effects on hearing
thresholds of 20 drop forge workers (in the same forge
as Phase I) were evaluated by manually administering
pure-tone threshold tests four times over two consecutive
days. In addition, nine employees were tested
following a five day work period. Subjects were divided
into two groups based on years of exposure and into
subgroups according to the type of ear protection employed.
Results revealed poorer resting thresholds at all
frequencies tested for the older men with longer exposure
to the noise. Temporary threshold shift (TTS) for both
groups was mild except for one subject who used no
ear protection. Small but inconclusive differences
between groups were noted. TTS for both groups varied
only slightly from one day to the next. Thresholds

returned to rested (pre-exposure) levels after a 16

Robert F. Lindberg

threshold shifts obtained from a large sample of hammer
shop employees exhibited serious hearing impairment after
ten to fifteen years of employment. Finally, both

rested (non-noise exposed) thresholds and TTS measure-
ments obtained from men with less than ten years of noise
exposure suggest that utilization of appropriate ear
protective devices substantially reduces the hazard to
hearing. The primary implication is that if hearing
conservation programs are initiated in drop forges, the
risk to the employee's hearing can be significantly

reduced.

DROP FORGE IMPACT NOISE: TEMPORARY AND

PERMANENT EFFECTS ON HEARING THRESHOLDS

BY

C
,.. I
.. .L
i.

Robert F. Lindberg

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Audiology and Speech Sciences

1971

ACKNOWLEDGEMENTS

In appreciation of the advice, knowledge, time
and talent so freely given, I gratefully acknowledge
Dr. William F. Rintelmann, my advisor; Dr. Judith
Frankmann; Mr. Donald Riggs; and Cpt. Donald R. Ciliax.
Equally cooperative were the following: Rehabilitation
Medical Center, Mr. Leonard Face, Director, for allowing
me accéss to the audiological records; the Michigan
Association for Better Hearing and Speech, Mr. Dennis Ortiz
Director, for the use of the special trailer for testing; and
last but not least, the Melling Forging Company, Mr. Kenneth
Brooks, Personnel Manager, and the men in the forge, for the
use of the forge, and their assistance and participation in
this study.

Finally, I most gratefully acknowledge my wife,
Barbara, for her moral support, typing skills and editorial

assistance, as well as taking care of three small children.

ii

LIST OF
LIST OF
LIST OF
Chapter

I.

II.

III.

IV.

TABLE OF CONTENTS

TABI‘ES O O O O C O O O C O O .
FIGURES . . . . . . . . . . .

APPENDICES O O O O C O O O O 0

INTRODUCTION . . . . . . . . .

Purpose of the Study . . . . . .
Importance of the Study . . . . .

REVIEW OF LITERATURE . . . . . . .

Problems in Measurement of Impact Noise
TTS Effects from Impact Noise . . .
PTS Effects from Impulse Noise . . -
Ear Protection in Impulse Noise. . .
Summary. . . . . . . . . . .

EXPERIMENTAL PROCEDURES . . . . . .
Phase I Noise Analysis . . . . .
Phase II TTS Measurement. . . . .
Phase III Retrospective Analysis . .

RESULTS AND DISCUSSION . . . . . .

Phase I Noise Measurement Results. .
Phase I Noise Measurement Discussion.

Phase II Temporary Threshold Shift Results.

Phase II Temporary Threshold Shift
Discussion . . . . . . . . .
Phase III PTS Descriptive Analysis .
Phase III PTS Discussion. . . . .
General Discussion . . . . . . .

iii

Page

ix

xi

l—‘

\IU'I

12
23
26
30

32
33
41
52
56
56
67
72
87

126
131

Chapter Page

V. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS . . 138

smary. O O O O 0 O O O O 138
Conclusions . . . . . . . . . . . 142
Recommendations for Further Research . . . 142
BIBLIOGWHY O O O O O O O O O O O I O 0 l4 5

APPENDICES . . . . . . . . . . . . . . . 153

iv

Table

10.

LIST OF TABLES

Ambient noise levels in dB in the testing room
of.the trailer as measured for the A
weighting scale, C weighting scale, and
octave bands . . . . . . . . . . .

Peak pressure means and standard deviations
at the three positions for the first and
eighth strokes on the 3000 lb. hammer . .

Peak pressure means and standard deviations
at the three positions for the lst, 3rd, 5th,
and 11th strokes on the 2000 lb. hammer . .

Rise time and total duration in msec for
each impact of a series of six strokes out
of a total of eleven produced on the 2000
lb. hammer . . . . . . . . . . .

Means and standard deviations for linear and
dB A scale measurements made of overall
noise levels during hammer shop Operation .

Means and standard deviations of combined
right and left ear resting thresholds for
the one to ten year exposure group
according to type of protection employed .

Means and standard deviations of combined
right and left ear resting thresholds for
the 15 to 25 year exposure group . . . .

Mean TTS and standard deviations in dB
following one day's exposure for each of
the two exposure groups . . . . . . .

Mean TTS and standard deviations in dB
following the second day of exposure for
each of the two exposure groups . . . .

Mean TTS and standard deviations in dB
following 16 hours recovery time from
initial exposure period . . . . . . .

Page

47

58

58

61

66

73

74

80

84

85

Table

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

Page

TTS in dB following exposure on Monday,

Tuesday, and Friday for nine subjects

working the full week . . . . . . . . 86
Age distribution for 71 drop forge workers

seen for hearing evaluations . . . . . 96
Means and standard deviations for combined

right and left ear thresholds by age groups

along with difference between the thresholds

obtained and the presbycusic factor . . . 97
Years of eXposure distribution for 71 drop

forge workers seen for hearing evaluations . 102
Means and standard deviations for age and

years of exposure of subgroup . . . . . 103
Means and standard deviations for combined

right and left ear thresholds by years of

eXposure . . . . . . . . . . . . 105
Summary of combined ear thresholds, slope

of loss, and number of employees for each

position by drop forge company . . . . . 111
Age, years of eXposure, average hearing

thresholds and standard deviations for the

right ear, left ear, and combined ears

as a function of the employee's position . 113
Mean hearing thresholds and standard

deviations for the right and left ears

combined as a function of position in

forge . . . . . . . . . . . . . 114
Mean thresholds and standard deviations for

the right and left ears combined for

hammermen by years of exposure . . . . 117
Mean thresholds and standard deviations for

the right and left.ears combined for

heatermen by years of exposure . . . . . 119
Mean hearing thresholds and standard

deviations for hammermen as a function of

protection . . . . . . . . . . . 122
Mean hearing thresholds and standard

deviations for heatermen as a function of

protection . . . . . . . . . . . 124

vi

Table Page

F1. Impact peak pressure measurements made

at the hammerman's position on Hammers I

and II 0 O O O O O O O I O O O O 168
F2. Impact peak pressure measurements made at

the heaterman's position on Hammers I

and II C O O 0 O O O O O O I O O 169
F3. Impact peak pressure measurement made at the

straightener's position on Hammers I and

II. o o o o o o o o o o o o o o 169
F4. Impact octave band measurements made on the

8th stroke of Hammer I . . . . . . . . 170
F5. Impact octave band measurement made on the

3rd stroke of Hammer II. . . . . . . . 170
F6. Linear and dB A scale noise measurement

at five locations. . . . . . . . . . 171
F7. Continuous background noise measurement . . . 172
F8. Peak SPL and dB A measures on Hammer III. . . 173
Gl. Subject data and resting threshold data for the

right and left ear of the 0-10 years

exposure group. . . . . . . . . . . 175
62. Subject data and resting threshold data for

right and left ears of the 15-25 year

exposure group. . . . . . . . . . . 176
H1. Temporary threshold shift for each of the 19

subjects tested on Monday, Tuesday AM

and Friday PM . . . . . . . . . . . 178
11. Subject data for employees from Forge A . . . 180
12. Subject data for employees from Forge B . . . 181
I3. Subject data for employees from Forge C . . . 182
I4. Subject data for employees from Forge D . . . 183
J1. Resting threshold data for employees from

Forge A . . . . . . . . . . . . . 185
J2. Resting threshold data for employees from

FOrge B o o o o o q o o o c o o o 186

vii

Page

Table
J3. Resting threshold data for employees
from Forge C . . . . . . . . . . . 187
J4. Resting threshold data for employees from
Forge D . . . . . . . . . . . . . 138
K1. Hearing loss attributed to presbycusis by
190

age and frequency. . . . . . . .

viii

Figure

l.

10.

11.

LIST OF FIGURES

Block diagram of instrumentation for
determining rise times, total duration
and pictures of wave form . . . . . .

The diagram of two adjacent drop hammers
and furnaces and the relationship of the
positions of the operating crew . . . .

Photograph of the eleven stroke series
of the 2000 lb. hammer taken from the
screen of the storage oscilloscope . . .

Photograph of the final (11th) stroke
of the series on the 2000 lb. hammer
taken from the screen of the storage
oscilloscope . . . . . . . . . .

Octave band analysis of impact noise from
two drop forges . . . . . . . . .

Octave band analysis of steady state furnace
and blower noise from the hammer shop
of a drop forge . . . . . . . . .

Mean resting thresholds for the 0-10 and the
15-25 years of exposure groups . . . .

Mean resting thresholds according to the
type of protection employed by the 0-10
years of exposure group. . . . . . .

Mean resting thresholds for the 15-25
years of exposure group according to the
type of protection employed . . . . .

Monday and Tuesday TTS for subjects in
0-10 years exposure group who shifted
more than 4 dB at any test frequency . .

Monday and Tuesday TTS for subjects in 15~

25 years of exposure group who shifted
more than 4 dB at any test frequency . .

ix

Page

34

37

60

62

64

68

75

77

79

82

83

Figure Page

12. Typical steeply sloping audiometric con-
figuration of a subject (A13) with 24
years exposure. . . . . . . . . . . 93

13. Typical gentle sloping audiometric con-
figuration of a subject (C13) with 16
years exposure. . . . . . . . . . . 94

14. Mean combined hearing thresholds and loss
from presbycusis for each age group. . . . 98

15. Mean hearing levels for combined ears for
each age group. . . . . . . . . . . 101

16. Composite audiogram of mean combined ear
threshold grouped by years of exposure. . . 106

17A. Distribution of hearing loss in drop forge A . 108
17B. Distribution of hearing loss in drop forge B . 108

17C. Distribution of hearing loss in drop forge C . 109

17D. Distribution of hearing loss in drop forge D . 109
18. Distribution of hearing loss for all four

forges . . . . . . . . . . . . . 110

19. Composite audiogram showing mean combined
thresholds by working position . . . . . 115

20. Composition audiogram of mean combined
thresholds for hammermen according to
years of exposure groups . . . . . . . 118

21A. Composite audiogram of mean better ear
thresholds for heatermen by years of
exposure. . . . . . . . . . . . . 120

213. Composite audiogram of mean combined ear
thresholds for heatermen by years of
exposure. . . . . . . . . . . . . 120

22. Composite audiogram of mean combined
thresholds for hammermen by type of
ear protection. . . . . . . . . . . 123

23. Composite audiogram of combined ear
thresholds for heatermen by type of
ear protection. . . . . . . . . . . 125

X

LIST OF APPENDICES

Appendix
A. Employee Screening Questionnaire . . .
B. Subject Data Form . . . . . . . .
C. Notices given to Subjects for Pre-Exposure
Tests and Post-Exposure Tests. . . .
D. Pre- and Post-Exposure Testing Procedures
E. Threshold Recording Data Sheet . . . .
F. Noise Analysis Raw Data . . . . . .
G. Phase II: Subject and Threshold Data. .
H. Phase II: TTS data. . . . . . . .
I. Phase III: Permanent Threshold Shift
Subject Data . . . . . . . . .
J. Phase III: Permanent Threshold Shift
Threshold Data. . . . . . . . .
K. Hearing Loss Attributed to Presbycusis .

xi

Page
154

156

158
161
165
167
174
177

179

184

189

CHAPTER I

INTRODUCTION

The human auditory system can be damaged in many
ways. Among the different causes of hearing impairment,
noise has been recognized as a major contributor for
many years (Bunch, 1937). While noise is present in
many phases of our environment, the industrial setting
generally has been considered particularly hazardous
(Kryter, 1950). Recent legislation by the federal
government regarding noise level control further emphasizes
the industrial noise problem (Walsh-Healey, 1969).

Within the industrial settings, the drop forge industry
is one which has been shown to provide a hazardous
environment which can result in hearing loss (Fox, 1953).
It is this industrial setting with its impact noise with
which the present study is concerned.

Impulse or impact noise, which for purposes of
this study are considered synonomous, is a short burst
of sound which reaches a maximum sound pressure level
(SPL) and then drops in intensity within one or two

seconds or less, i.e., gunshots, drop forges, etc.

Industrial impulse noise can be divided into three
general types as described by Coles and Rice (1967).

The first type consists of discrete impulses that are
separated in time by two or more seconds, such as is
found in pile driving and drop forging. The second type
is the semicontinuous. In this case impulses occur at
ten or more times per second and are found in riveting,
fettling, and pneumatic caulking. The final type of
industrial impulse noise is the repetitive discrete
impulses which occur at a rate of one impact every two
seconds or up to ten per second. This kind of noise is
found in blanking and other types of hammering, such

as drop forging.

In an effort to limit the hazard of exposure to
the various types of industrial noise, guidelines have
been developed specifying the physical characteristics
of the noise and the length of time men can be exposed
without incurring hearing loss. These guidelines are
referred to as damage risk criteria (DRC). In order to
develop risk criteria, data regarding the physical
characteristics of the noise are needed including: SPL,
duration of the impact; and rate of occurrence. Studies
relating the physical characteristics to temporary thres-
hold shifts (TTS) resulting from exposure to the noise
are also required. TTS is defined as a reduction in

hearing threshold resulting from noise exposure provided

that threSholds return to pre—exposure levels with time
after cessation of the noise exposure. These TTS studies
should be done both in the laboratory and in the actual
industrial environment if possible. Finally, data
regarding the permanent threshold shift (PTS) should
be obtained. PTS is defined as the reduction in hearing
threshold which remains after exposure to noise has
occurred and any temporary shift has recovered. When
considering DRC, one also must keep in mind that the use
of ear protection devices, such as ear plugs or ear
muffs, will reduce the effect of the noise upon both
TTS and PTS.

DRC have been developed for continuous (steady
state) and intermittent noise (Kryter, Ward, Miller,
and Eldredge, 1966; AAOO, 1969; Walsh-Healey, 1969).
For impulsive type noise, particularly high intensity
sounds such as gunfire, enough data have been obtained
for the development of DRC (Coles, Garinther, Hodge,
and Rice, 1968) with modifications by the Committee on
Hearing, Bioacoustics and Biomechanics (Ward, 1968)
and suggestions by Coles and Rice (1970). Studies have
also been completed showing the effects of ear protection
on gunfire impulse noise (Coles and Rice, 1966; Forrest

and Coles, 1970).

The use of current DRC for impulse noise, developed
primarily for gunfire, has several disadvantages if
applied to industrial types of impulse noise. First, the
DRC have limiting criteria. The number of impulses is set
at 1000 per exposure which is less than the average number
of impulses per day to which the average drOp forge worker
is exposed. In addition, the number of exposures per
year is limited to about 10 in the DRC while the industrial
worker must be exposed daily. Second, in studies which
have been conducted using simulated impact noise conditions,
discrepancies have been found between their results and
those predicted by the current DRC for impulses. Third,
as Cohen, Kylin, and LaBenz (1966) have indicated, the
typical industrial situation has a combination of both
steady state and impulse noise. Information regarding
the addition of steady state noise to the industrial
setting with impact noise needs to be acquired if a
damage risk criterion is to be applicable. Finally, and
most importantly, the exact nature of the hazards from
the various types of impact noise in industry are still
basically unknown, particularly drop forge noise. The
lack of sufficient evidence supporting the relationship
of drop forge noise and hearing loss makes generalization

to the current DRC questionable.

In order to aid in the establishment of a damage
risk criterion for industrial impact noise, particularly
drOp forge noise, basic information is needed (Coles
and Rice, 1967; Coles, et_31., 1968; Noble, 1970). The
present investigation was an attempt to gain some of
this basic information regarding drop forge noise and
its effects on hearing. Such datanare needed for later
development of more encompassing DRC. This study also
sought to obtain some notion as to the effectiveness of

ear protection in the drop forge industrial setting.

Purpose of the Study

 

This study sought basic information concerning
the nature of drop forge noise and its effects upon the
hearing thresholds of men exposed to such noise. The
investigation was conducted with three purposes in mind:
1. To obtain basic information regarding the nature of

impulse noise produced in the hammer shop of a

local drop forge through appropriate measurements of

the physical parameters. Questions delimiting the

research for Phase I were as follows:

a. What are the peak sound pressure levels resulting
from drop hammers?

b. What are the rise times and total duration of the

impacts from drOp forges?

At what frequencies is the energy concentrated
for impact noise in the drop forge?

What are the intensity level and spectral make-up
of the constant background noise present in the
drop forge?

What is the typical exposure for a hammer man in
terms of number of impulses for work day, week,

etc?

To compare hearing threshold measurements obtained

using standard pure-tone audiometry at the start of

a week's work with post exposure thresholds after

eight hours work, after a sixteen-hour recovery period,

after a second eight hour work period, and after five

days' work. The following questions governed the research

for Phase II.

a.

What are the changes in resting threshold as a
functiOn of years of exposure and type of ear
protection?

What differences exist in TTS following eight
hours of exposure within age groups with different
types of ear protection?

Are differences in TTS obtained between age group-
ing when using similar types of ear protection

devices?

d. Are sixteen hours of recovery time sufficient for
total recOvery of any TTS resulting from one day’s
exposure to drop forge noise?

e. Does TTS vary from one day's exposure to another?

f. Is the TTS present on the fifth day greater than
that obtained after one day's exposure?

3. To analyze the permanent hearing threshold data obtained
from records of drop forge employees seen for hearing
evaluations at a hearing and speech clinic. Questions
of interest for Phase III were:

a. Are there typical audiometric configurations of
workers exposed to drOp forge noise?

b. Does the degree of loss reach a terminal level
after many years of exposure to drOp forge noise?

c. Does the audiometric configuration or degree of
loss differ for exposure to impact noise from
different drop forge companies?

d. Is the use of ear protection devices reflected in
the degree of loss demonstrated by employees at

different work positions?

Importance of the Study

 

While the fact that drOp forge workers do have
hearing losses has been documented by Fox (1953), the
need exists for a more complete analysis of the nature

of the loss as represented by the pure-tone audiogram.

Coles and Rice (1970) have stressed the need for
data from the actual source of the hazard, the drop forge.
TTS and PTS measures need to be obtained and related to
physical parameters if apprOpriate DRC are to be established
or current DRC expanded and modified.

The use of sound attenuating devices in noise
has long been recognized as a major contribution to the
protection of hearing. While studies have shown the
benefit of ear protection to gunfire, the effectiveness
of these devices in the industrial setting, particularly
drop forges, is apparently unknown. This knowledge
would be useful not only for scientific reasons but also
for the benefit of the men employed in the industrial

setting.

CHAPTER II

REVIEW OF LITERATURE

In this chapter four areas of impact noise are
considered. First, problems associated with the
measurement of impact noise are discussed. Second,

TTS effects from impact noise are reviewed. Third,
PTS effects from impact noise are discussed and, finally,
the use of ear protection for this type of stimulus is

considered.
Problems in Measurement of Impact Noise

The rapid onset and decay of an impact noise
present particular problems in measurement not found
with steady state noise. These problem areas include
rise time, frequency response, operational characteristics
of the measurement instruments and position of the

instruments at the time of testing.

Rise Time

 

The rise time capability of a measurement system
must exceed the rise time of the impact if an accurate

measurement is to be obtained (Harbold, et a1., 1965;

10

Coles, et a1., 1968). This is particularly important for
the microphone employed, but must also be considered for

the entire system.

Frequencpresponse

 

The frequency response of a microphone or measure-
ment system will strongly influence the results obtained.
First, as Harbold, et_al. (1965) indicated, the rise time
is inversely proportional to the upper limit of the
frequency response. Thus, rise time and frequency
response are highly related. Coles, et al. (1968) recommend
a response curve of from 100 Hz to 70 kHz. Second, the
wider the response curve, the higher is the natural
frequency of the system. The natural resonant frequency
results in ringing or overshoot in the measurement of
transients (Harbold, et_al., 1965). These must be kept

to a minimum.

Operational Characteristics

 

Even though the rise time and frequency response
of the system has been considered, other problems may
occur. For example, in the past, sound level meters could
not be used because the ballistics of the needle on the
VU meter in the fast response made was not rapid enough to
accurately measure the peak level of most impact sounds.
In addition, the impact meters used until recently

necessitated the use of integration time constant and

11

thus gave peaks lower than those measured with the
oscillosc0pe. (Coles, et al., 1968). The development
of the new Bruel and Kjaer Type 2204 S sound impact
meter has eliminated the above mentioned two problems.

Some attempts have been made to record impact
noise for later analysis. However, the rise time
response characteristics as well as signal-to—noise
ratio and peak clipping problems have generally resulted
in poor recordings (Harbold, et_§1., 1965).

Coles, eE_al. (1968) further noted that problems
with the equipment can develop if it is not durable enough
to withstand acoustic damage from the pressures being
measured. They also noted that unless the equipment
is prOperly mounted, problems with vibrations and

microphonics can occur.

Position of Microphone

Coles, e£_31. (1968) stressed the need for the
microphone to be at an angle of 90° to the travel of
the impact wave. They cited differences between micro-
phones of 2 - 10 dB for peak measures when using 0°
incidence. Better agreement was found between micrOphones

at the 90° angle of incidence. If different angles were

12

used to measure the same sound, additional problems would
be present.
As can be seen, the problems of measuring impact

noise are many. Using the newest instrumentation avail-

able commercially, as many of these problems as possible

were eliminated in this investigation.

TTS Effects from Impact Noise

 

Unlike continuous steady state noise, impact
noise presents a more complicated group of physical
variables or parameters which must be considered regard-
ing their relationship to TTS and PTS if appropriate
DRCS are to be developed. Loeb, Fletcher, and Benson
(1965) indicated that these variables include peak
pressure, pulse duration, rise and decay time, number
of impulses, spacing of impulses and repetition rate.
The problems of attempting to study all these variables
are many, considering the numerous types of impulse pro-
ducing equipment in industry. Therefore, studies have
been done primarily in the laboratory in order to control
as many of the noise variables as possible. These methods
include the following: gunfire (Kryter and Garinther,
1965; Fletcher and Loeb, 1965; Rice and Coles, 1965),

discharging a capacitor across a speaker (Ward, Selters,

13

and Glorig, 1961), opening and closing toy crickets
(Ward, et_31., 1961), electrical spark gap (Loeb, §t_31.,
1965; Fletcher and Loeb, 1967), taped drop hammer noise
(Chisman and Simons, 1961), taped mechanically driven
hammers (Walker, 1970), slapping a board on a table
(Cohen, Kylin and LaBenz, 1966), or using short bursts
of noise (Spieth and Trittipoe, 1958). The latter
four methods were used to attempt to relate laboratory
noise to industrial noise. Most research, however, has
been concerned with gunfire noise. While gunfire research
is of interest, studies related to industrial type noise
are of major concern in this review. 7
Temporary threshold shift studies have considered
four main areas: growth as related to physical parameters,

recovery, reliability and susceptibility.

Growth of TTS

 

Initial studies by Ward, eE_al. (1961) revealed
that the growth of TTS with time follows a linear function.
Their investigation, using the shift at 4000 Hz to
impulses presented at 2.4 second intervals, supports the
notion that growth of TTS is proportional to the number
of impulses presented. Carter, Ball and Kryter (1962)
and Carter and Kryter (1962) using impulses of one msec
duration obtained similar findings of a possible linear
function with time. Kryter and Garinther (1965) reported
the linear function at least for the frequencies of 1000-

3000 Hz while studying the effects of gunfire.

14

In 1965, Loeb, ep_§l., using impulses of 800
microseconds duration and 156 dB peak intensity, reported
that growth of TTS was greater than a linear function
with time but less than logarithmic. Walker (1970)
reported similar findings when employing levels of 124-127
dB and 20 msec impulses. Walker (1970) suggested, however,
that the function depended on the total exposure time,
the total number of impulses, and the rate at which the
pulses were presented. While contradicting the findings
of a linear function, the studies do indicate a growth
of TTS which is Somewhat different from that generally
found for continuous noise. The differences obtained
among impulse noise studies would appear to be related
to other parameters as suggested by Walker (1970). These
parameters affecting growth of TTS include temporal
spacing and repetition rate, peak level, spectrum and
duration.

