A NEW METHGD FOR EXPRESSINS {FAWN AEEE FEEWRAAAME CEAE’, ACE‘ERESLKCS AND ‘ FOB SGNBEC‘I‘ERS Ea? “ARENG AH} EVAA'UA'EEQEAS Thesis for fin: Degree 65 951 D. M'CLEGM SHE-E S‘AE‘JEESET‘.’ Danie! R. Schumaier 9372 .3; LIBRARY M lshfgan State . University 152.3" ’ This is to certify that the thesis entitled A NEW METHOD FOR EXPRESSING HEARING AID CHARACTERISTICS AND FOR CONDUCTING HEARING AID EVALUATIONS ' presented by Daniel R. Schumaier has been accepted towards fulfillment of the requirements for Ph. D degree in .AisdiologL. /4-.L.7.fi%.. I Major professor Date September 25J 1972 0-7639 ~— _ I2 '(5 M ABSTRACT A NEW METHOD FOR EXPRESSING HEARING AID PERFORMANCE CHARACTERISTICS AND FOR CONDUCTING HEARING AID EVALUATIONS By Daniel R. Schumaier This thesis was comprised of two investigations. The first sought to compare the present HAIC procedure for measuring and reporting the acoustical characteristics of hearing aids, to a new procedure developed by the present investigator. It was argued that the HAIC procedure does not accurately represent the performance of a hearing aid under normal listening conditions. The new method for evaluating and reporting hearing aid character- istics was designed to overcome the basic criticism of the HAIC procedure. The results of both methods were compared on the following specific acoustical parameters: (1) gain, (2) maximum power output, (3) frequency range and (4) frequency response curve. The second investigation was performed in order to deter- mine if differences could be observed between hearing aids using CNC monosyllabic speech discrimination materials. Normal hearing Daniel R. Schumaier subjects responded to these speech materials after they were re- corded under five different listening conditions through nine different instruments. The listening conditions employed were: (1) CNCs in quiet, (2) + 10 signal-to-noise ratio, (3) + 10 signal- to-competing message ratio, (4) 0 signal-to-noise ratio, and (5) 0 signal-to—competing message ratio. The hearing aids employed were selected from three types of frequency response groups: flat, irregular and high frequency emphasis. All subjects listened to the recorded materials at a 50 dB sensation level, which was essentially equal to a normal conversational speech level (70 dB SPL). It was reasoned that if differences were seen between hearing aids, these differences could be attributed to the hearing aid's frequency response and associated acoustical distortion. The following conclusions were drawn from the first investiga- tion: (1) The HAIC Method tended to over-estimate actual "usable" gain by approximately 21 dB; however, because considerable variability existed between instruments, a constant correction factor could not be subtracted from the HAIC gain in order to predict the average gain derived by the new method; (2) Both methods gave approximately the same average maximum power output; (3) The frequency range derived by both methods was similar; however, the new method tended to raise the low frequency cut-off by approximately 100 Hz; (4) Both methods gave identical frequency response curves. The following conclusions were drawn from the second experi- ment employing normal listeners: (1) Differences between hearing Daniel R. Schumaier aids could not be adequately demonstrated using CNC monosyllabic words in a quiet listening condition; (2) The + 10 secondary signals (noise and competing message) equally depressed speech discrimination scores. Moreover, these listening conditions did not demonstrate differences between hearing aids; (3) Both 0 primary-to-secondary signals resulted in substantial depression of speech discrimination scores, with the poorest scores attained by the noise condition. Also under both of these conditions differences between hearing aids were observed; (4) When hearing aids were grouped by frequency response, (flat, irregular and high frequency emphasis) differences were not found between groupings for any of the five listening conditions. Based on the outcome of this investigation a hearing aid evaluation procedure was recommended involving the utilization of speech discrimination materials recorded through hearing aids on tape. A NEW METHOD FOR EXPRESSING HEARING AID PERFORMANCE CHARACTERISTICS AND FOR CONDUCTING HEARING AID EVALUATIONS By .. Daniel R§“Schumaier 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 1972 of) “N GO Accepted by the faculty of the Department of Audiology and Speech Sciences, College of Communication Arts, Michigan State University, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. s \ ‘ ThBSiS COMittBEiflVm—s ;- I iM‘u—U— Director William F. Rintelmann, Ph. D. X/(t g/o‘t “WM/K Daniel S. VBeasley, Ph. D. fig /C//..; May E. fiin, Ph. D. MM Verling C. Troldahl, Ph. D. ACKNOWLEDGEMENTS In appreciation of the guidance and time so freely given, I would like to thank Dr. William F. Rintelmann, my advisor; Dr. Daniel S. Beasley; Dr. May E. Chin; and Dr. Verling C. Troldahl. I also gratefully acknowledge Miss Sabina A. Kurdziel for her help in evaluating the hearing aids used in this study. Finally I would like to acknowledge my wife Marty, for her financial and moral support and typing skill. ii TABLE OF CONTENTS Page LIST OF TABLES O O O O O O C O O O O O O O O O O . C v LIST OF FIGURES . . . . . . . . . . . . . . . . . vii LIST OF APPENDICES . . . . . . . . . . . . . . . . ix Chapter I. INTRODUCTION . . . . . . . . . . . . . . . 