Temporal spacing and repetition rates.--Murray

 

and Reid (1946) reported that more TTS was produced by
firing one shot at a time than when firing the same
number of rounds rapidly. Smith and Goldstone (1961)
supported that finding in another gunfire study. Ward,
pp_§l. (1961) reported that when laboratory produced
impulses occurred at intervals of less than one second,

less TTS was reported than when impulses were presented

15

at one or more per second. Carter and Kryter (1962) found
that one impulse per second produced the most severe TTS
of the different intervals studied. Loeb, §E_a1. (1965)
further supports these results. Walker (1970) while
noting the influence of the inter-pulse-interval on

TTS also concluded that the rate was more important for
the most suspectible subjects.

The cause of the lower TTS at the faster rates
and shorter intervals has been attributed to the influence
of the middle ear muscle acoustic reflex by all of the
investigators. If impulses occur at a fast enough rate,
the muscles in the middle ear do not have sufficient time
to relax completely. Thus, some measure of protection
is carried-over to the next impulse. Differences in
individual relaxation rates have been hypothesized as
a possible factor in suspectibility to impulse noise.
(Ward, gp_al., 1961; Carter and Kryter, 1961).

Ward, ep_al. (1961) have also reported that with
impulses widely spaced in time, less TTS occurs. In a
study of this phenomenon Ward (1962) found that intervals
of l, 3, and 9 seconds were equivalent in the production
of TTS. Intervals over 9 seconds resulted in less TTS,

a finding Ward attributed to the effects of recovery.
Kryter (1970b) suggested that after only five seconds

the auditory system starts the recovery process.

16

Peak SPL.--With continuous noise, the more intense
the noise the greater the amount of TTS. Ward, gp_al.
(1961) indicated that the same process holds for impulse
noise. However, data are somewhat unclear concerning
the relationship with different types of impulse noise.
For gunfire, Kryter (1970a, 1970b) reviewed a number of
studies and made the necessary conversions to equate the
results. He reported that for gunfire the relation of
peak SPL to TTS could be shown as a straight line up
to approximately 170 dB SPL. Above this point of 170
dB, rapid growth in TTS has been observed (Kryter and
Garinther, 1965; Kryter, 1966).

When Kryter (1970b) attempted to equate the non-
gunfire studies, such as the spark gap data by Fletcher
and Loeb (1967), the loudspeaker data by Ward, gp_al.
(1961) and Carter and Kryter (1962), and the mechanical
clicker data by Ward, gp_al. (1961), he found that the
amount of TTS was not systematically related to the peak
pressure of the impulses. Thus, while a systematic
relationship has been observed with gunfire noise
between TTS and peak pressure, more data are needed on
other types of impulses.

In one study, Carter and Kryter (1962) used

impulses of one msec duration and reported a three decibel

17

increase in TTS with each six decibel increase in peak
pressure. Other studies reviewed have not substantiated
this relationship, particularly for the industrial setting.

Spectral Effects.-—One of the problems in the

 

measurement of impact noise noted by the American Standards
Subcommittee Z24—X-2 (1954) was spectral analysis.
Measurement problems have hindered the study of the
effects of spectrum on the TTS from impulse noise. There
are now methods of measurement and computation which can
be used. In addition, there is a method of estimating
the spectral characteristics based on the rise time,
duration, and peak SPL of the pulse (Kryter, 1970b).
Because of the difficulty in measuring and establishing
the Spectrum of impulses, Coles, gp_al. (1968) and
CHABA (Ward, 1968) do not include this measurement as
part of their DRC.

Gunfire studies by Kryter and Garinther (1965),
Plomp (1966), and others have generally shown the greatest
TTS to occur above 3000 Hz. Little shift is noted in the
lower frequencies. The spectral analysis of the four
weapons employed by Kryter and Garinther (1965) indicated
peak pressure to be about one octave below the area of
greatest TTS. This finding suggests that the maximum shift
resulting from impact noise is located in the same area
as the maximum shift of an octave band continuous type

noise (Ward, Glorig and Sklar, 1959).

18

When considering impulse noise other than gunfire,
Ward, e£_al, (1961) reported a range from 3000 to 10,000
Hz as being most affected by impulse noise. The greatest
shift was seen at 4000 Hz, but the frequency of maximum
shift varied with the individual and in the same person
over repeated exposures. Walker's (1970) data supported
the 4000 to 6000 Hz range.

Since duration has an influence on the spectral
composition, different durations of pulses should result
in different areas of maximum TTS. Fletcher and Loeb
(1967) employed pulse duration times of 36 microsec and
92 microsec. With the longer duration of 92 microsec
more TTS was obtained in the lower frequencies. The
authors, however, indicated that frequencies up to 18,000
Hz are affected for both durations. Loeb, pp_al. (1965)
also using very short impulses, found a range of greatest
TTS from 8000 to 12,000 Hz. Loeb and Fletcher (1968),
on the other hand, found that frequency of maximum shift
appeared independent of impulse duration. They observed
that the maximum loss occurred from ll,000-l6,000 Hz
but that shifts had also occurred at 3000 and 4000 Hz.

While some question remains about the various
parameter effects, the place of maximum threshold shift
is from 3000 Hz to 6000 Hz for gunfire and from 3000 Hz

to 18,000 Hz for other types of impulse noise.

19

Duration of Impulses.--Rice and Coles (1965) found

 

that the duration of the impulse resulted in different
degrees of hazard when firing on the open range and an
enclosed range. The longer the duration, the more
hazard from gunfire. In the development of the DRC for
impulses (Coles, gp_al., 1968) and the modifications

by CHABA (Ward, 1968), duration was quite important.
Rice (1968) indicated that levels well above the 140 dB
maximum permissible ceiling can be tolerated depending
upon the duration of the impulse and total number of
pulses (Kryter, Ward, Miller and Eldredge, 1966; Walsh-
Healey, 1969).

Studies specifically designed to test the effect
of pulse duration on TTS have strongly supported the
detrimental effects of longer impulse duration. Fletcher
and Loeb (1967) noted that 10—25 impulses at 92 microsec.
had the same effect as 75—100 impulses at 36 microsec.
duration. Loeb and Fletcher (1968) found further
support indicating greater shift from longer impulse
duration. These changes occurred with no modification
of intensity. The effects of different durations on
other types of impulse noise have not been determined.
The only attempt to relate duration to industrial noise

is through the current DRC for impulse noise.

20

Recovery of TTS from Impact Noise

 

In 1958, Ward, Glorig and Sklar reported that
the process of the shifted threshold returning to pre-
exposure levels followed a logarithm of time basis for
continuous noise. Ward, §E_a1. (1961), found that the
pattern of recovery for this very different kind of
noise followed the same function. This pattern was
similar for TTS of 40 dB or less. When TTS exceeded
this level, a change in the pattern of recovery occurred.
Carter, §E_al. (1962) and Ward (1962) supported the
logarithm of time recovery function for impulse noise.

A recent study by Fletcher (1969) found that
the recovery of TTS took longer for the impulsive type
noise than for continuous noise. Even after seven hours,
recovery was not completed in the high frequencies, up
to and including 14,000 Hz. While the results indicate
longer recovery time was for the impulse noise, it must be
noted that the impulsive noise produced a greater amount
of TTS initially; therefore, recovery would be expected to
take longer.

With this one exception, the bulk of the literature
on recovery of TTS supports the logarithm of time recovery

pattern.

21

Susceptibility to TTS

 

One of the major findings of the studies on
impulsive noise has been the wide range of susceptibility
to TTS. Murray and Reid (1946) reported large variability
in TTS from gunfire as did Rice and Coles (1965). With
laboratory types of impulse noise, findings have also
indicated a large degree of variability (Ward, ep_al.,
1961; Carter, e£_al., 1962; Carter and Kryter, 1962;
Hecker and Kryter, 1964). For example, Carter and
Kryter (1962) found a total TTS range of 55 dB for impulse
noise but only a 20 dB range for continuous octave band
noise. This large range of susceptibility to TTS
impulse noise. This large range of susceptibility to TTS
from impulse noise, coupled with the fact that the
impulse TTS did not correlate with the continuous noise
TTS, has lead most authorities to be extremely cautious
in exposing subjects to high intensity noise. As a means
of control and analysis for laboratory studies, Rice
and Coles (1965) recommended dividing subjects into
three groups according to their susceptibility to impulse

noise.

Reliability of TTS

 

The question of the reliability of TTS from
impulse noise, while being a part of most studies of

repeated measure, has been specifically studied by

22

Hodge and McCommons (1966) and Hodge, McCommons and
Blackmer (1966). Their findings indicate that individual
TTS's are not reliable enough to generalize as to the
effects of the noise. However, group mean TTS was
found to be a reliable indicator for both normal hearing
and hard-of-hearing subjects. These findings were
demonstrated for a number of different impulse conditions.
The source of the impulse noise was gunfire. TTS from
industrial impact noise has not been considered in terms
of reliability and would be difficult to do so as the
nature of the stimulus and a man's exposure time varies
considerably between exposures.

As is evident from the review of the TTS studies,
most investigations have focused their attention upon
the gunfire type of impulse noise or upon intensities
and durations similar to that found with gunfire. Studies
by Walker (1970) and R01, Sporr, and van Dishoeck (1967)
have considered repetitive discrete impulses and
simulated pneumatic drill noise respectively. Walker
(1970) reported that the TTS resulting from 127 dB peak
impulses produced more TTS than would be predicted on the
basis of the current DRC for impulses. Rol, §E_al. (1968)
raised the question as to the influence of combined
steady state noise and the impact noise of the pneumatic

drill. This question had been considered by Cohen, et a1.

23

(1966). Their findings indicated less TTS from combined
steady state noise and impact noise than from impact
noise alone. This occurred as long as the level of
the steady state noise was below 110 dB. Their TTS
results disagreed with that predicted by the DRC for
impulse noise.

The only study found to date which reports
some data on TTS from drop forge workers in the actual
setting is the Z24-X-2 subcommittee report of the American
Standards Association (1954). With 35 drOp forge
workers tested, results indicated sizable threshold
shifts. There was, however, no differentiation of PTS
and TTS in the data given. As is evident, few data
exist concerning the TTS effects of impact noise as

found in the drop forge.

PTS Effects from Impulse Noise

In studying the effects of permanent loss,
specific information can only be obtained from studies
in the field, where little or no control can be developed
by the experimenter. Thus, the first problem in field
studies is that the test environment situation must be used
as it is found (Burns, 1969). The only practical method
at present to estimate the degree of PTS over several

years of exposure is to measure the hearing of persons

24

who have been exposed to specific types and amounts of
noise over a known period of time. Burns (1969) has
indicated that the problem of presbycusis as well as
other physical and biological factors can confound the
results of this type of study. Thus for control of
these factors, there is need for precise information as
to the nature of the noise and the duration and pattern
of exposure and the need for a complete background of

a person's work in noise and his history with guns.
Control of biological factors include the need to know the
state of hearing before exposure, the present physical
condition of the ear, and a general medical history.
These problems and an inability to control the above
factors make studying permanent hearing loss difficult
to relate to the various types of industrial noise

and to the different impulse noise parameters.

As with TTS studies, the majority of PTS studies
have been accomplished with regard to the effects of
gunfire, apparently as an outgrowth from military needs.
Studies concerned with hearing loss resulting from
gunfire are numerous and, while of interest, the area
of priority in this review is industrial impact noise.
Generally, the characteristics of hearing loss resulting
from gunfire are as follows: 1) the hearing loss

increases rapidly in the first few years; 2) there are

25

large individual differences to susceptibility to PTS;
3) the maximal shift occurs at 4000 Hz or 6000 Hz;

4) the slope of the loss is very steep, with little or
no loss below 2000 Hz; and 5) the left ear loss is
somewhat greater than the right ear loss (Ward, Fleer
and Glorig, 1961; Fletcher, 1963; Coles and Knight,
1965; Livesey, 1965; Plomp, 1967).

Other types of impulse noise-induced PTS have
generally not been investigated thoroughly, other than
to establish the fact that a hazard exists. Only one
study was found to date in the literature which deals
with the permanent effects from drop forge noise. Fox
(1953) reported on 62 drop forge employees who had been
seen for otologic and audiologic evaluations and who
had claimed hearing loss resulting from drop forge noise.
His results revealed that 1) the hearing loss found
was proportional to exposure time; 2) wOrkers in the
fourth and fifth decade of life had equal loss when
determined using a percentage basis; 3) employees with
limited exposures showed mild to moderate deficits at
4000 Hz; 4) moderate to severe losses involved the speech
range with greater losses in the higher frequencies;
and 5) the hearing impairment involved both ears equally.
Because losses were reported in a percentage, the relation-

ship of the loss to the spectrum of the noise and the peak

26

levels cannot be assessed. Another problem with this
investigation is the lack of control regarding the
previous noise history of the workers seen. For example,
Fox indicated that eight of his subjects were between

60 and 70 years of age. One of these subjects had

worked only seven years in the drop forge. The question
that comes to mind is this: Prior to working in the

forge did he work in a different noisy environment? There
is need for more documented data in the area of drop

forge noise.

Ear Protection in Impulse Noise

 

In high intensity impact or impulse noise where
noise level reduction and equipment isolation cannot
reduce the noise to acceptable levels, some form of
protection for the ears is required. Protection means
reduction in the amount of sound energy reaching the
cochlea by some device (e.g. ear plug) or method of
utilization of the natural protective mechanism of the
ear.

As previously noted, the natural protective
mechanism of the auditory system for intense acoustic
stimuli is the acoustic reflex. In those studies consid-
ering spacing of impulses and repetition rate, less
TTS occurred with the rates faster as opposed to rates

slower than one per second. This finding was related

27

to the residual effects of the acoustic reflex (Ward, ep_al.,
1961; Loeb, gp_al., 1965; Walker, 1970). Cohen, ep_al.
(1966) attributed the lower TTS obtained from combined
steady state and impact noise to the improved sustaining
power of the reflex. In these studies, the protection
of the aural reflex was evident after the onset of the
Istimuli and the reflex were established. However, one
of the limitations of the aural reflex is its latency
period (Loeb, 1963). Impulsive sounds have a faster
rise time than does the reflex; thus the effects on the
cochlea occur from single pulses before the reflex can
provide its protection.

Fletcher and RiOpelle (1960), in order to counter
this problem, found that the aural reflex could be activated
by a tone just prior to a gun shot. In this way protec-
tion could be obtained. This approach has been further
documented by Fletcher (1965) and Chisman and Simon
(1961). Fletcher and Loeb (1962b) reported that the
use of the noise was a better eliciting stimulus than
pure tones. Fletcher (1961), however, found that when
comparing the protection of the aural reflex to that
of ear plugs, more protection resulted from the ear
plugs in the high frequency region. As more TTS occurs
in the high frequencies, the use of devices such as

ear plugs would appear to be the best method of protection.

28

A large variety of ear protecting devices have
been employed for both impact and continuous noise.
These devices include ear plugs, ear muffs and helmets.
Of these methods, cotton plugs have generally been shown
to be the least effective for continuous noise, whereas
ear muffs have been most effective. How the devices
protect the ear from impact noise has not been entirely
determined.

Of the methods for evaluating the effectiveness
of different ear protection, the threshold reduction
method advanced by Fletcher and Loeb (1962) and stressed
by Rice and Coles (1966) appears best for impact noise.
In this approach, TTS is measured first with no protection
and then with the protector of interest. The difference
in shift is the effectiveness of the protector.

How effective the ear protectors have been
against impulse noise compared to continuous noise
has been studied by Cohen (1961) and Fletcher and Loeb
(1962a). Cohen reported that a helmet was less effective
against impulse noise than ear plugs. On the other
hand, Fletcher and Loeb (1962a) found both plugs and
helmet to be equal in effectiveness of protection. The
use of cotton for both continuous and impulsive noise

is generally considered ineffective.

29

Studies using ear protectors for impulse noises
alone have generally been completed using gunfire as the
noise source (Ogden, 1950; Forrest and Coles, 1970;
Brasher, 1969; Plomp, 1967). The studies have found
that the use of ear protection is sufficient to reduce
TTS from gunfire to insignificant amounts. Thus, for
gunfire the use of protection has been evaluated and
found acceptable.

The fact that even if protectors are used, the
amount of protection they can provide varies in efficiency
according to the type used, the fit, and the way in
which they are used (Coles, 1969). Some of the variance
in efficiency has been noted in exposure to high intensity
explos1ve sounds. For example, Flottorp and Quist-
Hanssen (1960) found that for some types of ear plugs
better attenuation was obtained in the lower frequencies
for impact sounds. Coles and Rice (1966) have also noted
the low pass filter plugs, designed to improve communica-
tions in noise, were not as efficient in long duration
impulse sounds as they were for short impulse sounds.

No studies in the literature have been found
which indicate the effectiveness of ear protection in
industry, particularly drOp forge industries. All of

the studies have concerned themselves with gunfire effects.

30

While the need for protection is evident in industry
where impact noise is prevalent, research is needed to
determine whether the protection devices employed in that

setting are adequate.

Summary

Numerous instrumentation problems have delayed
until recently the study of the effects of impact noise
upon the human auditory system. Some of these problems
include: slow rise times; poor frequency responses;
the inability of instruments to follow the wave front,
to withstand the high intensities, or to accurately
reproduce the stimulus; and poor directional placement
of microphones.

The effects of impulse and impact noise are evident
in both temporary and permanent changes in hearing.
Unlike steady state noise, the influence of many para-
meters in TTS and PTS must be considered. Studies of
impact and impulse noise have primarily been concerned
with gunfire, although some controlled laboratory studies
using high intensity impulses have been completed.

While TTS studies have considered the effects of
some of the various parameters of impulse noise, few
studies have investigated the parameters of the noise
as it occurs in industry. In addition, no evidence was
found in the literature regarding TTS resulting from

daily exposure in the drOp forge industry.

31

While evidence regarding permanent effects of
gunfire noise is substantial, only one study was found
regarding permanent hearing loss from drOp forge noise
exposure. Fox (1953), while showing that hearing loss
did result from exposure to impact noise, did not
sufficiently describe the hearing losses so that
relationShips to noise parameters and DRC could be
made.

The use of ear protecting devices for impulse
noise has been found to be generally adequate for gun—
fire exposure, but there are differences in the
effectiveness of the various types of protectors. No
studies have been found to date dealing with the use
of ear protectors in the drop forge industry.

The over-riding question remains: What are the
temporary and permanent effects of drOp forge impact
noise upon the hearing thresholds of employees working

in the hammer shop?

CHAPTER III

EXPERIMENTAL PROCEDURES

To study drop forge noise and its effects upon
men working in that type of environment, a three part
investigation was performed. Phase I involved the
measurement of the drop forge noise using current methods
to ascertain peak levels, rise times, duration, repetition
rates, and frequency spectrum. The continuous background
noise in the forge was also measured. Phase II of the
investigation involved the testing of selected employee's
pure-tone hearing thresholds prior to their entering
and upon leaving the work area after a day's shift.
Resting thresholds and TTS measures were obtained several
times during a five day work period for each employee.

No attempt was made to control the noise, to hinder the
employee during his regular duties, or to control his
use of ear protection. Phase III of the investigation
consisted of descriptive analyses of documented hearing
loss cases seen at the Hearing and Speech Department of
the Rehabilitation Medical Center. Those cases selected

for review were drop forge personnel who had worked in

32

33

the forge for a known period of time. The objective
was to analyze resting or permanent thresholds on a

relatively large sample of workers.

Phase I Noise Analysis

 

Instrumentation

 

In the drop forge two types of noise are present:
impact noise from the drop hammers and continuous noise
from furnaces and blowers. Measurement of the continuous
noise was made using a precision sound level meter
(Bruel and Kjaer, Type 2204 S) with a one-half inch
condenser microphone (Bruel and Kjaer, Type 3134) and
an octave band filter (Bruel and Kjaer, Type 1613).
Calibration was completed using a pistonphone (Bruel
and Kjaer, Type 4220) with its accompanying barometer.

Measurement of impact noise has been a problem
in the past (Larsen, 1946). Problems related to fast
transients, high peak levels, and measurement devices
have been noted (Harbold, Tegt, and Standeven, 1965;
Coles and Rice, 1967; Coles, gp_al., 1968; and Rice,
1968). In the selection of the instrumentation, an
effort was made to assure proper rise times, frequency
responses and intensity limits. The instrument for
making peak level measurements and spectrum analyses
included the following: one—half inch condenser micro—

phone (Bruel and Kjaer, Type 3134), sound level meter

34

(Bruel and Kjaer, Type 220459 set for the peak/hold
circuitry, and the octave band filter (Bruel and Kjaer,
Type 1613). Corrections were made to the octave band
measurements on the basis of the response curve of the
microphone provided with the instrument. Rise times
and total duration of the impacts were measured using
a second one-half inch condenser microphone (Bruel and
Kjaer, Type 3134) with a cathode follower (Bruel and
Kjaer, Type 2615) coupled to a microphone power

supply (Bruel and Kjaer, Type 2801) and to a single
beam dual channel storage oscilloscope (Tektronic

Model 564) (see Figure l). The waveform of the

impact was photographed using a Polariod camera coupled
to the oscilloscope. The film employed was Polariod
Type 107, 3000 speed. The overall frequency response
of the measurement system was flat from 10 Hz through
40,000 Hz. The sound level meter and the condenser
microphone and cathode follower were each mounted on a

microphone stand (Bruel and Kjaer, Type UA 0049).

 

 

MicrOphone
Power Supply

Condenser MicrOphone
with Cathode Follower

 

 

\/

 

 

 

 

 

\/

ICamera Storage I
L Oscilloscopel

FIGURE l.--Block diagram of instrumentation for determining
rise times, total duration and pictures of wave
form. A

 

 

 

 

/\

 

 

35

To obtain measures of repetition rates and an
estimate of the number of impulses the men were exposed
to daily, the noiSe was recorded using a dynamic micro-
phOne (Electro—Voice Model 635 A) coupled to a magnetic
tape recorder (Ampex Model 601). Scotch Brand 201
magnetic tape was employed. The measurement of the rate
and number of impacts was obtained by using the tape
recorder in conjunction with a level recorder (Bruel
and Kjael Type 2305). In this way, an accurate number

of the impulses over a known period of time was calculated.

Procedure

 

In a local Lansing, Michigan,drop forge hammer
shop, noise measurements were obtained during the normal
working period for both the impact and the ambient noise.
Impact peak linear measurements were obtained initially
on two drop hammers during the normal morning break—
time. A different hammer was tested each day for two
successive days by having one hammer crew work while the
rest of the hammers were shut down. In this way, impact

measurements were made on a 3000 pound board hammer1 and

 

1A board hammer is a gravity fall hammer in which
the ram is attached to a hard wood board. The ram is
raised by motor driven rolls and by friction against the
board.

36

a 2000 pound air hammer.2 At the time of testing, bearing
parts were being forged from one and three-quarters inch
hot steel bars on the 3000 pound hammer, and metal elbows
were being forged from one and five—sixteenths leaded
steel bars on the 2000 pound hammer. The peak linear
measurements were made at the hammerman's, the heaterman's,
and the straightener's position (see Figure 2). For

the 3000 pound board hammer, ten readings were made at
the hammerman's position for the first and eighth strokes
of the series required in forming the piece being made.
Five readings were made at the straightener's and heater-
man's positions for the same strokes. For the 2000 pound
hammer, peak linear readings were made for the first,
third and fifth strokes at the three positions. Additional
readings were made on the eleventh stroke at both the
hammerman's and straightener's positions. The 2000 pound
hammer was forging two pieces from each bar of steel.

Five strokes were required for the first half of the bar
and six strokes for the second half of the bar. Thus
eleven strokes were required to forge each bar. The

sixth stroke on the last half of the bar was required as

the steel cooled somewhat and was harder. Readings of

 

2An air hammer is a gravity fall hammer in which
the ram is attached to a metal rod and cylinder. The
ram is raised by air pressure.

 

37

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Furnace A Heaterman's position
B Hammerman's position
A X C Straightener's position
X Noise measurement points
M
X
m
X 8'2
HTU B
C DIE
Furnace
A
E?
B
O.
c OE
urn
at:
Furnace

 

 

 

FIGURE 2.——Diagram of two adjacent drop hammers and furnaces
with the positions of the operating crew shown.
(Also noted are the points at which peak level
measurements were taken for the three positions.)

38

the last stroke were made to determine if the peak intensity
was different from the other strokes measured. Because

it was found that the repetition rate for both the

hammers tested was equal to or more than one stroke per
second, ten peak linear readings and ten RMS A scale,

slow response readings were made on a 3000 pound air

hammer at the hammerman's position. The peak readings

were made on the last stroke of a series of seven. The

dB A readings were made by visually determining the average
level for the series of seven strokes.

Octave band measurements of the impact noise were
completed at only the hammerman's position for both the
2000 pound and 3000 pound hammer using the octave band
filter coupled to the sound level meter. The peak
level for each of the octave bands from 31.5 to 31,500 Hz
was obtained using the peak hold circuit on the sound
level meter. Two readings were made at each octave band.
As the octave band characteristics for just one impact
could not be obtained without additional specialized
instrumentation, readings were made on the same stroke
for a number of series. For the 3000 pound board hammer,
readings were made on the eighth stroke, and for the
2000 pound air hammer readings were made on the third
stroke. Corrections were made for the use of the pro—
tective grid on the microphone and the frequency

responses.

39

The rise time and the total duration of the
impacts were determined by photographing the waveform
from the screen of the storage oscilloscope and calculating
the time required for the waveform to occur. Rise time
was defined as the time required from the onset of the
impulse for the intensity to reach its peak positive
amplitude. Total duration was defined as the total
time required for the amplitude to return to the level
of the background noise. The rise time and total
duration were calculated for the last six strokes of
the 2000 pound air hammer. The microphone was placed
at a 90° angle of incidence during all testing of
individual hammers.

During the measurement period, a recording of
the impact noise in the shop near the hammer being
measured was made for analysis of repetition rate and
number of impacts during a specified period of time.

The tape recorder was then coupled to the level recorder
and a record was obtained of the number of impulses

which occurred during a 15 minute period for the 2000
pound hammer singly and a 5 minute period for the hammer
when the whole shop was operating. The repetition rate
was then calculated and the number of impulses over a

working day estimated.

40

In addition to the readings on individual hammers,
overall RMS linear levels and A weighting scale levels
were obtained when the whole hammer shop was operating.
During each shift, eight drop hammers were in Operation.
Three readings were made at two locations on each of the
two days of testing. Thus, readings were made at four
locations in the hammershop.

The continuous background noise of the furnaces
and blowers was measured during a work break. At this
time, all hammers were shut down but the fans and
furnaces continued to Operate at a level similar to that
of the normal work period. Sound pressure level measure-
ments were made with the sound level meter and the octave
band filter. Readings were made with the linear and
the A scale weighting response at five different locations.
Four different readings were obtained for each of the
octave bands from 31.5 Hz through 31,500 Hz. The mean
levels were then‘calculated.

In summary, the following impact noise measure-
ments were made on two hammers: Peak levels, rise times,
duration, repetition rate, and frequency spectrum. In
addition, certain measurements were made while eight
hammers were Operating simultaneously and while no hammers

were operating.

41

Phase II TTS Measurement

 

 

The second phase of this investigation concerned

the effects of drop forge impact noise upon the hearing

thresholds of employees exposed on a daily basis.

Subjects

Based upon information obtained from a question—

naire (See Appendix A) given to all hammer shop employees

of a drop forge, twenty subjects were selected from the

sixty-two men responding. Selection was based on the

following criteria:

1.

An accurate account of the type and use of
ear protection by the subject was known.

He was not exposed to other types of
industrial noise or to gunfire either in the
armed services or in civilian life on a
habitual basis.

There was no unaccounted for periods in his
work history to assure that working in the
drop forge was his chief occupation.

There was no history of congential hearing
loss or of loss prior to his working in the

forge.

42

5. There was no history of conductive pathology.

6. There was no evidence of a unilateral hearing
loss.

The 20 subjects selected were placed into two
sub-groups according to age and number of years of exposure
as follows:

1. Subjects between twenty and thirty years of
age were placed in Group 1 and those between
thirty-five and fifty years of age were put
in Group 2.

2. Group 1 subjects had to have between one and
ten years of exposure to drop forge noise;
whereas, Group 2 subjects must have had
between fifteen and twenty-five years of
exposure.

The men with between one to ten years of exposure
ranged in age from 21 years 9 months to 27 years 11 months,
with a mean age of 24 years 11 months. The men with
between 15 to 25 years of exposure ranged in age from
35 years 8 months to 50 years 6 months, with a mean age
of 43 years 8 months. Each group was then further sub-
divided according to the type and use of ear protection.
Group 1 was divided into three sub-groups: those who
had used cotton or no protection (N=3); those who had
always used plugs or muffs (N=2); and those who had

shifted from cotton to plugs or muffs (N=5). Group 2

43

was divided into two sub-groups: those who had always
used cotton (N=3) and those who had shifted from cotton
to plugs or muffs (N=7). Of the older men who had
shifted from cotton to another type of protection, all
had used cotton between ten to twenty—three years.
Their use of plugs or muffs ranged from two weeks to
five years.

Description of the Work Area
and Exposure Periods

 

 

All of the subjects selected were employed in the
hot hammer shop of a local drop forge. The shop contained
sixteen drop hammers and twenty-one furnaces. Only eight
of the hammers were in operation during any one shift.

The hammers ranged in size from 1600 pounds to 5000 pounds.
The five largest hammers had two furnaces supplying them.
The shOp is composed of two large rooms connected to

form a 90 degree angle. The older portion of the shop

had two rows of hammers with a walkway between. The back
of the hammers faced the outside wall. In the newer
section of the hammershop, the hammers were located in

one row with a wide corridor on the front side of the
hammer. There was no acoustic treatment of the area

for sound reduction.

44

Each hammer had a crew of three or four men: the
hammerman, one or.two heatermen, and the straightener.
Other men in the shop were those transporting steel to
and from the hammers. These latter men had the most
variable exposure. It should be pointed-out that all
new employees generally started at the position of trans-
porting steel so that all men in the forge have had
that type of eXposure as well as working on the hammer
crew. Of the men used as subjects in this phase of the
study, ten were heatermen, seven were hammermen, and
three were straighteners.

The noise to which these men were exposed is
similar to that described in Phase I of this investigation.
The employees were exposed to the forge noise for up
to six and three-quarter hours a day. Time away from
the work area included a fifteen minute break after
two and one-half hours work and a thirty minute lunch
break after an additional one and three-quarters hours
of work. Exposure time following the long break was
approximately two and one-half hours. Exposure to the
intense impacts from one hammer varied daily depending
on work conditions, mechanical failure or changes in
the work schedule. As a consequence of these factors,
exposure varied from one and a half hours up to six
and three-quarter hours on different days while the study

was in progress.

45

Instrumentation

 

A pure-tone portable audiometer (Beltone 12C)
with THD-39 earphones equipped with MX 4l/AR cushions
and a bone conduction transducer (Radioear B-70A white
dot) was employed to administer the pure-tone tests.

The audiometer was calibrated in accordance with the
International Standards Organization (180) 1964
standards. Linearity of the attenuator was accurate
in two decibel steps. As a result, all testing was
completed using 2 dB steps.

In order to assure proper calibration through-
out the testing period, calibration was checked at
regular intervals. No systematic changes were noted.
The equipment employed for calibration included the
following: a sound level meter (Bruel and Kjaer, Type
2204), an octave band filter (Bruel and Kjaer, Type 1613),
an artifical ear (Bruel and Kjaer, Type 4151), and a
condenser microphone (Bruel and Kjaer, Type 4144). Bone
conduction calibration was completed with an artificial
mastoid (Beltone Model M5B) and its associated amplifier
coupled to a microphone amplifier (Bruel and Kjaer,

Type 2603). The Hearing Aid Industry Conference (HAIC)
Interim Standards for bone conduction were used for

0 dB hearing threshold level (Lybarger, 1966).

46

All pure-tone tests were administered in an
acoustically treated trailer constructed for the Michigan
Association for Better Hearing and Speech. The trailer
was located adjacent to the employee's entrance of the
forge so that subjects could come directly from work
to be tested and return to complete their shift with
a minimum loss of time. The ambient noise levels were
measured in the trailer test room on five occasions.
Readings were made using the A and C weighting scales
of the sound level meter as well as octave bands. The
levels noted in Table I were measured during the busiest
time of the day to assure that the room could be used
in the worst conditions that could occur. Instrumentation
employed for measuring the ambient noise included the
following: a condenser microphone (Bruel and Kjaer,

Type 4145), a sound level meter (Bruel and Kjaer, Type
22048), and an octave band filter (Bruel and Kjaer, Type
1613). In View of the highest readings (see Table I,
reading 4), it was necessary to arrange with the forge
company to close all doors in the vicinity of the trailer
during the testing time. When all precautions were taken,
the only frequency being tested which was affected by
noise was 500 Hz and only those subjects with normal
hearing at that frequency were affected. No correction
was made to the thresholds at 500 Hz. However, the
ambient noise problem at 500 Hz was considered in the

interpretation of the results.

47

.Ammmav x00 an OOOHHOmOU mam>ma Eseflxme Scum OHMOQMBm

Hmman<m¢ paw vomHTOmH cmmzumn mocmHmmeU mcfluomuundm we pmcflmuno mam>mq «E
.cmmo mums mmum mosw on muoop COBB mme mumz mocflpmmm «

 

0.0m o.Hm o.mm 0.0m o.mN o.mm o.Hm «avomHIOmH
me o ummu on “memo
CH mam>ma ESEmez

o.mo o.mm 0.0H o.oa 0.0H o.mH 0.0m o.m¢ o.mv o.vm o.mo m
o.wm o.mv o.oa o.va o.mm 0.0m o.Hv o.mv o.Hm o.vm o.mw xv
o.mm o.mm 0.0H o.oa o.vH o.om 0.0m o.Hv o.mv o.vm m.mm m
o.wo o.vm 0.0H o.oa o.oa o.mH 0.0m o.ov o.mv o.vm o.mo N
o.mw o.ov 0.0H o.va o.om o.v~ m.mm o.vv 0.0m o.¢m o.vw «a

 

ooom ooov ooom oooa oom 0mm mmH mo m.am
mamom U mamom a woswsvmnm Hmucmu Osmm m>muoo mmcflpmmm

 

.mOGMO m>muoo Ocm .mamom mcfiugmAOB O .mamum mcflunmflmz a esp no“ Omnsmmme
mm Hmaflmup may mo Eoou mcflummu may CH mp 2H mam>ma mmwos ucmHOE¢TT.H mqmde

48

Administration

 

All testing in this phase of the study was
completed in a four week period. Five subjects were
tested each week. Care was taken to assure that both
day and night shifts as well as men from both age groups
were tested each week. On the Friday preceding the
week of testing, each subject was contacted and a second
history form was completed (see Appendix B). In
addition, he was provided a notice of the test time
(see Appendix C).

When the subject reported for work on Monday
of the week he was to be tested, he was given a brief
otoscopic check to insure that ear canals were free
from excessive cerumen, foreign bodies or physical
abnormalities of the outer ear. When cleared by otosc0pic
examination, each subject was given the pre-exposure
pure-tone test. This pre—exposure test took place
between 6:15 AM and 7:00 AM for the day shift, and
between 2:45 PM and 3:30 PM for the evening shift. Mon-
day was selected to start the testing because of the
need to have at least 48 hours away from the noise to
assure accurate resting (non-noise exposed) threshold.
Following the first test, each man was given a reminder
notice for the other tests during the week (See Appendix

C).

49

Post-exposure test number one was obtained just
before the end of the subject's work shift. The schedule
was arranged so that as each hammer crew completed its
quota for the day, the subject came immediately to the
testing trailer, was tested, and then returned to his
position and completed the rest of his shift. The average
time elapsing between the time the subject left his
position and the time the test started was 5.9 minutes
for all subjects on the Mondav test.

Each subject returned the following day prior to
starting work. At that time, post exposure test number two
was given to determine if recovery of the initial (Monday)
TTS had occurred. At the end of the work shift following
the second day's exposure, the subject returned for
testing. Average time elapsing between the stopping
of work and testing averaged 6.6 minutes the second day.
The time elapsing between work stoppage and testing was
markedly influenced by two subjeccs who because of
mechanical breakdowns stopped work twenty and thirty
minutes before the experimenter was available for testing.

On Friday of the test week, nine subjects were
seen for testing following their work shift to determine
if any accumulated TTS could be determined over the week.
Only nine subjects were tested as some subjects did
not work the full week, other subjects were injured

or became sick, or work stopped because of the extreme

50

heat. Because of these problems, a total picture of TTS
on Friday could not be obtained.

During the pre—exposure test, auditory thresholds
were obtained at 500, 1000, 2000, 4000 and 8000 Hz for
each ear. The test was completed twice in order to
obtain some measure of reliability and to provide some
subject training for subsequent tests. Results of the
second test were used as the resting threshold for each
ear. Bone-conduction thresholds were obtained without
masking at 500 and 1000 Hz for each ear to determine if
an air-bone gap was present. If an air-bone gap of
greater than 8 dB was found, the subject was excluded
from the study. One subject was found to have a unilateral
conductive loss and was replaced by another subject. In
the initial test, the subject's better ear was tested
first. If no preference was noted, then the right ear
was tested. Time required for the initial test was 12
to 15 minutes.

For all post-exposure threshold tests, pure-tone
air-conduction thresholds were obtained on only the
better ear. The better ear was selected on the basis
of the average hearing level for the four octaves from
500-4000 Hz. When both ears were the same, one ear
was arbitrarily selected. Twelve right ears and eight

left ears were tested. The frequencies tested in the

51

four post-exposure tests were the same as those initially
measured. However, the order of the frequencies tested
was rotated to minimize recovery time order effects on
TTS measurements. The time required for the TTS measures
was three minutes or about 30 seconds per frequency.

Administration of all pure-tone tests was completed
manually by the experimenter. The earphones were placed
on the subject by the tester to insure proper alignment
with the ear canal. The testing method employed was
the ascending method advocated by Carhart and Jerger
(1959) but using 2 dB steps instead of 5 dB steps. Each
subject was instructed as follows:

This test is a brief evaluation of your hearing
for tones. The object of the test is to find the point
where you can just barely detect the tone. Some tones
will be very high pitched and some low pitched. Regard-
less of the pitch of the tone, the important thing is
that you indicate every time you hear it. You can signal
that you hear the tone by raising your hand. Keep you hand
raised as long as you hear the tone. When the tone is
gone lower your hand. We will test your ear first
and then your ear. Do you have any questions?

The same instructions were given before each of
the post-exposure pure-tone tests. An outline of the
testing procedure for the pre-exposure test and the post-
exposure tests is provided in Appendix D.

Threshold level in this study was defined as the

lowest intensity at which the sound was reported to have

52

been heard by the subject at two successive presentations,
or at two out of three presentations. All threshold results
were recorded on a data form, as shown in Appendix E.
TTS was calculated by subtracting the resting thresholds
from the post-exposure thresholds at each frequency.
The differences noted between tests indicated the amount
of TTS which occurred despite the use of the various
types of ear protection employed.

In summary, pure-tone thresholds were obtained
on four occasiOns over a two day period for 19 drop forge
workers and following five day's exposure for 9 of the
subjects in order to evaluate temporary threshold shift

from impact noise.

Phase III Retrospective Analysis

 

The third portion of this study involved a review
of the audiological records of drop forge workers who
had been referred for audiometric testing and if

necessary, fitting of a hearing aid.

Selection of Cases

 

From a population of 167 drop forge workers
referred for testing at the Rehabilitation Medical Center,
Edward W. Sparrow Hospital, Lansing, Michigan, 71 cases
were selected for retrospective analysis of their pure-

tone hearing thresholds. All of the subjects were seen

53

for testing between January 1961 and March 1971. Test-
ing was done by staff audiologists.

Selection of the cases for review was based

upon the following specific criteria:
. 1. Each subject was employed in the hammer shop
of the forging companies3 in the local area.

2. His chief occupation was that of working in
the drop forge hammer shop.

3. He was not habitually exposed to other types
of high intensity noise, either steady state
or.impact.

4. He did not have a congential hearing loss or
a hearing loss which pre-existed to working
in the drop forge.

5. There was no history of conductive pathology
or unilateral hearing loss.

6. His use of ear protection was documented 6.9.,
type and length of use.

7. The forging company at which the case was
employed was known.

8. The position (specific job) of the individual
subject was documented.

For some cases insufficient data were found in the

audiological records. Therefore, the individual or the

forge at which he worked was contacted and the missing

 

3Subjects were drawn from four drop forges in the
Lansing, Michigan area.

54

information obtained. Cases for which any of the above

data could not be obtained were excluded from this study.

Procedure

 

From each of the 71 cases selected from the

populations of drop forge workers tested, the following

data were obtained:

Name

Address

Age

Years of employment (or exposure)
Position held

Employing company

Type of protection used and how long
Pure-tone air-conduction thresholds.

mNQWDWNH
O O O O

The data obtained were placed on individual data
sheets. Each subject was coded according to the forge
at which he was employed. Recorded on the data sheet was
a copy of the audiOgram, including the pure-tone thresh-
olds at 250, 500, 1000, 2000 and 4000 Hz for both ears.
Data regarding thresholds at 6000 and 8000 Hz were found
to be omitted from a considerable number of cases due
to differences in testing procedures over the ten year
testing period. Therefore, these two frequencies were
excluded from the analysis. They are included, however,
in the data tables in Appendix J. All pure-tone thresholds
were converted to ISO-1964 standards to facilitate handling

and interpretation of the data.

55

For each subject, the average loss for the four
frequencies of 500, 1000, 2000 and 4000 Hz was calculated.
In addition, the slope of the loss across the three octaves
from 500 to 4000 Hz was determined for each subject
by dividing the total decibel change by the number of
octaves.

Once the data had been compiled, the data were
grouped according to age, years of exposure, types of
positions held, employing drop forge and the type of
ear protection used. The mean loss and standard deviation
for each frequency for the right and left ears combined
were determined in order to ascertain if particular trends

were evident for each of the variables noted.

 

CHAPTER IV
RESULTS AND DISCUSSION

This chapter contains the basic data obtained for
each of the three phases of the study and discussions of
their significance with regard to the experimental
questions posed. Each phase is considered separately
followed by a discussion of the relationships between the
different phases and relevant areas of interest.

Since the goal of the investigation was to study
the nature and effects of drop forge noise upon the
auditory system, elaborate statistical procedures were
ruled out in favor of obtaining an overview of the
problems involved and determining trends where evident.
Results are presented primarily using means and standard

deviations.

Phase I Noise Measurement Results

 

 

The raw data obtained during the noise survey at
the drop forge are presented in Appendix F, Tables Fl-F8.
The measurements of interest regarding impact noise were:
peak SPL, rise time and duration, spectral composition,

and some indication of the typical exposure as revealed

56

57

by repetition rate, the number of impulses per exposure
period, and the overall noise levels in the drop forge.
Continuous noise measurements of the ambient background
noise involved linear and dB A readings as well as an

octave band analyses.

Impact Peak SPL

 

The mean peak SPL and standard deviations obtained
on Hammer I (3000 lb. board hammer) and Hammer II (2000 lb.
air hammer) are presented in Tables 2 and 3. Several
findings should be noted. First, the peak pressure
levels vary with the position at which the employee stands
in relation to the hammer. The hammerman and the straight-
ener are exposed to the greatest intensity peaks. The
mean peak levels ranged from 122.7 dB to 143.6 dB for
both hammers measured. Second, the peak levels vary
according to the stroke of the hammer. As is illustrated
in Figure 3, the initial stroke of a series was always
less intense than the strokes which followed. Tables 2
and 3 also show the variation of peak levels by stroke.
This result is consistent with the nature of the forging
operation as the initial stroke is made on a hot piece
of metal of large diameter. A type of cushion occurs
between the dies on the first stroke. As the metal
becomes thinner and cools, the peak intensity increases

accordingly, with the final stroke usually being the

most intense. Third, while the intensity varies between

58

TABLE 2.--Peak pressure means and standard deviations* at
the three positions for the first and eighth strokes on the
3000 lb. hammer.

 

 

Strokes
Position lst 8th
Mean SD Mean SD
Hammerman** 128.0 1.0 143.0 0.9
Heaterman 124.3 0.3 139.0 0.7
Straightener 125.5 1.9 143.6 0.9

 

it
In dB re 0.0002 microbar
**
Ten readings made at the Hammerman's position.
Five readings made at each of the other positions.

1:
TABLE 3.—-Peak pressure means and standard deviations at

the three positions for the lst, 3rd, 5th, and 11th strokes
on the 2000 lb. hammer.

 

 

 

Strokes
POSltlon lst 3rd 5th 11th
Mean SD Mean SD Mean SD Mean
Hammerman** 128.4 0.8 140.6 1.2 139.4 0.9 141.1
Heaterman 122.7 0.8 135.4 0.5 134.0 0.9

Straightener 126.1 0.7 143.4 1.2 141.8 0.8 143.6

 

*
In dB re 0.0002 microbar

**
Ten readings made at the Hammerman's position.

Five readings made at each of the other positions.

59

strokes, the peak levels for a particular stroke in a
different series on the same hammer were found to be
extremely consistent as indicated by the small standard
deviations for both Hammers I and II. Fourth, the

peak levels for both Hammers I and II are comparable
despite the difference in type of hammer, weight of the

ram, and piece being forged.

 

Ordinate: .5 volts per division,
Abscissa: 1.0 second per division

FIGURE 3.-—Photograph of the eleven stroke series of the
2000 lb. hammer taken from the screen of a
storage oscilloscope.

60

A third hammer, a 3000 pound air hammer was meas-
ured for peak levels on the last stroke of a series
of seven for further comparison between hammers. The
mean peak level obtained was 145.8 dB with a standard
deviation of 1.3 dB. This higher peak level indicates
some differences between hammers. This finding will be
discussed in a later section of this chapter.

Since initial measurements on both hammers showed
repetition rates of more than one stroke per second,
dB A slow response readings were made on a third (3000 lb.
air) hammer to compare with the peak readings. The
mean dB A level was 113.2 dB for 10 series. Each read-
ing was the average level for a series of seven strokes,
determined visually from the VU meter of the sound level

meter.

Impact Rise Time and Duration

 

Rise times and total durations were determined
from photographs taken of individual impacts for the
last six strokes of a series of eleven by the 2000 lb.
hammer. Rise times as indicated in Table 4 are between
6 and 10 msec. The six strokes measured were those
required to forge one piece of metal on the hammer. Three

strokes were made in each of two dies. As evident from

61

TABLE 4.—-Rise time and total duration in msec for each
impact of a series of six strokes out of a total of eleven
produced on the 2000 lb. hammer.*

 

 

Stroke Rise Time Total Duration

(msec) (msec)
6 10 355
7 7 272
8 6 270
9 10 230
10 9 225
11 6 207

 

*

Strokes represented series required to form the
last half of a metal bar.

Strokes 1-5 formed the first half of the bar.

the rise times in Table 4, the first stroke in each of
the two dies had a slightly greater rise time. As the
metal became thinner the rise time decreased.

The duration of the impact was difficult to deter-
mine because an intense blast of air occurred immediately
after the hammer blow. It was not possible to differ-
entiate the duration of the impact from the air blast.
However, since the employee was exposed to both the
impact of the hammer and the air blast, a decision was *
made to compute the combined duration of those two noise

sources. Hence, the total duration shown in Table 4 is

62

for the impact of the hammer plus the air blast. The
duration ranged from 355 msec on the first stroke mea-
sured in a series to 207 msec for the last stroke of the
series. The duration became progressively shorter with
each stroke of the series. A typical waveform of the
last stroke of the series of six on Hammer II is shown

in Figure 4-

 

Ordinate: 0.5 volts per division
Abscissa: 50.5 msec per division

FIGURE 4.--Photograph of the final (11th) stroke of the
series on the 2000 lb. hammer taken from the
screen of the storage oscilloscope.