1 Purpose of the Study . . . . . . . . . Significance of the Study . . . . . . . Ul-L‘ II. BACKGROUND INFORMATION AND LITERATURE REVIEIJ O I O O 0 O O O O O O O O O O O 0 O 6 Hearing Aid Response Characteristics. . 6 Gain . . . . . . . . . . . . . . . . 8 Maximum Power Output . . . . . . . . 10 Frequency Range and Frequency Response Curve . . . . . . . . . 15 Clinical Hearing Aid Evaluations. . . . 18 Evaluation Procedure . . . . . . . . 18 Specific Questions Asked . . . . . . . 24 III. EXPERlbiENTAL DESIGN 0 O O O O 0 O O O O O . 26 Procedure for Describing Hearing Aid Characteristics . . . . . . . . . . . . 26 Selection of Hearing Aids. . . . . . 26 Equipment Used for Analysis of Hearing Aids . . . . . . . . . . 29 New Procedure Used for Obtaining and Reporting Hearing Aid Characteristics . . . . . . . . . 29 New Frequency Response Curve and Cain Values . . . . . . . . . . . 32 New Frequency Range . . . . . . . . 32 New Maximum Power Output . . . . . . 33 iii Speech Stimuli and Recording Procedure . . . . . . . . . . . . . . Stimulus Materials Recorded . . . Primary and Secondary Signals . . Listening Conditions . . . . . . . Selection of Hearing Aids . . . . Recording Equipment . . . . . . . Procedure Used for Recording . . . Experimental Design . . . . . . . . . Subjects . . . . . . . . . . . . . Testing Equipment . . . . . . . . Testing Procedure . . . . . . . . IV. RESULTS AND DISCUSSION . . . . . . . . . Hearing Aid Response Characteristics Gain . . . . . . . . . . . . . . Maximum Power Output . . . . . . Frequency Range . . . . . . . . . Frequency Response Curve . . . . . Speech Materials for Hearing Aid Evaluations . . . . . . . . . . . . . Listening Conditions . . . . . . . Hearing Aids . . . . . . . . . . . Listening Conditions by Fre- quency Response Grouping . . . Discussion. . . . . . . . . . . . . . Clinical Implications . . . . . . . . V. SUMMARY, CONCLUSIONS AND RECOMMENDATIONS summary I 0 O O O O O O O O O O O O O COHClUS ions 0 O 0 O O O O I O O O O 0 Recommendations . . . . . . . . . . . REFERENCES 0 O O O O O O O O I O O O 0 O O O O O API)ENDICES O I O O O O O O O O O O O O D O O I 0 iv 34 34 35 35 36 37 43 44 44 44 46 49 49 50 54 57 59 61 63 68 68 70 77 8O 80 82 84 85 89 Table 5. LIST OF TABLES Page Manufacturers' HAIC specifications and HAIC specifications obtained at Michigan State University Audiology Research Laboratory. . . . . . . . . . . 28 Comparison of gain values obtained with HAIC Method for determining gain and the new method for determining gain. The difference in gain between the two methods for each hearing aid is also shown . . . . . . . . . . . . . . . 51 Mean differences, standard deviations and range of differences in dB between the HAIC Method and the new method for determining gain. Values are given by the three original gain categories and for the total sample of eighteen hearing aids . . . . . . . . . . . . . . 52 Comparison of maximum power output (MPO) obtained with the HAIC Method and the new method. Values represent the three frequency average of 500, 1000 and 2000 Hz. The difference between the two methods is given for each hearing aid. Also, the peak frequency MPO obtained with the new method is shown. . . . . . . . . . . . . . . . . . 55 High and low cut-off frequencies ob- tained with HAIC Method and the new method for determining frequency range. The difference in Hertz between both methods is also shown . . . 58 Summary of analysis of variance for hearing aids and listening con- diticlls O O O O O O O O C O O O C C O O 64 Table Page 7. Mean discrimination scores (in percent correct) for each of nine hearing aids for five listening conditions. Means for each hearing aid across all listen- ing conditions and the means for each listening condition across all hearing aids is also shown. In addition, standard deviations (SDs) for listen- ing conditions are presented . . . . . . . 65 8. Summary of analysis of variance for frequency response and listening conditions . . . . . . . . . . . . . . . . 69 9. Mean discrimination scores (in percent correct) by frequency response group- ings (flat. irregular and high fre- quency) for the five listening con- ditions. The mean for each of the three groups across all five con- ditions is also shown. . . . . . . . . . . 