 

63

Impact Octave Band Spectrum

Figure 5 presents the spectrum of the octave band
measurements of the impacts from Hammers I and II. Read—
ings were taken from the same stroke of two series of
impacts for each octave band. The raw data are presented
in Appendix F, Tables F4 and F5. Results indicated a
peak intensity in the 2000 Hz octave band for both
hammers. However, the levels were high over a broad
frequency range. Levels were greater than 120 dB SPL
from 125 Hz through 31,500 Hz. Below 125 Hz, the levels
exceeded 110 dB SPL for Hammer I and 105 dB for Hammer II.

Impact Repetition Rate and
Number of Impacts

 

 

The repetition rate was calculated in two ways.
Initially the rate was’approximated from Figure 3, which
gave one series of impacts over a particular period of
time. Using this method, it was determined that a
rate of more than one stroke per second occurred. The
second method entailed the use of a tape recorder coupled
to a level recorder. The number of strokes occurring
from Hammer II over a 10,000 second or 16 minute and
40 second period was determined. The mean number of

strokes per minute was 53.7 or one stroke every 1.1

seconds. As the strokes occurred in series of eleven

 

: 0.0002 microbar

re

SPL in dB

 

64

 

 

 

150 w
140 i
130 w
120 « ’,
{I
110 “ .
I
100 “ Hammer I (3000 lb)
""' Hammer II (2000 lb)
90 «
t+§$£$i+4§§d
m 3
. m c> c: :E M M M M -
HMNLnOr-{Nfi‘mkor—l
MKOr—lNLO r-im
Octave Band Center Frequency
in Hertz
Figure 5. Octave band analysis of impact noise from

two drop forge hammers (Stroke 8 on Hammer I
and Stroke 3 on Hammer II).

65

strokes per piece of metal, 4.9 pieces were forged each
minute. Between each series of strokes, a period of
2.35 seconds elapsed. When the time the hammer was not
being operated is subtracted from the total time, the
actual repetition rate of the hammer was 1.1 strokes per
second. These findings apply only to Hammer II.

The total number of impacts to which an employee
was exposed was determined by taking the average strokes
per mihute and calculating the number of strokes per
hour, day, week and month. This of course, assumes
that the employee operates that particular hammer and
job constantly during the work period in question. When
calculated, the number of impulses produced was 3222 per
hour, 20,943 per day, 104,715 per week, and 418,860 per
month.

When other hammers were operating, the number
of impacts reaching the hammerman of Hammer II was 2.4
per second. This rate is an approximate since peaks on
the level recorder writeout do not indicate if two
hammers struck simultaneously. Assuming this rate of
2.4 strokes per second to be an average rate of 144 per
minute, the hammerman would be exposed to 56,160 impacts
per day or 280,800 impacts per week. The peak intensities
from the other hammers at Hammer II's location were not

determined.

66

Overall Shop Noise

 

The general level of the noise in the hammer shop
when the hammers were running was obtained using both
the linear response and the A weighting scale of the
sound level meter. The means and standard deviations
are shown in Table 5. Raw data are presented in Appendix
F, Table F6. Results indicate that at four locations
in the hammer shop, the levels are fairly constant for
both the linear and dB A readings. The only reading not
consistent with the others is location five. Here the
readings were somewhat more intense because this loca-
tion was close to two 3000 pound hammers and one 5000
pound hammer. The standard deviations obtained for the
readings at each position indicate that when the hammers

are in operation the levels are fairly constant.

TABLE 5.--Means and stardard deviations for linear and dB
A scale measurements made of overall noise levels during
hammer shop operation.

 

 

 

 

Locations* Linear dB A
Mean SD Mean SD
1 106.0 1.0 101.7 0.6
2 103.7 1.5 99.3 0.6
3 105.7 0.6 100.7 0.6
4 105.7 0.6 98.3 0.6
5 110.3 0.6 105.3 0.6

 

*
Three readings made at each location. The five

locations were simply five typical work areas in the hammer
shop.

67

Continuous Background Noise

 

The second type of noise found in the drop forge
is the continuous noise of the furnaces and blowers
which is present at all times during the work period.
Figure 6 illustrates the levels obtained in each octave
band as well as the linear and dB A scale measurements.
The most intense components of the noise appear to be
below 1000 Hz. Levels below 31.5 Hz could not be tested
due to the limitations of the equipment. Actual levels

obtained are presented in Appendix F, Table F7.

Phase I Noise Measurement Discussion

 

The questions asked at the start of this study
regarding the nature of the drop forge noise were as
follows:

1. What are the peak pressure levels from drop

forge noise?

2. What are the rise times and total duration of

the impacts from a drop hammer?

3. At what frequencies is the energy concentrated

in drop forge impact noise?

4. What is the intensity and spectral makeup of

the background noise in the forge?

5. What is the typical exposure for a hammerman

in terms of number of impulses per exposure

and their repetition rate?

0.0002 microbar

re:

SPL in dB

 

68

 

 

 

 

 

110"
100” T—
90» T
80-
70' ’8 a:
2 m
60T— )3 'U
50-
I 1 l L L 1 1 l 1 1 I
31.5 63 125 250 500 1K 2K 4K 8K 16K 31.5K
Octave Band Center Frequency
In Hertz
Figure 6. Octave band analysis of steady state furnace

and blower noise from the hammer shop of a
drOp forge.

69

The results presented in the preceding section
answer each of these questions. Restatement of the
results here is unnecessary; however, several aspects
of the noise must be discussed further.

Previous study of peak levels in drop forges by
Bolt, gp_al. (1952) revealed average levels of 138 dB
:5 dB. Present findings indicate that peak levels on
the 2000 and 3000 pound hammers generally are consistent
with the earlier findings. However, not only did a
third hammer reach higher mean levels, 145.8 dB as a maximum
peak (see Appendix F8), but all hammers measured varied depend-
ing upon the stroke of the series being used to forge the
piece of metal. Thus, there is considerable variability
to be expected not only between hammers, but also, from
a single hammer. Fox (1953) indicated that the variation
in peak levels from one hammer to another is not strictly
related to the size of the hammer. Factors such as the
construction of the machine and its mode of operation
contribute to the production of intensity peaks.

The remarkable small variation in peak levels
for the same stroke of different series on the same
hammer suggests a consistency which might be useful in
the establishing of DRC. This is assuming that other
factors such as the influence of other hammers and the

background noise are also constant.

70

The rise times, as calculated for the impacts
studied, ranged between 6 and 10 msec. These times are
somewhat longer than the rise times of 2 msec reported
by Bolt, g£_al. (1952). When comparing the hammer rise
times to the near instantaneous rise times of gunfire,
it is evident that this type of impact noise requires
more time to reach maximum intensity. However, the fact
that the rise time is more than 0.5 msec reduces the
hazard (Coles and Rice, 1967). The same type of finding
was made for the total duration of the impact noise.

That is, drop forge noise requires a longer decay time

than does gunfire. In the current study, the exact dura-
tion of the impact could not be established because a

blast of air which followed the impact became a part of

the envelope. As the impacts were made within an

enclosure, reverberation also prolonged the impact intensity.

The octave band spectrum of two hammers is an
unusual measure for impact noise, in that, the literature
has generally disregarded this type of measurement. The
primary reasons for not using octave band analysis or
other spectral methods in the live situation have been the
difficulty in measuring the variable noise levels and
the inability of the instrumentation to capture the peak
levels. More recent advances in instrumentation have

permitted more complex spectral analysis. However, due to

71

equipment limitations, the octave band approach was the
only method at hand which could be employed in the live
situation. The consistency of the peak levels for
particular strokes from one series to the next suggested
the method employed was feasible. Differences might
exist, however, because of different pieces of metal being
forged. The results indicated a broad band spectrum with
its peak at the 2000 Hz octave band. On the basis of

the rise times and durations, one would expect to find
more energy in the lower frequencies. Why this did not
occur is unknown at this time.

The background noise of the forge is continuous
and was measured in the conventional way. The dB A level
measured exceeds current DRC limits (Walsh-Healey, 1969).
Thus, considering the noise from the blowers and
furnaces alone, a level sufficient to cause hearing
impairment exists. The noise of the hammers naturally
adds to the total intensity in the shop area.

The question of what is a typical exposure for
a hammerman is a difficult one to answer. The number
of impulses he is exposed to will vary from one day to
the next, depending on the type of metal product being
forged, the number of hammers operating in the forge,
and.the number of times his hammer needs repair or
reset for a new job. The numbers given in the results

snection were estimates based upon slightly over 15

72

minutes of actual work time on only one hammer doing one

type of work.

Phase II Temporary Threshold Shift Results

 

 

The second phase of this study was divided into
two parts: obtaining resting thresholds on two groups
of drop forge employees and measuring TTS on the same
groups. The raw data obtained are in Appendices G and

H.

Resting Thresholds

 

The mean combined right and left ear resting
thresholds for the 0-10 year group and the 15-25 year
group are presented in Tables 6 and 7 respectively.
These thresholds are graphically presented in Figure 7.
The 0-10 year group had an average 3.6 years exposure
to the noise while the 15-25 year group had an average
of 20.2 years of exposure.

Results indicate that the younger group had a
mild deficit at 4000 and 8000 Hz with essentially normal
hearing in the lower frequencies. The older group showed
a mild to moderate deficit with a 15 dB per octave slope
from 500-4000 Hz. Since there were only 10 subjects in
each group, insufficient data were available to compile

a progressive audiogram showing resting thresholds at

73

TABLE 6.--Means and standard deviations of combined right
and left ear resting thresholds* for the one to ten year
exposure group according to type of protection employed.

 

 

 

Protection Number Frequengy in Hertz
Type of
Subjects 500 1000 2000 4000 8000
Mean 8.7 2.7 3.7 15.0 23.7
Cotton or
None 3
SD 3.1 1.2 3.1 10.5 7.0
Mean 15.0 14.0 10.4 19.2 14.0
Cotton to
Plugs or
Muffs 5
SD 11.1 12.3 8.7 16.5 9.8
Mean 4.5 2.5 6.5 4.5 20.5
Always
Plugs or
Muffs 2
SD 6.4 3.5 9.2 2.1 12.0
Mean 11.0 8.3 7.6 15.0 18.2
All 85 10
SD 9.0 10.3 7.4 13.5 9.5

 

dB hearing level (re:ISO, 1964).

74

TABLE 7.--Means and standard deviations of combined right
and left ear resting thresholds* for the 15 to 25 year
exposure group.

 

 

 

Protection Number Frequency in Hertz
Used of
Subjects 500 1000 2000 4000 8000
Mean 12.3 28.3 36.3 59.3 60.3
None or
Cotton 3
SD 11.8 5.8 15.3 21.0 48.8
Mean 20.9 37.3 49.4 59.4 57.6
Switched
to Plugs
or Muffs 7
SD 11.2 17.9 21.1 24.2 30.8
Mean 18.3 34.6 45.5 59.4 58.4
A11 85 10
SD 11.5 15.5 19.7 22.1 34.1
*
In dB hearing level (re: ISO, 1964).

1964)

Hearing Level in dB (ISO,

75

10* MN
20-

3o-
40-
so-

60“

70‘ C¥——{LO - 10 Years of Exposure

80.. o———{>15 - 25 Years of EXposure

90—

 

 

T

100 I I ' ' '
250 500 1000 2000 4000 8000

Frequency in Hertz

Figure 7. Mean Resting thresholds for the 0-10 and the

15-25 years of exposure groups. (N=10 subjects
per group).

76

different years of exposure. (This, however, is shown

in Part III.) A second finding is that the older group

had larger standard deviations at all frequencies

than the younger group. This result demonstrates that
there is a large variation among the older men in degree of
loss from essentially similar exposure to drop forge noise.

Resting Threshold and
Protection Used

 

Tables 6 and 7 also contain the mean thresholds
for the subjects in the younger and older exposure groups
respectively, according to the type of protection used
while working in the forge. As expected, the men with
greater exposure demonstrated greater hearing loss than
did the men in the group with less than 10 years exposure.
As demonstrated in Figure 8, the 0—10 year exposure group
showed some differences when compared by protection.

The two subjects using muffs since beginning work showed
no deficit at 4000 Hz with only a slight drop at 8000 Hz.
The three subjects who had used no protection or cotton
indicated a slight deficit at 4000 Hz and 8000 Hz as

did the five men who had switched from cotton to plugs
or muffs. The latter group also evidenced poorer
thresholds at 500 and 1000 Hz than did the other two
¢groups. All means for this group were within normal

fuearing limits (<25 dB ISO).

Hearing Level in dB (ISO, 1964)

77

 

 

 

0 ..
10 ‘
20 _
30 -
40 —
50 _
60 ‘
7O
£3Cotton/None (N=3)
80 a C>Cotton/Plugs or Muffs
(N=5)
90 ~ [JPlugs or Muffs (N=2)
100 1 I V I r v
250 500 1000 2000 4000 8000
Frequency in Hertz
Figure 8. Mean resting thresholds according to type of

protection employed by the 0 - 10 years of
exposure group.

78

For the 15—25 years exposure group (see Figure 9)
the three subjects who had employed cotton since they
started working in the forge showed better thresholds up
through 2000 Hz than did the group that switched from
cotton to plugs or muffs. Only at 4000 and 8000 Hz were
the thresholds similar. Thus for both groups, the men
who switched from cotton to muffs or plugs showed the
greatest loss. This finding was unexpected because cotton
provides very little attenuation. Why this occurred will be
discussed later.

Temporary Threshold Shift
After One Day's Exposure

 

 

As shown in Table 8, small mean TTS values were
obtained for both the 0—10 and 15-25 year exposure groups.
The small TTS values found apparently are a result of
‘the use of protection. The one subject in the group
who did not use protection demonstrated a shift of 46 dB
at 4000 Hz after one day's exposure. The older subjects
demonstrated greater shift in the lower frequencies
where generally their resting thresholds were better,
while the younger men showed higher threshold shifts at
4000 Hz.

The mean shifts did not exceed 10 dB for either
group at any frequency. However, there were a number
of subjects that exceeded the acceptable levels of 10 dB
below 1000 Hz, 15 dB at 2000 Hz, and 20 dB at 3000 Hz

and above according to the CHABA DRC (Kryter, et a1.

1964)

(ISO,

Hearing Level in dB

10 7
20-7
30'j
40 ‘
50 ‘

60'1

79

 

70.. (BCotton (N=3)

 

()Cotton/Plugs or Muffs

 

80 ~ (N=7)
9o 9
100 I U V U I I
250 500 1000 2000 4000 8000

Figure 9.

Frequency in Hertz

Mean resting thresholds for the 15 — 25 years
of exposure group according to type of protection
employed.

80

TABLE 8.--Mean TTS and standard deviations in dB following
one day's exposure for each of the two exposure groups.

 

 

 

Exposure Frequenzy in Hertz
Groups 500 1000 2000 4000 8000
Mean 4.9 3.1 5.3 7.0 4.0
0 - 10 yrs.
SD 4.1 2.8 5.6 14.6 9.4
Mean 7.0 7.2 5.2 2.2 2.6

15 — 25 yrs.
SD 5.4 5.8 4.8 2.2 6.1

 

1966). In interpreting the shift at 500 Hz, however,
one must recall the ambient noise levels in the test
room exceeded recommended levels for testing 0 dB hearing
level. In most cases, however, the men's thresholds
were sufficiently elevated that testing as low as 0 dB
was not required. Individual TTS patterns for the younger
and older men are illustrated in Figure 10 and 11,
respectively.

When considering the younger subjects by type
of protection, the two subjects using cotton or no pro-
tection showed the greatest shift in the higher frequencies.
When plugs were employed, the one younger man demonstrated
small amounts of TTS primarily in the lower frequencies.
The three subjects using muffs showed variable TTS at
all frequencies. In no case did TTS exceed acceptable
limits (CHABA working group #46) when muffs or plugs

were employed.

81

Among the older subjects, the greatest TTS for
the men that employed cotton for protection occurred in
the lower frequencies. The majority of the men in the
15-25 year exposure group had switched to the use of
plugs within the past 5 years. TTS obtained from those
men revealed small shifts at each frequency,but the

amount was variable from man to man.

TTS after Second Day's Exposure

 

The mean TTS following the second day‘s exposure
is presented in Table 9 for each group. The mean shifts
are fairly constant with those obtained after the first
day's exposure. Only at 4000 and 8000 Hz for the 0—10
year exposure group was any real change noted. By com-
paring the shifts made by Subject 3 on Figure 10 for the
two days, one can note why the change in mean threshold
shifts occurred at 4000 and 8000 Hz. Again, the older
group showed the greater shift for the lower frequencies
while the younger group demonstrated a more consistent
shift across frequency. No large change was noted for

most subjects from one day to the next.

{gecovery from One Day's TTS

 

Thresholds were obtained on both groups sixteen
hcours after the first day's exposure. Results shown in
Table 10 indicate that thresholds had essentially returned

'UD pre-exposure levels for both groups of subjects.

82

mo s can» once coumflnm

mm a“ hocmsvmum
mm .x« xN xa Mm.

 

mmnu
hmno
omnw

wwwsz

um cw hocosvmum

0N

om

ov

 

 

Mm x¢ MN xa Mm.

  

fille|1 wl.

  

mm 0
mm 0

mmAQ

mums:

om

OH

:om

om

ov

 

om

8P u; 511

HP u; 511

N: am xocmswoum
mm .Mv mm ma Mm.

d 1

. Gll‘lfl . P
.///Q\\x9//lq -roa

 

 

mmsHm gem

M<QmmDB mom mBB

mm cfl wocwskum
Mm mm xm Ma Mm.

 

7|1A¥\\)7//AW a i o
IILWIIAV .Aoa
low
1
1 cm
«mnw J ov
mODHm . om

 

><QZOE mom mBB

8P “T 811

HP U? 811

.hocmsvoum ummu ham um
0:3 muouo musmomxm munch oHuo :fl muomnndm now was mmpmmsa can amccoz

us ca mucoavmum

ME‘ x4 am (mm m.
4 {is

 

mmnu
HmAV

mcoz Ho couuou

am am mocmsvmum

 

an. x4 xN ma am.
— q

, \ .mm o
q Hmcq

ocoz no couuoo

A__l

 

n
__‘_

!

 

om

om

 

.oa magmas

OH

ON

on

ov

om

0H

.ov

on

8? u? 311

HP “T 811

83

mo v away once pwumwnm 0:3 msoum chamomxw mo ammo» mNImH

um ca aocmsvmuh
xm xv mm ma Mm.

 

- fi 4

mammw

mums:

a: CH aocmsvmum
Mm xv xN ma Mm.

)1

1

 

 

\Xl.

mHmAV

mums:

 

OH

on

on

oe

om

0H

cm

on

ov

om

mm :a mocmnvmum
Km xv mm &H xm..

 

 

o
1 3
m 1
3 8m- ; om
m. 3m 0
p m o 1 on
a S 1

onG gov

 

macaw ..om

w<DWMDB mom wBB

N: am xocwsgmum
xm. me xm ma xm.

 

 

 

o
. 3
L l
m
.1 ON
m. omml .1
9 2m 0 .1 on
8
hamno
. 3
V mam Q
mmsHm gem

MdQZOE mOh mBB

HP “T 811

8? “T 511

.hocmavmum and» ham as
aw muomnbdm MOM was umpmmaa can wanna:

u: cw wocoavoum

 

 

mHmnu
«Hmnu
HHmAV

couuoo

um ca wocmavmum
mm x¢

gm ma Mm.

3m o
«G 0
Sad

covuoo

 

1

L14

 

 

.HH munmflm

OH

on

on

ov

cm

on

ov

om

8? “T 811

HP u? 511

84

TABLE 9.--Mean TTS and standard deviations in dB following
the second day of exposure for each of the two exposure
groups.

 

Frequency in Hertz

 

 

Exposure
Groups 500 1000 2000 4000 8000
Mean 4.0 2.9 4.4 4.6 0.4
0 - 10 yrs.
SD 4.2 3.9 4.2 8.2 4.
Mean 6.2 7.0 6.0 1.6 2.4
SD 5.9 6.1 4.3 3.1 5.5

 

TTS after Five Days of Exposure

 

Only nine of the twenty subjects completed five
consecutive work days during the week that they were
being tested. Reasons for those not working five days
included injury, illness, or work stoppage because of
extreme heat. Also, a couple of subjects forgot to report
for hearing testing despite reminders. Table 11 presents
the TTS of each of the nine men for Monday, Tuesday, and
Friday. As can be seen, TTS did not vary greatly from
one day to the next except in rare instances. The
important finding, however, is that TTS after five days
of working in the forge was essentially the same as the
TTS after one day of exposure. Thus, there appeared to

be no accumulative effect during the work week.

85

TABLE lO.--Mean TTS and standard deviations in dB following
16 hours recovery time from initial exposure period.

 

Exposure Frequency in Hertz

o
rouPS 500 1000 2000 4000 8000

 

Mean 0.7 0.4 0.4 1.1 -1.1
0 - 10 yrs.
SD 2.8 1.9 1.7 1.8 1.1

Mean 1.0 -0.2 -0.6 —l.0 0.2
15 - 25 yrs.
SD 2.2 2.4 1.0 1.1 2.4

 

Several of the subjects were tested prior to
entering the shop on the last day of the week. While these
results are not presented, the thresholds indicated that
hearing levels at the start of the fifth work day were
essentially the same as the resting thresholds taken at
the start of the week. Subject 3, the one who shifted
46 dB after one day¥5exposure, also started the last day

of the week with no accumulative TTS.

TABLE ll.--TTS in dB following exposure on Monday, Tuesday,

86

and Friday for nine subjects working the full week.

 

 

 

Exposure 85 Date of Frequency in Hertz
Group Test 500 1000 2000 4000 8000
Monday 6.0 4.0 12.0 8.0 -2.0
6 Tuesday 6.0 10.0 10.0 2.0 -2.0
Friday 6.0 12.0 10.0 10.0 —2.0
Monday -2.0 0.0 0.0 0.0 2.0
0 - 10 yrs 8 Tuesday -2.0 -2.0 0.0 -2.0 0.0
Friday -4.0 -2.0 0.0 0.0 -2.0
Monday 2.0 0.0 —2.0 0.0 -2.0
10 Tuesday 0.0 0.0 -2.0 0.0 -4.0
Friday 0.0 4.0 0.0 0.0 -4.0
Monday 16.0 18.0 8.0 2.0 0.0
11 Tuesday 12.0 14.0 6.0 0.0 0.0
Friday 18.0 10.0 4.0 0.0 -2.0
Monday 2.0 2.0 0.0 0.0 0.0
14 Tuesday —2.0 2.0 -2.0 2.0 -2.0
Friday 4.0 0.0 0.0 4.0 0.0
Monday 4.0 6.0 0.0 0.0 10.0
15 Tuesday 0.0 6.0 4.0 6.0 2.0
Friday 8.0 8.0 2.0 0.0 8.0
15 — 25 yrs
Monday 0.0 4.0 6.0 2.0 -2.0
17 Tuesday 4.0 4.0 8.0 0.0 -2.0
Friday 8.0 8.0 4.0 2.0 6.0
Monday 10.0 8.0 10.0 2.0 ----*
18 Tuesday 12.0 8.0 12.0 -2.0 ----*
Friday 8 0 6.0 8.0 0.0 ----*
Monday 12.0 10.0 10.0 2.0 16.0
20 Tuesday 12.0 10.0 6.0 0.0 14.0
Friday 10.0 10.0 6.0 2.0 8.0

*
Ss did not respond at this frequency for any pre or

POSt;exposure test.

87

Phase II Temporary Threshold Shift Discussion

The questions asked at the start of the investigation
concerning TTS resulting from drop forge noise were as
follows:

1. What are the changes in resting threshold

as a function of years of exposure and type
of ear protection?

2. What differences exist in TTS following eight
hours of exposure within age groups with
different types of ear protection?

3. What are the differences in TTS obtained
between age groups when using similar types
of ear protection devices?

4. Is sixteen hours of recovery time sufficient
for total recovery of any TTS resulting from
one day's exposure to drop forge noise?

5. Does TTS vary from one day's exposure to another?

6. Is TTS on the fifth day of exposure greater
than that resulting from one day's exposure?

In considering the first question regarding resting
threshold changes due to years of exposure, the mean
thresholds for the 15-25 year exposure group were influenced
greatly by the results of subject 4. This subject had
23 years of exposure to drop forge noise and as shown

in Appendix G, Table G2, his hearing is exceptionally

88

good. As a result,because of the small sample size,
the means for the men that changed protection types

as well as the older group as a whole showed less loss
than was generally found over all. Variability was
also demonstrated in the 0-10 years of exposure group
individual thresholds although not to the degree that
the older group varied. The younger men.having the
benefit of protection early in their work history,show
less loss and less variability. The older men generally
had not employed protection other than cotton until
after 15 years of exposure. This work period allowed
individual differences in susceptibility to be demon-
strated.

The resting thresholds according to work posi-
tion of the men were not compared as the men were not
equally divided by position between groups. Most of
the hammermen were in the older group because that
position is a seniority position, while the heatermen
and straighteners were in the younger group of men.

In Figures 8 and 9, it was previously noted
that the men in both groups who had switched from cotton
to plugs or muffs demonstrated a slightly greater hear-
ing loss than did the groups that used just cotton or
had used only muffs or plugs. While, again, the sample
size was too small to generalize, one might conjecture

that these men switched to plugs or muffs when available

89

because they were not satisfied with the protection
provided by cotton. The approach of looking at resting
thresholds as a way of determining effectiveness of
protection is relatiVely new and has several limitations.
These will be discussed later in this chapter. It is,
however, one way of determining what long term effects
ear protection has on threshold changes.

The second question regards TTS after 8 hours
of exposure for men in the same exposure group with
different types of protection. For the men in the
0-10 years exposure group, the subjects using cotton
or no protection demonstrated the greatest shift.

The TTS occurring when plugs and muffs were used was
generally small, exceeding 15 dB at only one frequency
for one subject (see Figure 10). Again, the N was

too small to make an adequate statistical comparison.
Nonetheless, the use of protection does appear to reduce
TTS.

For the older men, the use of cotton for pro-
tection resulted in TTS occurring primarily in the
lower frequencies. A contributing factor, however, is
the fact that resting thresholds were poorer in the
higher frequencies and thus less TTS occurred in those
frequencies. When using plugs, wax impregnated or
valve type, the older men showed mild TTS, 0-16 dB;

but,both the low and the high frequencies were shifted.

90

The use of muffs for one of the older men also showed
mild shifts in both the low and high frequencies. In
general, the TTS resulting from one day's exposure is
reduced but not eliminated through the use of ear
protection devices, preferably plugs and muffs.

When comparing TTS between the two age groups,
the only differences occurred in the cotton or no
protection subgroups. The older men demonstrated TTS
in the lower frequencies while the younger men showed
the greatest shift in the higher frequencies. This
appears to be related to the amount of hearing loss
present before exposure.

The time needed for recovery from one day's
exposure apparently was less than 16 hours for all
subjects. Thus, the fourth question was answered
affirmatively.

Variability from one day's exposure to another
is shown graphically in Figures 10 and 11. Small
variations did occur for most of the men,but the changes
were small. Subject 3, however, demonstrated consid-
erable change at 4000 Hz. The cause for the large
difference in TTS for subject 3 is probably due to the
marked difference in type and amount of exposure over
the two day period. The first day he spent as a heaterman
on a 3000 pound hammer for 6% hours. The second day,

because of equipment breakdowns, he was exposed to his

 

91

particular hammer only 1% hours, plus the background
noise from other hammers and fans and furnaces. The
exposure varied for other men also, but not as dramatically.
The exposure changes which occur from one day to the
next must be considered in more detail if the relation-
ship of hearing loss to impact noise is to be determined.
The results indicated little change in TTS from
one day to five days exposure under actual working
conditions. Even pre-exposure thresholds showed no effects
of accumulation of TTS over the five day period. These
findings suggest that when using ear protection, each
man tends to start at his resting threshold each working
day. What happens when no protection is employed is
unknown at this time. However, because of the recent
surge of interest in protection, availability of sub-
jects for a study of that type would be limited.

Phase III Permanent
Threshold’Shift

 

 

This phase involved reviewing the audiologic
findings of 71 drop forge employees seen for evaluation
at the Rehabilitation Medical Center. Results based
upon the pure-tone thresholds are presented according
to: configuration of loss, age, years of exposure,
employing forge, position worked, and type of protection
worn. Raw data regarding personal data and hearing

thresholds are presented in Appendix I and J respectively.

92

Configuration of Loss

 

For each subject selected, a four frequency
average (500-4000 Hz) was calculated along with a two
frequency average (250-500 Hz) and the slope of the loss
for the octaves from 500-4000 Hz. After reviewing each
case, arbitrary criteria were established for categoriz—
ing the configuration of the loss according to amount
of slope. This was done to differentiate the steeply
sloping losses from those with gradual or flat configura-
tions. The criteria were as follows: a) a two frequency
average equal to or less than 40 dB hearing level; b) a
difference between the two frequency and four frequency
average of greater than 20 dB; and c) a slope equal to
or greater than 8.3 dB per octave. When all three of
these criteria were met in one or both ears,the subject
was placed in the steeply falling configuration category
(See Figure 12). When one or more of the criteria were
IM)t met, the subject was placed into the gradual sloping
category (See Figure 13). When applied to the cases
selected for this study, 69% had hearing losses with
steeply sloping configurations while 31% did not meet
these criteria and hence demonstrated a flatter config-
uration while being exposed to similar amounts of drop
forge impact noise. While the amount of slope of loss

varied somewhat, these two patterns predominated.

1964)

Hearing Level in dB (ISO,

10

20

3O
4O
50
6O
7.
80

90
100

Figure 12.

 

93

C) Right Ear
X Left Ear

 

X
X
X
r T I I I
250 500 1000 2000 4000 8000

Frequency in Hertz

Typical steeply sloping audiomatric configura-
tion of a subject (A13) with 24 years exposure.

Hearing Level in dB (ISO, 1964)

10 ‘

40 ~

50

6O ‘

70"

80 j

90 j

 

100

Figure 13.

94

0 Right Ear
X Left Ear

(PX

22><:;

—‘

F f l I l 1

250 500 1000 2000 4000 8000

Frequency in Hertz

Typical gradual sloping audiomatric
configuration of a subject (C13) with 16
years exposure.

95

Hearing Loss by Age

 

For purposes of analysis, the men were grouped
according to age. The groups were patterned after Corso
(1963) with cases falling between Corso's age classifi-
cations being placed in the closest category. Corso's
data Were selected as they provided a homogenous population
with little or no noise exposure. The age distribution
is presented in Table 12. As is evident, very few
cases are in the age group from 26 to 32 years. The
reason for this lack of cases was that very few younger
men were referred to the hearing clinic from which the
cases were selected.

Table 13 presents the mean thresholds and
standard deviations for each of the age groups by frequency.
In addition, the table presents the difference between
the mean thresholds and the presbycusic factor according
to Corso (1963). To explain, the difference between the
actual threshold and predicted loss due to presbycusis
is the amount by which the hearing loss was greater than
the age factor alone. For example, the mean threshold
at 4000 Hz for the 59-65 year age group was 74.4 dB.

The presbycusic loss for that age group is 34.8 dB
(See Appendix K). The difference between the two is
39.6 dB which represents the amount that the loss due
to noise was greater than the age factor. The data

for each age group are presented graphically in Figure 14.

96

TABLE 12.--Age distribution* for 71 drop forge workers seen

for hearing evaluations.

 

 

Age Group
in Years No. of Cases
26 32 2
34 40 14
43 49 24
51 57 18
59 65 13

 

*

Distribution according to Corso's (1963) groups.

97

TABLE l3.--Means and standard deviations for combined right
and left ear thresholds by age groups along with difference
between the thresholds obtained and the presbycusic factor.*

 

Frequency in Hertz

Age Groups

 

 

250 500 1000 2000 4000
Mean 20.0 33.8 47.5 63.8 76.2

26 - 32
(N=2) SD 7.1 7.8 7.1 1.8 8.8
Diff. 19.6 31.6 46.9 62.7 68.7
Mean 20.2 32.1 49.8 57.8 70.9

34 - 40
(N=14) SD 10.3 15.3 8.8 9.5 15.3
Diff. 22.5 31.8 46.9 55.5 61.7
Mean 24.7 32.4 51.4 64.9 72.9

43 - 49
(N=24) SD 13.6 15.3 11.6 11.1 14.5
Diff. 20.5 29.5 44.8 53.5 60.4
Mean 29.4 40.1 51.7 64.2 74.0

51 - 57
(N=18) SD 9.7 8.4 14.7 15.7 16.2
Diff. 27.9 35.4 45.2 53.2 51.7
Mean 33.8 43.8 55.2 68.1 74.4

59 - 65
(N=l3) SD 19.2 17.3 15.0 8.7 10.1
Diff. 26.3 36.3 45.6 49.9 39.6

*
Presbycusic factor based on Corso's (1963).

98

10009

Eouw mcHuHsmmu

mm :H woum

-000?
LOOOZ
b-000‘l.'
PODS
—OSZ

 

./. j
/// 1

tic-II!

 

l I

Amummw melmmv

um cH Umum

-OOOZ
-OOOT
-OOS
-OSZ

 

 

OOH

1 cm
1 on
1 co
1 cm
1 0v
1 cm
. 1 ON
l// 1 OH

0/ o‘l‘w

 

Amummw mvlmvv

mmoH paw mpHonmwusu mcHummn Ham pwwH cam unOHu omcHnEoo cam:

OOH
OO

cm
on
Om
om
Ov
Om
ON

OH

000?

O

~OOOV

.msoum mom sumo now mHmsowbmmum

um CH kum

—OOOZ
—OOOT
-OOS
-0SZ

 

 

Amumow hmIHmv

mm :H gown

Z
0
0
O

/

910' O

T
0
O
0

_

OOS
OSZ

OOH.

om
on

J

l

ow

A:
f

ON
OH

I‘. I o

414,]

 

Amummw owlvmv

 

Ammma .omuoo

umuwgv mmoq :1.

OHmsoanmoum

cams onwn ocm
m>onm om 0:0

F—U-i

pHozmmune cows 0

am cH wmum

V Z .1
0 no 0 S Z
0 0 o 0 C.—
0 0 .0 0 0
t #11 _ _ b

 

/o"‘l\|l’.

Amummw NMIONV

[11

1

L4

 

.vH muswam

OOH
om

om
Oh
OO
om
ov
OM
ON
OH

'OSI) IeAaq burzeen

(V961

99

While the N is small for the youngest age group, the
configuration of the loss is similar for all five
groups. The loss at 4000 Hz appears to be similar for
all groups,suggesting it has reached a terminal level
of change already by the youngest age group. The presbycusic
effects according to Corso (1963) show the increasing
loss due to age. However, over the five groups, the
loss does not continue to get poorer at 4000 Hz, suggest-
ing that the effect of presbycusis has not been added
on the loss due to noise exposure. If presbycusis
would be a significant factor for a group of workers,
this can be observed (see Table 13) by the difference in
actual thresholds and the presbycusic factor becoming
smaller as a function of years of exposure. In other
words the longer a man works in noise, the resulting
hearing loss becomes less a loss from noise and more
a loss from age. Table 13 shows that this was not the
case. Instead, differences between actual thresholds
and presbycusic factors remained large, allowing the
conclusion that presbycusis was not an important factor.
Further comment will be made regarding this effect in
the general discussion section of this chapter.

A composite audiogram is presented in Figure 15
and illustrates the mean threshold changes resulting by
age group. The major changes which can be seen are in

the low frequencies. Both 250 and 500 Hz show changes

(ISO-1964)

Hearing Level in dB

 

100

 

 

I 26-32 Years
0 ‘ C) 34-40 Years
1 0 43—49 Years
0 .4
A) 51-57 Years
20 - 9 59-65 Years
30 -
40 s
50 n
60 ~
70 -
80 _
90 fl
100 Y
50 500 1000 2000 4000 8000
Frequency in Hertz
Figure 15. Mean hearing levels for combined ears for

each age group.

101

from the youngest to the oldest age group greater than
the loss from presbycusis alone. Frequencies from
1000-4000 Hz showed little change from the younger group
to the older group. This implies that maximum hearing
loss resulting from this type of noise exposure occurs
within a few years (in the youngest age group). Further,
the presbycusic factor never "catches up" with the

noise - induced portion of the hearing loss, even among

the older men.

Hearing Loss by Years of Exposure

The cases were subdivided into six groups accord-
ing to years of exposure to drop forge noise. The
results of this distribution are shown in Table 14. As
can be seen from Table 14, most of the cases had been
exposed between 15 and 35 years. Cases under 15 years
of exposure were scarce in the records reviewed for this
study. The reasons why more men between 1 and 15 years
of exposure have not been seen for audiological evalua-
tion can only be conjectured. This will be discussed
later in this chapter. Table 15 presents the mean ages
and years of exposure for each of the six exposure groups.
As can be seen, the variation in years of exposure within
each group is small. This is partly the result of having

groups based on six year intervals.

102

TABLE 14.--Years of exposure distribution for 71 drop forge
workers seen for hearing evaluations.

 

 

Years of
Exposure No. of Cases
8 - l4 4
15 - 21 25
22 - 28 24
29 - 35 ll
36 - 42 6

43 - 49 l

 

103

TABLE 15.--Means and standard deviations for age and years of
exposure of subgroup.

 

 

 

 

Age Years of Exposure
Years of Exposure Mean SD Mean SD
.8 — 14 35.0 6.4 11.2 2.2
15 - 21 42.1 4.0 18.6 1.8
22 - 28 51.5 6.3 24.3 1.5
29 - 35 54.8 3.0 31.5 2.3
36 - 42 60.5 2.7 40.9 1.1
43 - 49 65.0 ---* 48.0 ---*

 

N=l for 43-49 years of exposure group.

104

Table 16 presents the means for the combined
ear thresholds by frequency. These results are graphically
depicited in Figure 16 in a composite audiogram.

The lower frequencies, up to and including 1000 Hz show

a consistent pattern of increasing hearing loss as a
function of number of years of exposure. The frequencies
above 1000 Hz show a more variable pattern of loss by
years of exposure suggesting that high frequency thres-
holds are essentially maximally affected by this type

of noise after approximately eleven years of exposure
(See tables 15 and 16).

Thus, analysis of the data by age groups or years
of exposure demonstrates that the severity of the loss
changes only slightly after 10 to 15 years exposure.

The hearing loss after that time is primarily in the

low frequencies resulting in a flattening of the config-
uration. Finally, it would appear that the major hearing
loss resulting from drop forge impact noise has occurred
prior to the time that presbycusis becomes an important
factor. Further, hearing loss resulting from age alone
(presbycusis) is vastly "over-shadowed" by the loss

resulting from noise exposure (See Figure 14).

105

TABLE 16.--Means and standard deviations for combined right
and left ear thresholds* by years of exposure.

 

Frequency in Hertz

 

 

 

Years of
Exposure 250 500 1000 2000 4000
Mean 20.6 33.1 46.9 57.5 77.5
8 14
SD 10.1 14.3 13.9 15.1 14.3
Mean 22.0 30.4 49.4 60.5 71.0
15 21
SD 11.9 14.6 9.0 10.6 15.5
Mean 27.7 39.3 52.0 65.2 71.5
22 28
SD 11.6 6.1 12.6 13.2 12.9
Mean 30.5 42.5 55.2 61.4 75.9
29 35
SD 13.9 10.2 15.5 12.7 14.6
Mean 36.2 45.8 57.5 68.3 75.4
36 42
SD 23.1 22.4 18.6 11.1 12.7
Mean 42.5 55.0 55.0 72.5 80.0
43 49
SD ---— --—- ---— ---— —--—
*
In dB hearing level (re: ISO, 1964).

1964)

Hearing Level in dB (ISO,

.0309

10

20

30

4O

50

6O

70

80

90
100

8-14
15-21
22-28
29-35
36-42

 

106

 

 

I l ' I '

250 500 1000 2000 4000

Frequency in Hertz

 

years
years ------
years ————-- ————
years -
years ...........

 

Figure 16. Composite audiogram of mean combined ear

thresholds grouped by years of exposure.

107

Hearing Loss and Employing Drop Forge

 

To determine if differences existed between the
four drop forges at which the men were employed, a
comparison of hearing losses by company was completed.
Table 17 presents a summary of the combined ear thresholds,
the slope of loss and the number of employees for each
position by forging company. The results indicate that
the combined average loss (500-4000 Hz) for forges A,

B, and C are very similar although the standard deviations
are large. The fourth forge (D) shows a somewhat greater
mean loss but the small number of cases from that company
may account in part for the disparity. The mean slope

of the loss for all of the companies is consistently
greater than 10 dB per octave. Of the cases reviewed,

68% had slopes of 10 dB per octave or greater while 32%
had slopes of less than 10 dB per octave. Slopes ranged
from .8 dB per octave to 30 dB per octave.

The breakdown of personnel by forge indicates
that 51 men were employed as hammermen, 14 as heatermen,
and 6 were straighteners.

The distribution of hearing loss by drop forge
is depicited in Figure 17, A, B, C, and D. The overall
distribution is shown as Figure 18. The most evident
result is the spread of loss for each forge. The vari-
ability of loss is markedly influenced by age and

exposure time, but not in the progressive manner as one

108

10 - Drop Forge A (N = 25)
_ Mean Years of Exposure 23.9
8 _ Range 16—48 Years

 

 

 

 

No. of Employees
Ch

 

 

 

 

 

 

 

 

 

 

 

 

0 [1 _ _ 7 ,
’l l I F 1 I l l I I
25—29 35-39 45—49 55-59 65-69 75-79
30-34 40-44 50-54 60-64 70-74
Average Combined Hearing Loss in dB (Re ISO, 1964)

Figure 17A. Distribution of hearing loss in Drop Forge A.
10 - Drop Forge B (N = 21)

Mean Years of Exposure 28.2
Range l9-42.5 Years

 

 

 

 

No. of Employees
ox

 

 

 

 

 

 

 

_ [—1——

o //

 

| I I 1 I
25-29 I 35-39 I 45—49 I 55-59 I 65-69 I 75-79
30-34 40—44 50—54 60-64 70-74
Average Combined Hearing Loss in dB (Re ISO, 1964)

Figure 17B. Distribution of hearing loss in Drop Forge B.

109

DrOp Force C (N = 18)
10 I Mean Years of Exposure 21.4

 

 

 

 

 

 

 

 

 

 

 

 

 

Range 12-40 Years
112%
m _
3‘ ..
H 6
04 .—
5‘4.
'44 1.-
o
.2“
O A
Zen/l I I I I

I l I ‘ I
25-29I 35597 45-49 I 55L59I 65-69I 75-79
30-34 40-44 50—54 60-64 70-74
Average Combined Hearing Loss in dB (Re ISO, 1964)

Figure 17C. Distribution of hearing loss in Drop Forge C.
10 _ Drop Forge D (N = 7)

Mean Years of Exposure 22.4
Range 8-30 Years

of Employees
m

 

 

NO.

 

 

 

 

 

 

 

 

I | I I I T I l | I I
25-29 35-39 45-49 55-59 65-69 70-79
30-34 40-44 50-54 60-64 70-74

Average Combined Hearing Loss in dB (Re ISO, 1964)

Figure 17D. Distribution of hearing loss in Drop Forge D.

 

110

20"
I All drop forges (N=71)
18> Mean Years of Exposure
_ 24.4

16~ Range: 8-48 Years

 

14-

12*

 

10'

 

o. of Employees

N
ox
I

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0 i7

 

I I | I I I I I I
25-29 35-39 45-49 55—59 65-69 75-79
30—34 40—44 50—54 60-64 70-74

Average Combined Hearing Loss in dB (Re 180, 1964)

Figure 18. Distribution of hearing loss for all four forges.

 

111

TABLE l7.--Summary of combined ear thresholds, slope of loss,
and number of employees for each position by drop forge

 

 

 

company.
Combined Ears Position
Drop Forge PTA* Slope
Company Mean SD Mean SD Hammerman Heaterman Str.
A 54.9 12.5 11.4 4.3 19 3 3
B 56.1 9.4 13.8 5.0 15 6 -
C 55.5 12.0 11.3 6.3 10 5 3
D 64.0 7.2 12.9 6.1 7 - -

 

*
Pure-tone average 500-4000 Hz (re: ISO, 1964).

would expect. Some of the cases with better hearing are
among the older men who have had considerable exposure.
The consistency of the means for the three forges with
the larger N indicates that the losses generally are
comparable between forges. The fourth forge which shows
the greater mean loss also had the smallest N and,there-

forepis not directly comparable.

Hearing Loss by Position

 

Regarding hearing loss by position, data were
organized to consider the age, years of exposure, average
loss for the right and left ears separately and combined,
and average losses at each frequency. Table 18 summarizes
the data concerning age, years of exposure and average

for the ears separately and combined. Results indicate

 

112

that while the number of cases reviewed by position were
not equal, the mean age for employees working different

positions were comparable. There did exist, however,

an approximate 4 years difference in years of exposure.

When considered by position, the combined ear
hearing loss averaged 58.7 dB for the hammermen and 51.9
and 45.9 dB for the heatermen and straighteners respec-
tively. Apparently the position worked does influence
the degree of loss; however, this finding is also
influenced by the difference in years of exposure between
hammermen and the other two positions. Viewing the ears
separately, the mean average losses were within three
dB for each position as were the standard deviations.
These findings suggest that the losses for each ear
are essentially the same.

Table 19 presents the mean combined thresholds
for each frequency according to the position worked.
Figure 19 compares the three positions. The results
indicate that the hammermen have the greatest loss
while the heatermen have a parallel but somewhat lesser
hearing loss. The straighteners, on the other hand,
demonstrate less loss in the higher frequencies than
either of the other positions. It must be noted,
however, that the difference in number of cases and in
years of exposure may account for part of the disparity

between positions. Also, since the hammermen have usually

113

TABLE 18.--Age, years of exposure, average* hearing thresholds
and standard deviations for the right ear, left ear, and com-
bined ears as a function of the employee's position.

 

 

Years of Right Left Combined

Position Age Exposure ear ear ears

Mean Mean Mean Mean Mean
Hammerman 48.5 25.5 60.0 57.4 58.7
(N=Sl) (9.0)** (8.7) (11.5) (11.7) (10.8)
Heaterman 47.2 21.6 52.0 51.6 51.9
(N=l4) (6.6) (4.9) (9.5) (10.5) (9.7)
Straightener 47.2 21.7 44.4 47.4 45.9
(N=6) (5.4) (5.0) (9.0) (9.3) (8.7)

 

* 1
Average of thresholds 500 - 4000 Hz in dB (re:
ISO, 1964).

**
Standard deviation.

114

TABLE l9.--Mean hearing thresholds* and standard deviations
for the right and left ears combined as a function of posi-
tion in forge.

 

 

 

Fre uenc Position

in gertzy Hammerman Heaterman Straightener
Mean SD Mean SD Mean SD

250 27.8 14.5 22.7 10.1 25.8 11.9
500 38.7 14.9 30.4 14.5 33.3 11.5
1000 54.1 12.4 48.6 10.5 40.0 10.4
2000 65.7 12.2 60.9 9.6 51.7 12.7
4000 75.9 13.9 67.7 11.0 58.8 7.7

 

*
Average of thresholds at 500-4000 Hz in dB (re:

1964).

ISO,

115

 

 

0 _
10’—

3

g 20--

H

5' 30 —

U)

H

7’ 40 -

m

ro

C: 50 _

H

3', 60 —

>

o

1—1 70 1..

m

c

'Q 80 -

m

m

m 90 _

100

 

1 1 1 | I
250 500 1000 2000 4000
Frequency in Hertz
A H amme rmen

ElHeatermen
C)Straighteners

Figure 19. Composite audiogram showing mean combined
thresholds by working position.

116

worked at all three positions in the past, their loss is
a combination of exposure at all three positions. In
general,hammermen show the greatest loss of the men in

the forge.

Position by Years of Exposure

 

The mean thresholds for the combined ears for
hammermen calculated by years of exposure are presented
in Table 20, and graphically presented in a composite
audiogram on Figure 20. The results continue to show
the progressive loss in the lower frequencies and to a
lesser extent in the high frequency region. The number
of cases for each year of exposure group is not equal, a fact
which limits any general interpretation. A trend, however,
is present. The continuing finding of large standard
deviations supports the wide range of individual
susceptibility to impact noise.

Table 21 presents the same type of data seen in
Table 20, but for heatermen. Because of the small
number of cases reviewed,only three years of exposure
groups are included. However, as seen in Figure 21 (A
and B) the trend is for progressive loss at 250 and
500 Hz with smaller reductions or no change in hearing
threshold for the higher frequencies. For purposes of
comparison, the data were calculated both for average

loss for the better ear and for combined ears. Figure

117

TABLE 20.--Mean thresholds* and standard deviations for the
right and left ears combined for hammermen by years of
exposure.

 

 

 

Years Frequency in Hertz
of
Exposure 250 500 1000 2000 4000
8 - 14 24.2 39.2 52.5 65.0 82.5
=3 (8.8)** (9.5) (10.0) (2.5) (12.5)
15 - 21 22.0 33.0 51.4 60.6 73.0
N=16 (12.6) (14.3) (7.0) (10.8) (16.5)
22 - 28 28.8 38.6 55.0 69.1 76.6
N=16 (14.0) (14.3) (12.7) (12.8) (12.0)
29 - 35 30.6 42.2 55.3 66.7 77.5
N=9 (15.6) (11.2) (17.2) (15.6) (15.7)
36 - 42 36.2 45.8 57.5 68.3 75.4
=6 (23.1) (22.4) (18.6). (11.1) (12.7)
43 - 49 42.5 55.0 55.0 72.5 80.0
N=l

Hearing thresholds in dB (re: ISO, 1964).

**
Standard deviation.

118

 

 

 

 

 

 

0 _
10 ~
3
\O
m 20 _
H
0* 3o -
m
S
40 ‘
m
t
c 50 “
oa
H
m 60 —
>
3
70 *
6w
.5
H 80 -
m
3%
90 _
100
1 j 1 I T
250 500 1000 2000 4000
Frequency in Hertz
A 8-14 N=3
0 15-21 —————— - — 1:16
0 22-28 ----- N=16
A 29-35 N=9
0 36-42 1:6
I 43-49 - ——-- ——— N=l

Figure 20. Composite audiogram of mean combined ear
thresholds for hammermen by years of exposure.

119

TABLE 21.--Mean thresholds* and standard deviations for the
right and left ears combined for heatermen by years of

 

 

 

exposure.

Years Frequency in Hertz

of
Exposure 250 500 1000 2000 4000
8 - l4

N=0
15 - 21 21.9 25.8 45.8 60.3 67.5
N=9 (ll.3)** (15.6) (11.3) (10.6) (13.7)
22 - 28 20.0 35.0 50.0 61.7 67.5
N=3 (9.0) (9.0) (8.7) (11.8) (4.3)
29 - 35 30.0 43.8 55.0 62.5 68.8
N=2 (0.0) (5.3) (7.1) (3.5) (5.3)
36 - 42

=0
43 — 49

N:

 

*

*
Hearing thresholds (re:

it
Standard deviation.

ISO, 1964).

1964)

ISO,

Hearing Level in dB (re:

ISO, 1964)

(re:

Hearing Level in dB

120

10 «
E1 ()15-21 Years
(322—28 Years
[529-35 Years

   
  
  
 

20 -

30- A

50 ‘
60 ‘
70 q

80

 

 

I I l I 1

250 500 1000 2000 4000
Frequency in Hertz

Figure 21A. Composite audiogram of mean better ear thres-
holds for heatermen by years of exposure.

(DIS-21 Years
[322-28 Years
A29-35 Years

 

80

 

r I

250 500 100 2000 4000

Frequency in Hertz

Figure 21B. Composite audiogram of mean combined ear
thresholds for heatermen by years of exposure.

121

21.(A andlfl depicts the two composite audiograms. As
is evident, the patterns were the same with only small
differences found in the dB levels.

The data regarding straighteners were limited
(N=6) so that comparison by years of exposure could not

be done.

Effects of Protection by Position

 

The hearing thresholds for combined ears for
hammermen using cotton for protection and those not
using protection are presented in Table 22 and illustrated
in Figure 22. Results indicate an apparent protective
effect resulting from cotton protection in the lower
frequencies. However, there is a 4.6 years' difference
in age between the two groups and 6 years' difference in
years of exposure. These factors may partly account for
the differences seen.

The combined thresholds for heatermen using cotton
for protection or no protection are presented in Table
23. Results depicted in Figure 23 suggest a definite
difference between the two groups with the protected
group having the better hearing. While a two year differ-
ence in exposure to impact noise is present between the
group protected with cotton and the unprotected group,
results suggest cotton does help even if the difference
is small. Further study is needed, however, as the

number of heatermen is small (N=l4).

122

.mGOHuMH>m© Unmtcmum
**

.AvOmH .OmH "muv mUHonmmwnu mcHHmmm«

 

 

.AN.OHV An.mHv Am.HHO Av.mHO AN.NHV AO.FO Am.mv Omuz
N.mn m.vm o.Hm m.vm H.vm m.mm m.m¢ coupou
Av.HHv Am.OHv AN.NHO AN.OHV Ao.nHv AH.mv «*AO.mV mmuz
m.mh n.mw m.nm m.mv m.Hm m.mm m.om mcoz
ooov OOON OOOH oom omm muzmomxm mm< mewe
mo coHuomuoum
Nuumm CH wosmsqmwm mummy

 

.GOHpomuonm mo COHpocdw m
mm cmfiumfifims How mGOHDMH>mU pumpcmum tam *mpHonmmHeu msHHmmS :mm211.mm mqmda

123

 

 

O _
10 ~
$ 20 "
Ch
v—l
. 30 -
O
U)
U 40 —
Cf.)
'0 50 -
C‘.
H
H 60 F
G)
5
.4 7o -
U‘
C
-r-1 80 ‘
L1
(0
3% 9o -
100

 

I I I ' I
250 500 1000 2000 4000

Frequency in Hertz

A No protection
C)Protection with cotton

Figure 22. Composite audiogram of mean combined ear thresholds
for hammerman by type of ear protection.

124

.AvmmH .OmH ”onv mp cH«

 

 

 

Am.mHv AH.OHO AH.mv AO.NHV Ao.mv muz
m.mo h.mm m.mv h.Hm m.hH m.om m.>v couuou
Am.mv Am.mv Am.mv Am.mHv Av.OHv mu
O.Hn H.¢O m.mm m.Om N.ON m.mm m.nw ocoz
ooos ooom oooa oom omm musmomxm mm¢ muse
mo COHuomuoum
npnmm SH moamswmum whom»

 

.sOHuomuonm mo GOHuocsm 8
mm cofinoumme How mCOHuMH>op pumpcmum paw *mOHOSmmHQu mcHumon cmm211.mm mqmde

1964)

(ISO,

Hearing Level in dB

10 —

20 -

30 -

4O -

60 r

70 -

90 _

 

100

125

 

250 500 1000 2000 4000 8000

Frequency in Hertz

1Q No protection
(3 Protected with cotton

Figure 23.

Composite audiogram of combined ear thresholds
for heatermen by type of ear protection.

126

Because of the small number of straighteners
reviewed, no comparison of protected versus unprotected
thresholds was completed.

Phase III
Permanent Threshold Shift Discussion

 

 

In reviewing the cases, the lack of men seen with
between one and fifteen years of exposure leaves a definite
gap in the data presented. One might argue that the
cases selected represent a restricted sample of the men
working in the forge and thus not a true indication of
the extent of hearing loss. It is true that in this study
only the older men are really considered, but this
limitation was imposed by the cases referred for testing.
The younger men were not referred unless they demonstrated
considerable hearing impairment. Nonetheless, the results
reported do represent the effects of the impact noise
in the drop forge for a fairly large sample, and thus
add to the body of information needed to develop protective
criteria.

A second factor which must be considered is that
most of the men reviewed were working in a forge at the
time of their hearing evaluation. Thus, the thresholds
might more accurately be called resting thresholds. In

addition, in this sample not all men had at least sixteen

127

hours rest from the noise prior to being seen for audio-
logical testing. However, it is the policy of the
center doing the auditory testing to try to avoid testing
any employee of a drop forge immediately following
work. Therefore, most but not all of the thresholds
analyzed in Phase III were "rested".
The questions asked at the start of the investiga-
tion concerning the permanent effects of drop forge
impact noise were as follows:
1. Are there typical audiometric configurations
of workers exposed to drop forge noise?
2. Does the degree of loss reach a terminal
level after many years of exposure to drop
forge noise?
3. Does the audiometric configuration or degree
of loss differ for various drop forge companies?
4. Is the use of ear protection devices reflected
in the degree of loss obtained on the differ-
ent exposure groups of men studied?
An additional question was asked after the data were compiled:
What differences in hearing result from working in different
positions near drop forge hammers?
In answer to the question regarding audiometric
configurations, the most frequently found configuration
was a steeply sloping loss starting at 250 or 500 Hz.

The slope of the loss did change with age and years of

128

exposure. The lower frequencies progressively became
poorer while the higher frequencies changed only slightly.
The frequency of 1000 Hz seemed to be a fulcrum below
which changes took place after several years of exposure.
The low frequency changes, however, did not alter
considerably the general configuration of the losses.
There were also a number of men whose audiometric con-
figurations were essentially flat even after a few years
of exposure.

The second question regarding the degree of loss
over time is a more difficult one to answer. First, the
variability as the degree of loss was great. Some younger
men had severe deficits while some older men displayed
mild to moderate losses. This variability was also found
in a previous drop forge study (Fox, 1953). The general
statement can be made that, as in previous noise studies,
no case was found in which total hearing loss in the
speech range resulted from exposure to drop forge noise.

The degree of loss and its change over the years
worked can be related to both age and years of exposure.
In considering the effects of age on the cases studied,
major deficits in hearing were found prior to fifteen
years of exposure to drop forge noise. Losses following
in latter years showed some evidence of more change in
the lower frequencies than the presbycusic effects

(Corso, 1963) would suggest. The higher frequencies

129

appeared to have reached the terminal level by the time
the men had worked 10 to 15 years. The results suggest
that the major part of the loss occurred for most of
the men before the effect of aging is generally consid—
ered a factor. Subtracting the presbycusic factor from
their thresholds particularly in the high frequencies
in the older age groups would suggest that their hearing
loss due to noise is reduced as they get older. If
presbycusis effects were additive, then one might expect
to see considerably poorer thresholds in the higher
frequencies for the older age groups; however, this did
not occur. Therefore, the data were not corrected for
presbycusis before considering other factors. (See general
discussion for further consideration of this problem.)

The interesting finding regarding the increasing
loss in the low frequencies after 15 years of exposure
suggests that the impact noise continues to affect the
auditory system. Why the loss in the low frequencies
continues can only be conjectured. But, it is possible
that the constantly fluctuating noise levels and chang-
ing of positions near the hammer might account for the
continuing low frequency loss.

The degree and slope of the loss occurring in

different drop forge companies were found to be quite

130

consistent for three of the four forges studied. The
fourth forge was not adequately represented to compare
with the other companies. The consistency of the mean
average loss and slopes between forges suggests that
while different companies have different equipment, the
resulting hearing loss from drop forge noise exposure
is quite similar. Thus, results can be generalized to
some extent for drop forges from different areas.
Further study is required to verify this finding.

The method used to evaluate effects of protection
in this study had not generally been employed in the
field. By considering differences in resting thresholds
between men protected and unprotected, assuming they
started out with normal hearing, one might gain some
insight into what long term effects the protection
provided. In the present study, small differences were
noted suggesting cotton did provide some protection.
However, difference in age and years of exposure could
have accounted for the dissimilarities noted.

In the current study, the hammermen evidenced
greater mean threshold losses than did the heatermen
or straighteners. This might be expected in light of
the closer position to the hammer. However, the results

are confounded in that most hammermen have been exposed

131

to the noise at both the heaterman's and straightener's
positions. The hammerman's job is obtained through
seniority, and then only if he wants to work that posi-
tion. The study does suggest that the hammerman's
position is the more hazardous as the majority of the
cases reviewed were working as hammermen or had worked

as hammermen at the time they were evaluated.

General Discussion

 

The nature of drop forge noise and its relation-
ship to current damage risk criteria for impulse noise
needs further elaboration. Coles, et_al. (1968) recognized
the differences between industrial noise and gunfire.
These differences included lower peak levels for drop
forge noise, longer rise times and durations, and greater
repetition rates and reverberation. The current DRC
are, therefore, Specifically for gunfire alone. Damage
risk criteria based strictly on peak level and duration
as well as a limited exposure period obviously cannot
be applied to the industrial setting appropriately.

The three approaches used in developing DRC for
industrial noise have included: attempting to relate
industrial noise to the gunfire criterion, using the
single figure limit for impacts or, as suggested by Coles

et a1. (l968),employing conventional steady state measures

132

and DRC. The latter approach was suggested because the
repetition rate and reverberation tend to make the noise
relatively continuous.

None of the above mentioned approaches appear
to be satisfactory for impact noise industrial applica-
tions at this time. Because of the numerous differences
between gunfire and drop forge noise, current criteria
are inadequate. The single level limit has been shown
by Cohen, et_al. (1965) and Walker (1970) to be
inappropriate as TTS was obtained from impact noise with
lower levels than the maximum set. The last approach,
that of using conventional criteria,was not found to be
adequate for fettling noise which has a repetition rate
of greater than 20 impacts per second (Martin, et_§l.,
1970). If not appropriate for fettling noise which is
more continuous than drop forge noise, then dB A would
hardly describe accurately the damage risk from drop
forge hammer noise. The additional problem of both
steady state and impact noise combined adds a further
confOunding factor to establishing impact noise protective
guidelines. Thus, further research is needed to resolve
this question of DRC for drop forge noise.

The second phase of the study, originally designed
to consider TTS resulting from drop forge noise, had to
be modified because the majority of the employees used

ear protection devices. No subjects were asked to go

133

without protection. Therefore, the TTS measured reflected
the effects of noise as attenuated by the ear protectors.
The confounding effects of the ear protection and the
variable exposure for each man limited the findings of
this study as to the relationship between the impact
noise and TTS. The results do indicate, however, that

in every day working conditions, TTS resulting from the
drop forge noise is mild if ear protection is used. The
fact that some TTS is present even with ear plugs or
muffs suggests that 20-35 dB additional shift might have
resulted if protection had not been employed. This is
illustrated by subject 3 (see Figure 10) who demonstrated
a 46 dB shift at 4000 Hz while going unprotected.

An important clinical implication regarding TTS
should be noted. All subjects' TTS had recovered to
resting threshold levels within a 16 hour period. Since
many of these individuals receive hearing evaluations
and hearing aid evaluations at audiological centers, the
assumption can be made that test results reflect resting
thresholds if 16 hours have elapsed since the person was
exposed to noise. With impact noise, this assumption
should only be made with employees using ear protection,

as unprotected TTS studies have not been completed.

134

In relating the noise measured to the PTS for
a group of men from different forges, one must assume
that the noise measured is representative of the noise
in all shops. While the hearing losses were found to
be similar in three of the four forges reviewed, one
cannot assume that the noise measured in one forge at
the present time has remained constant during the past
50 years or so. The long-term effects of the noise,
however, are demonstrated by the PTS data. The steeply
sloping high frequency loss is consistent with other
types of impact noise. Also, the impact noise exposure
appears to continue to reduce the hearing acuity in
the low frequency region up to the age of 65. In those
cases in which thresholds at 6000 and 8000 Hz were
obtained, the noise tends to affect those frequencies

equally or more than 4000 Hz.

An important finding regarding permanent effects
of the noise is that the results of Phase II clearly
demonstrated that with the exception of the low frequency
region, the terminal hearing loss from impact noise was
already reached by the time men had spent 10-15 years
in the hammer shop. In fact, it is possible that this
terminal loss for some men might even occur earlier,
but supporting evidence is not yet available for the

younger men.

135

The relationship between TTS from one day's
exposure and PTS from habitual exposure cannot be evaluated
in this study as unprotected TTS measures were not made
and the two phases were completed on two different
groups of subjects. The relationship between TTS and
PTS for steady state noise is uncertain, although many
researchers feel a given TTS occurring often enough
will result in a similar PTS. Impact noise has a
different effect on the auditory system in terms of TTS
growth rate and has more variability. The daily variation
in exposure to drop forge noise might possibly increase
the rate of PTS development. Coles, e£_al. (1968)
suggested that possibly the levels arriving at normal
incidence to the ear from other sources might actually
be more hazardous and cause a more rapid onset of PTS.
Their comments were made regarding gunfire noise but
would appear to be applicable to drop forge noise also.
Progression of hearing loss due to impact noise during
the first few years of exposure could not be studied
because cases with 0—15 years of exposure were not
available for study. The reason for only a few younger
men being referred for hearing evaluations is unknown.
However, it is speculated that these men might demonstrate
less difficulty hearing while at work and, therefore, not

be referred for audiological evaluation.

136

The findings of the TTS study suggest that the
use of protection devices results in shifts which for
the majority of cases do not exceed the CHABA (Kryter,
1966) limits. The method of study is limited, however,
since the unprotected TTS could not be obtained on each
man as a comparative measure. Comparison of hearing
loss on men using different protective devices in the
PTS study is a different approach to the consideration
of protection. Several assumptions must be met, however,
if this approach is to be used. One must assume that
each group of men start with equal hearing levels, that
the age and years of exposure of the groups are comparable,
and that the nature of the noise was the same for all
groups. These assumptions were not entirely met in the
current study. Thus, conclusions concerning protection
based on PTS data are also limited.

The influence of age upon hearing thresholds
has been considered by many researchers (Hinchcliffe,
1959; Glorig and Davis, 1961; Glorig and Nixon, 1962;
Corso, 1963; Spoor, 1967; Wagemann, 1967). All show

decreasing hearing resulting from the aging process.

137

In considering noise exposure, the effects of age have
been generally considered additive with no interaction
(ASA, 1954; Glorig and Davis, 1961). As a result,
compensation usually is based upon an allowance for the
hearing loss expected with advanced age.

In the current study, by using age groups based
on Corso's divisions (Corso, 1963) it was found that
younger subjects, 26-32 years of age, had losses compar-
able to the older group of men, particularly in the
higher frequencies. If presbycusis effects were additive,
then one might expect the loss at 4000 Hz to increase as
the men became older. The effect of 34 dB at 4000 Hz1
attributed to age was not apparent in the threshold differ-
ence from the youngest to the oldest group. The
difference at 4000 Hz between these two groups was small.
This suggests that the effects of presbycusis are "over-
shadowed" by the effects of the noise exposure.

One might conjecture that there is an interaction
effect of noise exposure and age, and that the damage
caused to the cochlea by the noise prior to the onset
of presbycusis affects the same anatomical and physiological
structures as does age. The result is that the damage
expected from age alone has already been done by the noise

and only minimal changes are evident from aging.

 

lPresbycusic factor (34 dB at 4000 Hz) for men aged
59-65 according to Corso (1963).

CHAPTER V
SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS
Summary

A review of the literature concerning industrial
noise revealed a dearth of research concerning the
effects of drop forge impact noise upon the human
auditory system. The few studies reported to date have
employed laboratory produced impact noise to investigate
temporary effects on hearing thresholds. Unfortunately,
these studies cannot be directly applied to actual
industrial noise in terms of predicting potential auditory
hazard because of the complex way numerous variables
(e.g. type, amount, duration of exposure, etc.) exist in the
field; but, these factors are controlled in the laboratory.
Thus, in order to determine the nature of actual drop forge
noise and its temporary and permanent effects on hearing
thresholds, a three phase study was completed.

Phase I entailed measurement of drop forge noise
and associated background noise in a local Lansing,
Michigan, drop forge. Noise parameters measured included
the following: peak SPL, rise time, duration, octave

band analyses, total number of impacts and repetitions

139

rates. Continuous noise measurement included linear
and dB A readings as well as octave band analyses. For
all parameters measured, several readings were made

and the means and standard deviations were reported.

In Phase II, twenty employees of a drop forge
company were given both resting threshold and TTS tests
over a five day work period. Subjects were divided
equally into two groups, 0-10 years of exposure and
15-25 years of exposure, and further subdivided accord-
ing to the type of ear protection each man used. Manual
conventional pure-tone tests were administered at the
start of a week's work, three times following work
shifts and once after 16 hours of recovery time. Mean
resting thresholds were compared for the two groups and
for the subgroups. TTS measurements after one day,
two days and five days of exposure were compared on an
individual basis.

Phase III was designed to study impact noise
effects on PTS. A retrospective study was completed
of the audiometric records of 71 drop forge workers.
Each case met strict criteria regarding work, ear
protection, and hearing loss histories. Results based
upon pure-tone threshold configurations were analyzed
according to age, years of exposure, employing forging
company, position worked, and type of protection used.

Because the hearing loss for each ear was essentially

140

the same, cOmbined right and left ear thresholds were
computed for each case. Four frequency averages (500-
4000 Hz) and slope of the loss per octave were also
determined. Central tendency statistics were employed.

The major results from this three part investiga-
tion were as follows:

1. Impact peak levels vary greatly from one
hammer to another and also for a given hammer. Levels
ranged from 120 dB to 145 dB.

2. Steady state noise levels within the drop
forge hammer shop are highly variablesbut remain higher
than 90 dB A during a work shift.

3. Daily exposure to impact plus continuous
noise is variable for each man depending upon his position
near the hammer, the relationship to other hammers and
daily changes in work procedures.

4. Differences between drop forge and gunfire
impact noise relative to rise times, durations, peaks,
and repetition rates reduce the applicability of current
impact DRC which are based upon gunfire noise parameters.

5. As evidenced by the PTS and unprotected TTS
results, the noise present in the drop forge is definitely

hazardous to the human auditory system.

141

6. When using appropriate ear protection:

(a) daily TTS found for the majority of
employees is reduced to within acceptable
limits according to current DRC.

(b) no residual TTS is present at the start
of the next work day.

(c) accumulative TTS over a five day work
week was found to be negligible.

7. Hearing loss from impact noise for both
temporary and permanent threshold shifts shows a high
degree of variability in terms of amount of loss.

8. The typical audiometric configuration resulting
from drop forge noise found in 69% of the cases, was a
bilaterally symmetrical steeply sloping hearing loss. A
gradual slope or flat configuration was exhibited by the
remaining 31% of the drop forge workers.

9. The primary loss of hearing resulting from
drop forge noise occurs during the first 10 to 15 years
of exposure. However, additional loss due to exposure
to noise continues tOIaffect the low frequencies until
the employee retires at about age 65.

10. The effects of presbycusis contribute only
a minor part to the hearing loss of drop forge workers;
however, presbycusis and drop forge noise appear to have
an interacting effect upon PTS instead of an additive

one .

142

Conclusions

 

Three primary conclusions from this study are
warranted. First, both impact and continuous (steady
state) noise levels measured in a drop forge were found
to be potentially hazardous to the workers' auditory
systems according to current DRC. Secondly, permanent
threshold shifts obtained from a large sample of hammer
shop employees exhibited serious hearing impairment
after ten to fifteen years of employment. Finally, both
rested (non—noise exposed) thresholds and TTS measure-
ments obtained from men with less than ten years of noise
exposure suggest that utilization of appropriate ear
protective devices substantially reduces the hazard to
hearing. The primary implication is that if hearing
conservation programs are initiated in drop forges, the
risk to the employee's hearing can be significantly

reduced.

Recommendations for Further Research

 

In View of the findings of the present investiga-
tion, the following recommendations for additional research
are made:

1. It is suggested that a comparison study of
both steady state and impact noise in different drop

forges be made. This would permit generalization as

143

to the average noise levels from one forge to another,
and permit conclusions regarding the relationship between
noise levels and PTS.

2. A study designed to compare different methods
of measurement and their relationship to hearing loss
is particularly needed if criteria for industrial
impact noise are to be developed. One such measure is
the use of statistical distribution analyses such as
employed by Dieroff (1966).

3. A follow-up study of the men tested in the
TTS phase of the current investigation would provide
needed data regarding hearing changes when using ear
protection. These types of data are needed for
the younger men.

4. Since the noise levels are constantly
fluctuating but remain at hazardous levels, a study of
the adaptation rate of the middle ear muscles to this
type of stimulus might provide important information
regarding the natural protection of the middle ear
mechanism.

5. A study designed to compare TTS and different
repetition rates of hammers on the job, or in the

laboratory, is needed. These data are needed to supplement

the findings of Ward (1962) regarding the growth of TTS.

144

6. A study designed to measure the incidence
of hearing loss in the drop forge is needed to determine
if all men are affected by the noise. This might be done
during the initial testing for a hearing conservation
program.

7. An investigation is needed to supplement the
current data regarding the PTS of men working in the drop
forge for l to 15 years. Obtaining threshold and other
data on a large sample of these men would help answer
the question regarding how rapid the hearing loss occurs
from impact noise. However, it is in this group of
younger men that ear protection has been introduced early
in their work histories. Hence, it would be difficult
to obtain PTS data tha¢;are comparable to that of the
older men, namely, long-term unprotected pure-tone
thresholds.

Studies such as those mentioned above are needed
in order to establish and conduct hearing conservation
programs. When this is accomplished, the profound hearing
losses that are presently an occupational hazard in

drop forge hammer shops will hopefully be eliminated.

BIBLIOGRAPHY

145

BIBLIOGRAPHY

AAOO., Guide for conservation of hearing in noise. Trans-
actions of American Academy of Ophthalomology and
Otalarypgology Supp. (1969).

 

 

American Standards Association, The relations of hearing
loss to noise exposure. Subcommittee Report
Z24—X—2 (1954).

 

Ball, J. H., Instrumentation for a study of temporary
threshold shifts caused by high-intensity acoustic
transients, Rept. 785. Cambridge: Bolt, Beranek
and Newman (1961).

 

Brasher, P. F., Modern weapons and the human ear. J.
Royal Army Med. Corp., 115, 163-171 (1969).

 

Bunch, C. C., The neural mechanism of hearing; "nerve
deafness" of known pathology or etiologY; diag—
nosis of occupational or traumatic deafness;
historical and audiometric study. Laryngoscope,
47, 615-691 (1937).

 

Burns, W., Noise and Man. Philadelphia: Lippincott
(1969).

 

Carhart, R., and Jerger, J., Preferred method for clini-
cal determination of pure tone thresholds.
J. Speech and Hearing Dis., 24, 330-345 (1959).

 

Carter, N. L., and Kryter, K. D., Studies of temporary
threshold shift caused by high intensity acoustic
transients, Rept. 949. Cambridge: Bolt, Beranek
and Newman (1962).

 

Carter, N. L., Ball, J., and Kryter, K. D., Studies of
temporary threshold shift caused by high intensity
acoustic transients, Rept. 916. Cambridge:

Bolt, Beranek, and Newman (1962).

 

Chisman, J. A., and Simon, J. R.I Protection against
impulse-type industrial noise by utilizing the
acoustic reflex. J. applied Psych., 45, 402—
407 (1961).

 

146

Cohen,

Cohen,

Coles,

Coles,

Coles,

Coles,

Coles,

Coles,

Coles,

Corso,

147

A., Temporary hearing losses for bare and pro-
tected ears as a function of exposure time to
continuous and firing noise. J. acoust. Soc.
Amer., 33, 858 (A) (1961).

 

A., Kylin, G., and LaBenz, P. J., Temporary
threshold shifts in hearing from exposure to
combined impact/steady state noise conditions.
J. acoust. Soc. Amer., 40, 1371-1380 (1966).

 

R. R. A., Control of industrial noise through
personal protection. Noise as a Public Health
Hazard, ASHA Rept. 4,(1969).

 

 

R. R. A., and Rice, C. G., High intensity noise
problems in the royal navy and royal marines.
J. Royal Naval Med. Services, 51, 184-192 (1965).

 

R. R. A., and Rice, C. G., Speech communication
effects and temporary threshold shift reduction
provided by V51R and Selectone-K earplugs under
conditions of high intensity impulsive noise.
J. Sound Vib., 4, 156-171 (1966).

 

R. R. A., and Rice, C. G., Hazards from impulse
noise. Ann. Occup. Hyg., 10, 381-388 (1967).

 

R. R. A., and Rice, C. G., Towards a criterion
for impulse noise in industry. Ann. Occup. Hyg.,
13, 43-50 (1970).

 

R. R. A., Rice, C. G., Garinther, G. R., and
Hodge, D. C., Criteria for Assessing Hearing
Damage Risk from Impulse-Noise Exposure.
Tech. Memo. 13—67, Aberdeen Proving Ground:
US Army Engineefing Lab. (1967).

 

R. R. A., Garinther, G. R., Hodge, D. C., and
Rice, C. G., Hazardous exposure to impulse noise.
J. acoust. Soc. Amer., 43, 336-343 (1968).

 

J. F., Age and sex differences in pure-tone
thresholds. Arch. of Otolaryng., 77, 385-405
(1964).

 

Cox, J. R., How quiet must it be to measure normal hear-

ing. Noise Control, 1, 25-29 (1955).

 

Dieroff, H. G., The problem of impulse-rich working

noise, its measuring, and the hearing losses
resulting from it. J. Inter. Audiol., 5, 339-
343 (1966).

 

148

Fletcher, J. L., Comparison of attenuation characteristics
of the acoustic reflex and the V-51R earplug.
J. Aud. Res., 1, 111-116 (1961).

 

Fletcher, J. L., Hearing losses of personnel exposed to
impulse and steady state noise. J. Aud. Res.,
3, 83—90 (1963).

 

Fletcher, J. L., Protection from intensities of impulse
noise by way of preceding noise and click
stimuli. J. Aud. Res., 5, 145-150 (1965).

 

Fletcher, J. L., Temporary threshold shift recovery
from impulse and steady state noise exposure,
Rept. 829. Fort Knox: US Army Medical Research
Lab. (1969).

 

Fletcher, J. L., and Loeb, M., Free-field threshold shift
and temporary shift reduction as measures of

efficiency of ear protection devices. J. acoust.
Soc. Amer., 34, 1629—1633 (1962a).

 

 

Fletcher, J. L., and Loeb, M., The influence of different
acoustic stimuli on the threshold of the contra—
lateral ear: a possible index of attenuation by
the intratympanic reflex. Acta Oto-laryng.,

54, 33-47 (1962b).

 

Fletcher, J. L., and Loeb, M., Relationships for
temporary threshold shifts produced by three
different sources. J. Aud. Res., 5, 41-45 (1965).

 

Fletcher, J. L., and Loeb, M., The effect of pulse dura-
tion on TTS produced by impulse noise. J. Aud.
Res., 7, 163-167 (1967).

Fletcher, J. L., and Riopelle, A. J., Protective effect
of the acoustic reflex for impulsive noise.
J. acoust. Soc. Amer., 32, 401-404 (1960).

 

Flottorp, G., and Quist—Hanssen, S., The effect of ear
protectors against sound waves from explosions.
Acta-laryng. Supp., 158, 286-294 (1960).

 

Forrest, M. R., and Coles, R. R. A., Problems of com-
munication and ear protection in the royal
marines. J. Royal Naval Med. Service, 56, 162-
169 (1970).

 

149

Fox, M. S., Evaluation of hearing loss in drop forge
workers. Laryngoscope, 63, 960—971 (1953).

 

Glorig, A., and Davis, H., Age, noise, and hearing loss.
Ann. Oto. Rhino. Laryng., 70, 556-571 (1961).

 

Glorig, A., and Nixon, J., Hearing loss as a function of
age. Laryngoscope, 72, 1596-1610 (1962).

 

Harbold, G. J., Tegt, R. P., and Standeven, B. S., Prob-
lems associated with measurement of acoustic
transients. Aerospace Med., 36, 767-773 (1965).

 

Harris, C. M.,(Ed.). Handbook of Noise Control. New
York: McGraw-Hill (1957).

 

Hecker, H. H. L., and Kryter, K. D., A study of auditory
fatigue caused by high—intensity acoustic trans-
ients, Rept. 1158. Cambridge: Bolt, Beranek
and Newman (1964).

 

Hinchcliffe, K., The threshold of hearing as a function
of age. Acustica, 9, 303-308 (1959).

Hodge, D. C., and McCommons, R. B., Reliability of TTS

from impulse-noise exposure. J. acoust. Soc.
Amer., 40, 839—843 (1966).

 

Hodge, D. C., McCommons, R. B., and Blackmer, R. B.,
Reliability of temporary threshold shifts caused
by repeated impulse-noise exposures. J. Aud.
Res., 6, 121—127 (1966).

Industrial machine noise. In Bolt, R. H., Lukosik,
S. J., Nolle, A. W., and Frost, A. D.,(Eds.).
Handbook of Acoustic Noise Control Vol. I.
Physical Acoustics, WADC Tech. Rept. 52-204,
Wright Air Development Center, Chp. 7, 157-
184 (1952).

 

 

Kryter, K. D., The effects of noise on man. J. Speech
and Hearing Dis. Monogr. Supp., 1, 1-95 (1950).

 

 

Kryter, K. D., Temporary threshold shifts in hearing
from acoustic impulses of high intensity.
J. Inter. Audiol., 5, 323-330 (1966).

 

Kryter, K. D., Evaluation of exposure to impulse noise.
Arch. Environ. Health, 20, 624-635 (1970a).

 

150

Kryter, K. D., The Effects of Noise on Man. New York:
Academic Press (1970b).

 

Kryter, K. D., and Garinther, G. R., Auditory effects
of acoustic impulses from firearms. Acta Oto-
Laryng. Supp., 211, 1—22 (1965).

 

Kryter, K. D., Ward, W. D., Miller, J. D., and Eldredge,
D. H., Hazardous exposure to intermittent and
steady-state noise. J. acoust. Soc. Amer., 39,
451-463 (1966).

 

Larson, B., Investigations on noise in certain factories.
Acta Oto-Laryng., 34, 71—77 (1946).

 

Livesey, B., Acoustic trauma as an occupational hazard
in infantrymen. Royal Army Med. Corp. J., 111,
188-193 (1965).

 

Loeb, M., Psychophysical correlates of intratympanic
reflex action. In Fletcher, J. L. (Ed.).
Middle Ear Function Seminar, Rept. 576. Fort
Knox: US Army Medical Research Lab. T1963).

 

Loeb, M., and Fletcher, J. L., Temporary threshold shift
in successive sessions for subjects exposed to
continuous and periodic intermittent noise.

J. Aud. Res., 3, 213-220 (1963).

 

Loeb, M., and Fletcher, J. L.I Impulse Duration and
Temporary Threshold Shift, Rept. 791. Fort Knox:
US Army Medical Research Lab. (1968).

 

 

Loeb, M., Fletcher, J. L., and Benson, R. W., Some pre-
1iminary studies of temporary threshold shift
with an arc-discharge impulse generator.

J. acoust. Soc. Amer., 37, 313-318 (1965).

 

Loring, J. C. G., Selected bibliography on the effects
of high intensity noise on man. J. Speech and
Hearing Dis. Monogr. Supp., 3, 1-38 (1954).

 

 

Lybarger, S. F., Interim bone conduction thresholds for
audiometry. J. Speech Hearing Res., 9, 483-
487 (1966).

 

Murray, N. B., and Reid, G.I Experimental observations
on the aural effects of gun blast. Med. J.
Australia, 33, 611—617 (1946).

 

151

Noble, W. G., A new concept of damage risk criterion.
Ann. Occup. Hyg., 13, 69-75 (1970).

 

Ogden, F. W., Effect of gunfire upon auditory acuity for
puretones and the efficacy of earplugs as pro-
tectors. Laryngoscope, 60, 993-1012 (1950).

 

Plomp, R., Hearing losses induced by small arms. J.
Inter. Audiol., 6, 31-36 (1967).

 

Rice, C. G., Deafness due to impulse noise. Philosphical
Trans. Royal Soc. London, 263, 279-287 (1968).

 

 

Rice, C. G., and Coles, R. R. A., Impulsive noise studies
and temporary threshold shift. 5th Inter. Cong.
of Acoust. Liege (1965).

 

 

Rice, C. G., and Coles, R. R. A., Design factors and use
of ear protection. Brit. J. Indust. Med., 23,
194-203 (1966).

 

Rol, C., Spoor, A., and van Dishoeck, H. A. E., Hearing
loss induced by non-steady sounds. J. Aud. Res.,
7, 129~135 (1967).

Smith, M. G., and Goldstone, G., A pilot study of
temporary threshold shifts resulting from
exposure to high intensity impulse noise, Rept.
TM 19—61, Aberdeen Proving Ground: US Army
Ordinance (1961). Cited in Kryter, K. D.,

The Effects of Noise on Man. New York: Academic
Press (1970).

 

Spieth, W., and Trittipoe, W. J., Temporary threshold
elevation produced continuous and "impulsive"
noise. J. acoust. Soc. Amer., 30, 523-527
(1958).

 

Spoor, A., Prebycusis values in relation to noise induced
hearing loss. J. Inter. Audiol., 6, 48-57
(1967).

 

Wagemann, W., Development and progress of the permanent
threshold shift during decades of exposure to
noise. J. Inter. Audiol., 6, 48-57 (1967).

 

Walker, J. G., Temporary threshold shift from impulse
noise. Ann. Occup. Hyg., 13, 51-58 (1970).

152

Walsh—Healey Public Contracts Act Amendment, Rules and

Ward,

Ward,

Ward,

Ward,

Ward,

Ward,

Regulations 50-204.10, Federal Register, 34,
(Tuesday, May 20, 1969).

 

W. D., Effect of temporal spacing of temporary
threshold shift from impulses. J. acoust. Soc.
Amer., 34, 1230-1232 (1962).

 

W. D.,(Ed.), Proposed damage-risk criterion for
impulse noise (gunfire), Rept. Working Group 57,
NAS-NRC Committee on Hearing,Bioacoustics, and
Biomechanics (1968).

 

W. D., Fleer, R. E., and Glorig, A., Characteristics
of hearing losses produced by gunfire and by
steady state noise. J. Aud. Res., 1, 325-356
(1961/61).

 

W. D., Glorig, A., and Sklar, D. L., Dependence
of temporary threshold shift at 4kc on intensity
and time. J. acoust. Soc. Amer., 30, 944-954
(1958).

 

W. D., Glorig, A., and Sklar, D. L., Temporary
threshold shift from octave—band noise: appli-
cations to damage-risk criteria. J. acoust.
Soc. Amer., 31, 522-528 (1959).

 

 

W. D., Selters, W., and Glorig, A., Exploratory
studies on temporary threshold shift from
impulses. J. acoust. Soc. Amer., 33, 781-
793 (1961).

 

APPENDICES

153

APPENDIX A

EMPLOYEE SCREENING QUESTIONNAIRE

154

EMPLOYEE SCREENING QUESTIONNAIRE

PLEASE ANSWER ALL QUESTIONS AS COMPLETELY AS POSSIBLE:

Name: Age:

 

 

Number of years working in the drop forge:

 

Types of positions you have held in the forge:
(from first to present position)

1. No. of Yrs.

 

2. No. of Yrs.
3. No. of Yrs.
4.

No. of Yrs.

 

 

 

Types of ear protection (Check the type you have used and
the length of time):

 

 

None: Time used:
Cotton: - Time used:
Ear Plugs: Time used:

 

(Brand name if known)

Ear Muffs: Time used:
(Brand name if known)

 

Other: Time used:
(Please Specify)

 

Use of Protection: Always:

 

Sometimes:

 

Rarely:

 

Have you worked in other noisy jobs? (Armed Services, other
factories, etc.)

 

If so, what kind of noise:

 

How long did you work there:

 

Did you use ear protection:

 

If so, what kind of protection:

 

Do you do much shooting:

 

If so, do you use protection:

 

If asked, would you be willing to spend about 5 minutes to
have your hearing tested before and after your work shift?

 

Thank you.
155

APPENDIX B

SUBJECT DATA FORM

156

SUBJECT DATA FORM

 

 

 

No.___-
Identification:
Name:
Birthdate: Age:
Present Position: Time on Job:

 

Noise History:
Years in the Drop Forge:
Previous Positions in Forge:

Time on Job:

 

 

 

Gunfire History:

 

Other Noisy occupations:

 

Ear Protection:
Presently using: (Type) Length of time
Consistency: Always: Sometimes: Rarely____
Previous Use of Protection:

Types Time:

 

 

 

 

 

 

Medical History:
History of H/L prior to working in the Forge:
Yes: No:
History of H/L in the family: Who:

 

 

 

 

History of Ear Pain:
Drainage:
Injury:
Surgery:

 

 

 

 

Tinnitus: Vertigo

 

 

 

Following exposure periods:
Monday Tuesday Friday
1. Time hammer stOpped: 2. 3.

 

Time of Test:

 

157

APPENDIX C

NOTICES GIVEN TO SUBJECTS FOR
PRE-EXPOSURE TESTS AND
POST-EXPOSURE TESTS

158

NOTICE GIVEN TO SUBJECTS FOR PRE-EXPOSURE TESTS

 

You are being asked to participate in a study here
in the forge in order to help determine the effects of
hammer noise on the employee's hearing. As noted in the
announcements on the bulletin board, your participation
involves having your hearing tested several times during
one week's work period.

The first of these tests is scheduled for next

Monday . This test is done prior to your

 

starting work and therefore, it will be necessary for you
to come to the forge a few minutes earlier than you would
regularly. The test to be given at that time will
require approximately 10—12 minutes.

Please report to the hearing test trailer, which
will be located on the drive outside the employee's
entrance, before you are scheduled to begin your shift.

Information regarding the other tests will be
given following the first test on Monday. These other

tests will take only 2-3 minutes each to complete.

YOUR TEST TIME ON MONDAY IS

 

 

Please try to be prompt as we do not want to affect the

start of your shift in any way.

159

160

NOTICE GIVEN SUBJECTS FOR POST-EXPOSURE TESTS

As previously indicated, this study involves having
your hearing tested several times during this week. The

DAYS and TIMES of these other tests are as follows:

TEST 2: Monday (today) please return for a short test
IMMEDIATELY FOLLOWING THE STOPPING OF YOUR
HAMMER FOR THE DAY. This test takes only 2-3
minutes and you can return to finish the shift
after the test is completed. (PLEASE NOTE THE
TIME THAT THE HAMMER STOPS AND YOU LEAVE THE SHOP).

TEST 3: Tuesday morning please stop for a short test
prior to starting work. Times scheduled are
6:30 AM or 3:00 PM for Heatermen and 6:45 AM
and 3:15 PM for other men depending on which
shift you are working.

 

TEST 4: Tuesday at end of shift: Same time as the test
after Monday's work shift. Come immediately
after your hammer stops for the day. Then
return to complete shift. (PLEASE NOTE THE
TIME THE HAMMER STOPS AND YOU LEAVE THE SHOP).

 

TEST 5: Friday at end of shift: Same time as Monday
and Tuesday after the day's run is completed.
(PLEASE NOTE THE TIME THAT THE HAMMER STOPS
AND YOU LEAVE THE SHOP FOR THE TEST).

 

APPENDIX D

PRE- AND POST-EXPOSURE TESTING PROCEDURES

161

PRE-EXPOSURE TESTING PROCEDURE

Check both ears with the otosc0pe for impacted wax or
other evident abnormality. Do not test subject if
problem is present.

Check questionnaire for any incompleted questions.
Complete quickly or note absence of data and time
will be taken during one of the following tests to
complete the form.

Give the following testing instructions:

THIS TEST IS A BRIEF EVALUATION OF YOUR HEARING FOR
TONES. THE OBJECT OF THE TEST IS TO FIND THE POINT
WHERE YOU CAN JUST BARELY DETECT THE TONE. SOME TONES
WILL BE VERY HIGH PITCHED AND SOME LOW PITCHED.
REGARDLESS OF THE PITCH OF THE TONE, THE IMPORTANT
THING IS THAT YOU INDICATE EVERY TIME YOU HEAR IT.
YOU CAN SIGNAL THAT YOU HEAR THE TONE BY RAISING YOUR
HAND. KEEP YOUR HAND RAISED AS LONG AS YOU HEAR THE
TONE. WHEN THE TONE IS GONE, LOWER YOUR HAND. WE
WILL TEST FIRST YOUR EAR AND THEN YOUR
EAR. DO YOU HAVE ANY QUESTIONS?

Test the better ear first, if one is better. If not,
test the right ear first.

When testing, use the Carhart and Jerger ascending
technique. However, when ascending use 2 dB steps
instead of 5 dB steps.
a. Present the tone at a level above threshold.
b. When response is obtained decrease tone in 10
dB steps.
c. Ascend in 2 dB steps until response is obtained.
d. Decrease in 4 dB steps until no response and then
ascend in 2 dB steps until response is obtained.
e. Threshold is the lowest point at which 2
consecutive responses are obtained or 2 out
of 3 reSponses are obtained.

 

Test the Frequencies in the following order: 1000,
2000, 4000, 8000, repeat 1000, and then 500 Hz.
Record the thresholds on the form provided, using the
second 1000 Hz threshold, in the section TEST 1.

Test both ears.

Repeat the test in both ears at the same frequencies
(once only at each frequency). Cover the TEST 1
results so that they do not influence TEST 2. Record
TEST 2 results in apprOpriate place on the form.

162

163

Test bone conduction at 500 and 1000 Hz bilaterally.
If air-bone gap of greater than 8 dB is present,
subject is not usable in study.

Give the subject the reminder slip for the other test
times. REMIND HIM VERBALLY HE IS TO RETURN FOLLOWING
THE SHUTING DOWN OF HIS HAMMER FOR THE DAY. (ASSUMING
THAT IT IS THE END OF THE RUN). HE MUST NOTE THE TIME
THAT THE HAMMER STOPS FOR THE DAY. Arrangements for
his leaving the shop for a few minutes have been
cleared with personnel office.

164

PROCEDURES FOR POST-EXPOSURE TESTS

Record the time that the hammer stopped according to
the employee.

Record the time that the test starts.

(On Friday only, ASK EMPLOYEE IF HE WORKED ALL FIVE
DAYS IN THE WEEK). If not,indicate so and do not
test subject.

Do air-conduction test on the one ear indicated on
the form. Test at the same frequencies as tested
previously but in the order indicated on the form.

Record results on the data sheet in the space provided
for the test being done.

Advise employee to return to the trailer prior to
starting his shift the following day (following Monday's
test) or

Advise employee to return immediately after the
stopping of his hammer (following Tuesday Morning's
test) or

Advise employee to return immediately after the
stopping of his hammer on Friday (following Tuesday
Afternoon's test).

APPENDIX E

THRESHOLD RECORDING DATA SHEET

165

THRESHOLD RECORDING DATA SHEET

PRE-EXPOSURE TEST

 

 

 

 

 

 

 

 

Right Ear Left Ear
Frequency Test 1 Test 2 Average Test 1 Test 2 Average
8000
4000
2000
1000 BC BC
500

 

 

 

 

 

 

 

 

 

4 Frequency Total

POST-EXPOSURE TESTS
Ear

 

 

Post-Exposure Test 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz

 

Thresholds
Monday Afternoon

 

Thresholds
Tuesday Morning

 

 

Thresholds
Tuesday Afternoon

 

Thresholds
Friday Afternoon

 

RESTING THRESHOLDS
(Monday Dre-Test)

 

 

 

TTS Monday PM

 

TTS Tuesday AM

 

TTS Tuesday PM

 

TTS Friday PM

 

 

 

 

 

 

166

APPENDIX F

NOISE ANALYSIS RAW DATA

167

168

smegma “am he ooom.

HOEEMQ UHMOQ QH OOOM..n

k.

unbouoHE mooo.ouou mp CH
um

 

 

 

 

s.mma s.ava o.oma m.mva m.sma oa
o.mma m.asa m.w~a o.msa a.mma a
m.mma o.mva «.mma o.asa o.mma m
m.mma m.mma m.mma H.maa m.mma e
o.ama m.ava m.mma m.mva o.sma o
v.asa N.Hsa o.mma m.mma o.ssa N.sma m
o.asa o.mma m.oea m.sma m.mva m.smH a
o.osa o.osH o.HsH o.sma m.vva o.sma m
v.asa o.mma s.mma m.mma m.msa o.mma m
o.asa ~.mma H.04H o.mma _o.msa o.oma a
mxouum sued mxouum cum mxoubm cum mxonnm uma meouum new mxouum bmm mcaommm
HH nmEEmm H umEEmm mo 02

 

+.HH umEEmm tam «4H woes m

so COHuHmom m.cmEumEEme men um meme v.mugoemuuafi@de madmmwum xmmm pomeEH11.Hm mqmde

TABLE F2.--Impact peak pressure measurements* made at the
heaterman's position on Hammer I and Hammer II.

 

 

 

 

Number Hammer I Strokes Hammer II Strokes
of
Reading
lst 8th lst 3rd 5th
1 124.0 139.0 122.0 135.4 134.5
2 124.3 139.0 122.5 135.2 133.4
3 124.7 139.5 123.9 135.0 133.3
4 124.0 138.2 122.9 136.2 135.4
5 123.9 140.0 122.0 135.0 133.3

 

*
In dB re:0.0002 microbar

TABLE F3.--Impact peak pressure measurements* made at the
straightener's position on Hammer I and Hammer II.

 

 

 

 

Number Hammer I Strokes Hammer II Strokes
of

Reading lst 8th lst 3rd 5th 11th
1 128.0 144.5 126.5 144.5 142.8 141.5
2 123.0 143.0 125.9 142.0 142.3 144.5
3 125.3 144.6 126.4 142.5 141.5 143.3
4 124.8 142.5 126.7 143.0 141.0 144.0
5 126.5 143.4 125.0 144.8 141.2 144.9

 

*
In dB re:0.0002 microbar

.CoHuHmom m.CmEHmEEmm
an

 

 

w.va e.mmH m.mmH m.HmH m.mMH m.omH m.omH m.>mH N.NNH O.mHH m.OOH m

m.mmH H.mmH m.omH m.mmH b.omH m.HMH O.mmH m.me n.mmH m.mHH e.m0H H

oomHm OOOOH ooom ooov ooom OOOH oom omm mmH O.mo m.Hm quECz
OCHpmmm

 

170

mm CH moCqumnm HmpCmU pCmm m>muoo

 

¥.HH HoEEmm mo oxonpm tum use Co meme mquEmHCmmoE OCBQ m>mpoo HommEH11.mm mqmde

.COHuHmom m.CmEumEEmm
a.

O.va H.OMH H.NMH m.VMH n.mMH m.VMH O.NMH O.me v.ONH O.NHH m.NHH m
O.mmH H.mNH H.VMH m.MMH m.mMH v.VMH m.MMH «.mNH O.vNH m.HHH N.HHH H

 

OomHm OOOOH ooow ooov OOON OOOH oom 0mm mNH O.mm m.Hm
umbECz

 

mm CH >0Cmswoum HmquU pCmm m>muoo OCHpmmm

 

¥.H HmEEmm mo mxonum new one Co meme mquEmHsmmoE prb m>mpoo uommEH11.vm mqmde

171

TABLE F6.-~Linear and dB A scale noise measurements* at
five locations in the hammer shop of the drop forge.

 

 

. No. of Linear dB A
Location Reading (Slow) (Slow)
1 105.0 101.0
A 2 106.0 102.0
3 107.0 102.0
1 104.0 100.0
B 2 105.0 99.0
3 102.0 99.0
1 105.0 100.0
C 2 106.0 101.0
3 106.0 101.0
1 105.0 98.0
D 2 106.0 98.0
3 106.0 99.0
1 110.0 106.0
E 2 111.0 105.0
3 110.0 105.0

 

f

*
In dB re:0.0002 microbar

172

.mCHCCCH mHmEEmC 0C :uHB Coxmu muCOEmusmmmz
.1

 

 

 

vm.m vb.N ON.N mm.m vv.m mm.H om.N HH.N mO.H om.m VH.N mH.v HO.H om
N.mm v.mn O.m> N.mh m.vm m.hm m.Hm O.Hm H.Hm m.mm m.mm m.~m m.MOH cam:

11 1- 11 -1 -1 1- 1- 1- -1 -1 -1 0.4a m.moa m
m.wm m.m> m.mn O.Nm m.vw H.Om m.vm m.mm 0.0m O.mm 0.0m m.vm O.NOH v
m.Hm O.mh m.Oh m.mh m.Hw v.0m 0.0m O.mm O.Hm m.hm O.mm m.om m.MOH m
v.mm m.mm O.Vh 0.0m m.nm m.mm v.Hm O.Hm m.mm 0.5m m.mm O.vm m.v0H N
m.mh O.mh m.mh O.mn m.mm m.mm m.mm 0.0m 0.0m 0.00H O.HOH m.Hm O.vOH H
oomHm OOOOH ooom ooov OOON OOOH OOm omm mNH mm m.Hm d mp HMOCHH

COHumooq

um CH wondvmum umuCoU UCmm m>muoo

 

.AHMQOHUHE Nooo.o on me CHO ba..U.Cm:ewusmswe mmHOC pCsoumxomb msosCHuC0011.hm mqmde

173

TABLE F8.--Peak SPL and dB A measures* on Hammer III.

 

 

Reading Linear Peak dB A
Number SPL

1 146.0 112.0

2 146.5 113.0

3 146.0 114.0

4 145.5 114.0

5 144.5 112.0

6 143.0 113.0

7 146.0 114.0

8 145.5 113.0

9 147.5 113.5

10 147.1 114.0

Mean 145.8 113.2

SD 1.3 0.8

 

*

Peak Measurement made on the seventh or last
stroke of the series. dB A is visually determined average
of all strokes in the series.

APPENDIX G

PHASE II: SUBJECT AND THRESHOLD DATA

174

]§75

.vomHIOmHnmu sane coHuosOcoouuHa
i

 

 

 

0.0m O.~H O.HH o.v 0.0H O.mm 0.0 O.mH 0.0 O.m mews: .uum m.H hm.wm OH
0.0H 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 mums: .num O.N m>.ON m
m.muuou
Hummmsz
O.w O.~H O.m O.v O.vH 0.0H O.m 0.0 O.v O.HH m.1mOCHm umEEmm O.m Nm.Om m
NIuou
anmmsz
O.NH O.vm O.vm 0.0m O.NH 0.0H O.NH O.mH 0.0m O.NH m1wmsz .num 0.0 mm.hm n
m.-msaa
O.vm O.~H O.VH 0.0N O.vm O.mm O.mN 0.0H O.om O.Hm m.H1mm:z .uum o.m O.mm O
H1mmsHm
O.v 0.0m O.m 0.0H 0.0H O.m O.mm O.m 0.0 0.0H Nummwsz Connor O.m mv.vm m
m.H1mmsHa
O.m O.m 0.0 O.v O.v O.mH O.o 0.0H O.v 0.0 H1uou Congo: m.m mm.mm v
0.0m O.v O.v 0.0 O.vH O.Nm O.v 0.0H O.m 0.0H mCoz Connor O.m mn.om m
O.HH 0.0m O.N O.N O.N 0.0m O.vm 0.0 O.m O.HH Coauoo amuse: O.m mm.vm N
O.mm O.vm 0.0 O.N 0.0 O.HH O.m 0.0 O.m 0.0 Couuou nmummm m.m ms.Hm H
OOOO ooov OOON OOOH oom OOOO ooov OOON OOOH Oom musmomxm umnEsz
.uoouonm .uHmom mumo mo< own :
Aumm ummqv um CH mucosqmum Aumm uanmv a: CH >oCmsgwnm w u .n m

 

.QSOCO mesmOme

mumm> OHIO mnu MOM mumm ummH pCm quHH men new must w.HoHoemeCu

mcHummu was «nap noansm11.Ho mamas

.mCHCEHum ou mmCommmu 0C m>mm mm CmsonuHm Hm>oH mCHummu EdEmeE mm Cm>Hm CHOCmmunag

.mCHm xm3 Coauoo cu mummmu msHm 3

.mmsHm mew» m>Hm> ou muommu msHm >

w

x.

vomH1OmH "on .mHCo CoHuonCooluH4
i

 

 

 

NH .uou
O.vm O.Hm 0.0m O.Hv 0.0N O.HN O.mv O.mv O.HH O.vN m msHm .> Hoummm O.mH 0.0v ON
NN .uou
O.Nm O.vn 0.00 O.Nm 0.0N 1O.OO 0.00 O.vn O.NO O.mm mnmsHm .> umEEmm O.mN mm.mv mH
ON1.uou
*«0.0m O.Hm O.Hv 0.0v 0.0H i0.0m 0.0m 0.0m O.HV 0.0H v msHm .> nmEEmm O.vN O.hv OH
mH1.uou
0.0m O.mO O.vm 0.0m O.Nm O.Nm O.Nn 0.00 O.Hm O.Hv m OSHA .># noummm O.HN hH.hv NH
,6 OH1.uou
.1 0.0¢ O.vm 0.00 0.0m 0.0N O.Hv o.mv O.mv 0.0 O.NH mmsHm .3 nmEEmm 0.0H hm.mm OH
11
HH1.uou
0.00 O.mm O.HO O.Nm O.mH O.vm O.Nm O.Hm 0.0v 0.0N Nnmwwsz noEEm: 0.0H NH.mm mH
mm-.noo
O.v 0.0H O.v o.o O.m 0.0 0.0N 0.0 O.N 0.0 msHm .3e nmEEmm O.MN om.mv «H
440.0m o.N> O.mm O.mN O.Nm 0.0m O.mv O.mH O.NN 0.0N Couuoo noumom O.NH om.om mH
O.N o.ov O.NN o.NH 0.0 0.0 0.0m O.HM O.mm 0.0 couuou HwEEmm 0.0N om.Ov NH
*«O.om O.mm 0.0m O.Nm 0.0 O.mm O.Nn O.mm O.mm O.v couuou Hoummm O.NN mN.mv HH
OOOO ooov OOON OOOH oom OOOO ooov OOON OOOH oom mudmomxm umnECz
.uomuoum .uHmom mama» mod uomn 5m
Aumm bmmqv a: CH moCmsvwnm Aumm UCOHCV mm CH NUCmswmum .b

 

use» mNImH mnu new mumm ume pCm

.msoum whomomxm

DCOHH now meme «UHOCmounu OCHummH OCm must uommnCmII.NU mqmda

APPENDIX H

PHASE II: TTS DATA

177

.OC30w mm3 wee 0C ponusooo wmcmco 0C m4 .mummu HHm Co a: OOOO um mCHSEHum Ou mmCoeme 0C m>mw mHmm
a;

.EHC How meHmuno on uOC pHsoo mump wee pCm mCHummu mo xmoB HmCHm we» OCHnsp pmquCH mm3 m uowmnsm«

 

178

 

 

 

 

O.VH 0.0 0.0 0.0H O.NH 0.0 O.N1 0.0 O.N 0.0 0.0H O.N 0.0H 0.0H O.NH ON
O.v1 O.N1 O.N O.N1 O.N1 0.0 0.0 0.0 O.v1 O.N1 O.N1 O.N 0.0 0.0 O.N mH
««mz O.N1 O.NH O.N O.NH ««mz O.N1 0.0 0.0 0.0 ««mz O.N 0.0H O.N 0.0H OH
O.N1 0.0 O.N O.v o.e O.N O.N1 0.0 O.v O.v O.N1 O.N 0.0 O.v 0.0 NH
O.m O.N O.NH 0.0H O.NH O.N1 0.0 O.N1 0.0 O.N 0.0 O.N O.NH O.vH O.NH OH
O.N 0.0 O.v 0.0 0.0 O.N1 0.0 O.N1 O.N1 O.N1 0.0H 0.0 0.0 0.0 O.v mH
O.N1 O.N O.N1 O.N O.N1 0.0 0.0 O.N1 O.N O.v 0.0 0.0 0.0 O.N O.N vH
O.N 0.0 O.v 0.0 O.N 0.0 O.N1 0.0 O.N1 O.N O.N1 O.N 0.0 O.N O.m mH
0.0 0.0 O.N O.NH 0.0 0.0 O.N1 0.0 0.0 O.N 0.0 O.N 0.0 O.N O.v NH
0.0 0.0 0.0 O.vH O.NH O.N1 0.0 0.0 O.N1 0.0 0.0 O.N O.N O.NH 0.0H HH
O.v1 0.0 O.N1 0.0 0.0 O.N1 0.0 O.N1 0.0 0.0 O.N1 0.0 O.N1 0.0 O.N OH
O.N1 O.m O.N 0.0 O.m O.N1 O.v 0.0 0.0 O.N O.N1 0.0H O.N O.v 0.0 m
0.0 O.N1 0.0 O.N1 O.N1 0.0 0.0 0.0 O.N1 O.N1 O.N 0.0 0.0 0.0 O.N1 m
O.m O.v 0.0 0.0 O.m 0.0 0.0 O.N O.N O.N1 0.0H O.v O.v O.m 0.0H N
O.N1 O.N 0.0H 0.0H 0.0 0.0 0.0 O.N O.N1 0.0 O.N1 O.N O.NH O.v 0.0 m
O.N O.N1 0.0H 0.0 O.N O.N1 0.0 O.N1 0.0 O.N1 O.N 0.0 0.0 0.0 0.0H v
O.N O.¢N 0.0 0.0 O.N1 O.N1 O.v O.N O.N 0.0 O.HN 0.0v O.¢H O.¢ O.N m
O.v1 0.0 O.N O.N O.v 0.0 O.N O.N 0.0 0.0 O.N1 O.N O.N O.N O.N N
O.N1 O.m 0.0 O.v 0.0 O.N1 0.0 0.0 O.v O.v 0.0 O.NH 0.0H 0.0 O.N H
OOOO OOOO OOON OOOH oom OOOm ooov OOON OOOH oom OOOO ooov OOON OOOH oom
Sm >mpmo59 Ed mmpmmde 2m mmpCoz uowhnsm

um CH xoCmswmum

 

.Zm xmpmosa pCm .24 ampmwse .zm umpCoz Co poummu smuowflQSm mH mew mo comm now muwHCm pHonmmuCB >umuomEmell.Hm mqmda

APPENDIX I

PHASE III: PERMANENT THRESHOLD SHIFT

SUBJECT DATA

179

TABLE Il.--Subject data for employees from Forge A.

 

 

gaggzgt Age Eiggzugg Position Protection
A l 36 18 Hammerman None
A 2 42 23 " "
A 3 55 25 " "
A 4 52 29 " "
A 5 65 48 " "
A 6 51 25 " None 20/Cotton 5
A23 60 23 " None 10/ Cotton 13
A 7 38 16 " Cotton
A 8 38 16 " "
A 9 36 17 " "
A10 48 18 " "
All 40 20 " "
A12 52 23 " "
A13 43 24 " "
A14 41 24 " "
A15 46 26 " "
A16 54 30 " "
A17 56 35 " "
A24 63 24 " "
A18 51 22 Straightener "
A19 53 24 " "
A20 43 26 " "
A21 46 19 Heaterman Cotton 17/ Plugs 2
A22 49 20 " Cotton 18/ Plugs 2
A25 63 23 " Cotton

 

180

181

TABLE 12.--Subject data for employees from Forge B.

 

 

gflggzgt Age Eiggzugi Position Protection
B l 37 19 Hammerman None
B 2 46 27 " "
B 3 54 31 " "
B 4 59 31 " "
B 5 65 42.5 " "
B 6 46 27 " None 25/ Plugs 2
B 7 58 4O " None 38/ Plugs 2
B 8 l 41 " None 39/ Plugs 2
B21 40 20 " None 19.5/Plugs .5
B 9 46 20 " Cotton
B10 56 27 " "
B11 50 32 " "
B12 55 35 " "
B13 59 42 " Damp Cotton
B14 43 21 Hammer 15 Cotton 19/ Plugs 2
Heater 6
B15 43 20 Heater None
B16 47 23 " "
B17 59 34 " "
818 44 20 " Cotton
B19 39 20 " "
B20 42 20 " "

 

182

TABLE I3.--Subject data for employees from Forge C.

 

Subject Years of

 

Number Age Exposure Position Protection
C_l 41 13 Hammerman None
C 2 41 18 " "
C 3 45 23 " None 10/ Cotton 13
C 4 45 24 " None 19/ Cotton 5
C 5 58 40 " None
C 6 62 40 " "
C 7 30 12 " Cotton
C 8 4O 20 " Cotton
C 9 42 20 " Cotton 16/ Plugs 4
C18 51 15 " Cotton
C10 45 24 Straightener None 23/ Plugs 1
C11 51 22 " Cotton 15/ None 7
C12 40 12 " Cotton 11/ Plugs 1
C13 42 16 Heaterman None
C14 39 16 " "
C15 46 16 " "
C16 48 25 " "

C17 54 30 " "

 

183

TABLE I4.--Subject data for employees from Forge D.

 

Subject Years of

 

Number Age Exposure Position Protection

D1 54 24 Hammerman Cotton

D2 50 26 " "

D3 29 8 " Cotton 7/ Muffs 1
D4 44 21 " Cotton 20/ Muffs 1
D5 52 29 " Cotton 28/ Muffs 1
D6 43 19 " None

D7 58 30 Hammerman 18 None 20/ Cotton 10

Heaterman 12

 

APPENDIX J

PHASE III: PERMANENT THRESHOLD DATA

184

AVOOH .OmH "may Hm>mH OCHHmw: mp CH«

 

185

 

 

 

11 OO OO OO cm on ON 11 ON OO OO Ov Om OH ONC
OO 11 OO Ow ON ON OO ON 11 OO OO om OH ON NNC
ON 11 me OO me O O ON 11 OO Ov OO O OH HNC
11 on OO OO OO OO Ov 11 OO OO OO ov Ov Om ONC
11 Oh ON ON OO Ov OO 11 OO OO OO OO Ov ov mHC
11 on OO Ov ON Om ON 11 On OO Ov Om om ON OHC
11 mz Oh Oh Om ON OO 11 OO On on ON OM OM «NC
11 mm mm OO OH ON ON 11 ON OO OH OH ON ON NHC
11 mz OOH OO OO Ow ON 11 mz Om ON OO ov Om OHC
11 m2 OOH OO OO ov ON 11 mz Om Om OO Ov ON OHC
11 ON on ON OO ow ON 11 OOH mm on OO OO ON vHC
11 mm mm OO OO ON ON 11 om OO OO OO Om OH OHC
11 mz OO mm on OO OO 11 mz om Os OO ov OO NHC
OO Os OO OO OH OH O me on OO OO OO ON OH HHC
11 mz on OO OO OH O 11 ON OO OO Ov OH O OHC
11 OO OO OO Ov ov ON 11 OO OO OO OO ov ON m C
OO 11 me mm mm OO OO On 11 OO OO OO OO OO O C
11 on OO OO OO Om ON 11 mm mm OO Os OO ON N C
OO OO OO ON OO Ow ON Ov Oh OO OO OO ow ON ONC
OO 11 OO OO OH OH O On 11 OO me me OO ON O C
11 OO ON on OO OO ca 11 m2 OO ON OO OO me O C
11 mm on ON OO OO OO 11 OO om OO OO OO me O C
11 mz OO Om OO ov OO 11 mz OOH Om OO OO om m C
OO 11 cm ON O» OO OO OO 11 OO Om OO on OO N C
On 11 OO OO OO ON OH ON 11 OO OO OO ON OH H C
OOOO OOOO OOOv OOON OOOH OOO OON OOOO OOOO OOOH OOON OOOH OOO OON
uomnndm
Aumm uwoqv a: CH mocosvmum Anmm quHmO N: CH NUCmamum

 

.C mouom Eouw mmmhoHQEm MOO «mumo UHOCmOHCB OCHummm11.HO mHmCB

 

186

mm 11 Oh Oh OO om ON OO 11 Oh Oh OO om ON ONm

 

 

 

mz mz Om ow ow O O mz m2 On OO OO OH OH mHm
mm 11 OO Oh Ow Om ON . ov 11 OO OO OO ON ON mHm
on 11 On OO OO ov om OO 11 Oh OO OO OV Om NHm
OO Oh Oh OO Ov ov OH Ow OO ON OO OO OH OH OHm
On 11 On ON OO ov OH ON 11 OO mm mm Ov ON OHm
OO OO Om OO OO om ON OO OO Om OO OO om Om va
On 11 OO . OO OO ON O OO 11 OO OO OO om O OHm
OO 11 OO OO OO Om om OO 11 OO OO OO Ow Om NHm
m2 11 OOH OO Oh Om om mz 11 OOH Om Ob OO Om HHm
mz 11 OOH om On Om Om mz 11 OO OO OO Om Om OHm
O» OO OO OO ON OH OH mz m2 OO ON OO Om OH O m
mz 11 OO OO OO ow ON OO 11 OO OO OO ov Om HNm
On 11 OO O» OO OO OO OO 11 OO ON OO OO ov O m
OO 11 OO OO OH O OH OO 11 OO OO ov OH O h m
OO 11 OO OO ON O OH OO 11 Oh OO OO O OH O m
om 11 OO OO OO OO OO mz 11 OO OO OO OO OO O m
mz 11 OO On OO om OH mz 11 OO OO OO Ov OH O O
OO 11 OO OO OO om ON OO 11 OOH OO OO ow ON m m
OO 11 On ov OO Ow OO ON 11 Oh Oh Oh OO OO N m
Ov 11 Oh OO Ov Om OH . OO 11 On OO OO Ov OH H m
OOOO OOOO ooov OOON OOOH OOO OON OOOO OOOO OOOO OOON OOOH OOO OON
Aumm uwmqv um CH OOCmsvmum Aumm quHmO um CH NOCmsvmnm pommnsm

 

.m mmuom Eoum mmmmonEm new mama CHOCmmHCB mCHummm11.Nb mamCB

 

.187

11 OO Oh OO OO Ov om 11 OO OO OO OO cm on OHU

 

 

 

11 OO on On ON OO OQ 11 OO Oh Oh OO Om ON OHU
mz 11 On On Ow Om Om mz 11 OO Oh ov Om Om OHU
11 OO OO OO om O OH 11 on OO OO Ov OH OH VHO
on 11 On OO OO OO ov ON 11 . Oh OO OO Ow Ov OHU
cm 11 OO Om Om OH OH ov 11 OO Om om OH OH NH0
11 Ov OO OO Ov om Om 11 OO OV Om om ov Om HHU
11 OO OO OO Ov ON OH 11 OO OO OO ov om OH OHU
mz 11 On OO OO Ov Om m2 11 ON OO OO ow om OHU
mz 11 OO Ov Om OH OH OO 11 OO ov ov OH OH O U
mz 11 OOH On Ov OH OH m2 11 OOH ON OO OH OH O U
11 OO Oh OO OO om ON 11 om ON OO OO ov ON O 0
OO 11 OO OO OO OO OO m2 11 On OO om On OO O U
11 m2 Om OO OO OO Ov 11 OO On OO OO OO OO O U
11 om OO on OO om ON 11 mz Om OO OO Om OH O O
OO 11 OO OO OO Om OH OO 11 OO OO OO Om ON m U
OO 11 OO OO OO Om ON OO 11 OO ON OO Ov ON N U
11 mz Om OO OO ov OO 11 mz Om ON ON OO Om H U
OOOO OOOO OOOv OOON OOOH OOO OON OOOO OOOO OOOO OOON OOOH OOO OON
poansm
Aumm uwmqv um CH >0Cmsvmum Aumm uCOHmv mm CH NUCmawmum

 

.0 mmuom Eouw mammoHQEm COO mama UHOCmmCCB OCHummm11.mO mquB

188

 

 

 

 

om 11 OO OO Oh OO OO ON 11 OO Oh OO OO OO O Q
11 OO Om OO OO OO OO 11 OO OO Oh OO OO OO O Q
OO 11 . OO OO OO ow OH OO 11 OO OO OO ow OH O Q
mz .11 OOH om OO ON om mz 11 mm mm OO Om om v o
mz 11 Oh OO ow om ON mz 11 OO OO Ov Om OH O Q
mm 11 om mp on mO om om 11 mm on om mq me N O
OO 11 OO OO OO Ov ON OO 11 OO OO OO Ov om H a
OOOO OOOO ooov OOON OOOH OOO OON OOOO OOOO OOOO OOON OOOH OOO OON
_ OomHnsm
Humm ummqv um CH >0Cmsvmum Aumm quHmO mm CH NOCmswmum

 

.a mmuow Eouw mmmonmEm How mumo CHOCmmHCB OCHummMI1.vO mHmCB

 

APPENDIX K

HEARING LOSS ATTRIBUTED TO PRESBYCUSIS

189

TABLE Kl.--Hearing loss in dB attributed to presbycusis by
age and frequency.*

 

 

 

Age Group
Frequency (in years)
(H2)
26-32 34-40 43-49 51-57 59-65

250 0.4 -2.3 4.2 1.5 7.5
500 2.2 1.0 2.9 4.7 7.5
1000 0.6 1.9 6.6 6.5 9.6
2000 1.1 2.3 11.4 11.0 18.2
4000 7.5 9.7 20.6 21.3 34.8
8000 6.0 9.2 12.5 19.0 27.7

 

it
After Corso (1963).

190