71 vi Figure 8. LIST OF FIGURES Family of gain curves for a specific hearing aid obtained with a differ— ent procedure than advocated by HAIC. A reduction in output is also shown for the full—on volume control adjust- ment 0 O O O O O O O O O O O O O O I O O O 0 Illustration of the new procedure for determining the maximum power output (MPO) of a hearing aid . . . . . . . . . . HAIC Method for determining the basic fre- quency response curve and frequency range of a hearing aid . . . . . . . . . . . . Illustration of proposed method for deter— mining the frequency range of a hearing aid. 0 O O O C C O O O O C O O O O 0 Simplified block diagram of hearing aid measurement system: Sine—Random Gen- erator (Bruel and Kjaer, Type 1124); Anechoic Chamber (Bruel and Kjaer, Type 4212); Frequency Analyzer (Bruel and Kjaer, Type 2107); Audio Frequency Spectro- meter (Bruel and Kjaer, Type 2112); Graphic Level Recorder (Bruel and Kjaer, Type 2305) . . . . . . . . . . . . Individual frequency response curves of three hearing aids with relatively flat frequency response characteristics between 500 and 3000 Hz. . . . . . . . . . Individual frequency response curves for three hearing aids with high frequency emphasis between 500 atld 3000 I12. 0 O O 0 O O O O 0 Individual frequency response curves for three hearing aids with irregular fre- quency response between 500 and 3000 Hz. vii Page 11 14 17 19 30 38 39 4O Figure Page 9. Mean frequency response curves from 500 to 4000 Hz for the three frequency response groups; high frequency emphasis, flat and irregular . . . . . . . . . . . . . . . . . 41 10 Simplified block diagram of equipment used for recording speech materials through hearing aids; Sound Level Meter (Bruel and Kjaer, Type 2204); Tape Recorder (Ampex, AG 600); Sine-Random Generator (Bruel and Kjaer, Type 1024); Clinical Audiometer (Maico 24) with it's associated tape deck, speaker and turn table . . . . . . . . . . . . 42 11 Mean audiogram, speech reception threshold, and pure-tone average for ninety normal listeners O 0 O O O O O O C O O O O O O O O O O 45 12 Illustration of frequency range as measured by the HAIC Method and the new method. The frequency response curves obtained from both methods have been superimposed for this illustration . . . . . . . . . . . . . 60 13 Comparison of the frequency response curves obtained with both procedures for the hearing aid exhibiting the smallest differ- ence and the hearing aid with the largest difference. The dashed frequency response curve represents the new procedure. . . . . . . 62 14 Mean discrimination scores from each hear— ing aid for all five listening conditions . . . 66 15 Mean discrimination scores for the three frequency response groupings for all five listening conditions . . . . . . . . . . . . . 72 16 Frequency response of left earphone on Grason-Stadler 162 speech audiometer. Pre-and post—experimental frequency res- ponse curves were identical . . . . . . . . . .101 viii LIST OF APPENDICES Appendix A. HAIC Standard Method of Expressing Hearing Aid Performance . . . . . . . . . . . . . . . 90 B. Northwestern University Auditory Test No. 6, 1801.111 B, Lists I’IV O O O O O O O O C O O O 95 C. Calibration of Equipment . . . . . . . . . . . . 98 D. New Method and HAIC Data for all Eighteen Hearing Aids. . . . . . . . . . . . . . . . . 103 ix CHAPTER I INTRODUCTION At the present time many hearing clinics employ a "tradi— tional" approach in the selection of a specific hearing aid for patients in need of amplification. With this approach the patient's performance is evaluated with several different hearing aids under "identical" listening conditions. The results are compared to deter- mine if one hearing aid provides more benefits than others for the individual with a particular hearing loss. In preparation for the hearing aid evaluation the audiologist examines the audiometric test results in order to select several appropriate hearing aids for trial by the patient. The level of the speech reception threshold (SRT) is used as a guide in selecting the needed gain of the hearing aid. The pure-tone audiometric con- figuration suggests the appropriate frequency response curve, while the threshold for discomfort dictates the maximum power output of the instruments. In the hearing aid evaluation the patient's performance is compared on several hearing aids by obtaining such measures as aided SRT, the aided speech discrimination score, and the aided tolerance level. The objective of such a hearing aid evaluation is to deter- mine if one particular hearing aid and receiver combination would benefit the patient more than other hearing aids. The hearing aid and receiver combination which yields the best SRT, the highest tolerance level, and the highest speech discrimination score is generally the type of hearing aid recommended for purchase. Several problems are inherent in this "traditional” procedure for hearing aid selection. Some of the more apparent problems are: l. The aided SRT is usually obtained after the volume control on the instrument has been adjusted to the level where the patient "judges" it to be most comfortable, or the volume control is arbitrarily adjusted by the audiologist. This setting may or may not be the optimal setting for the particular hearing aid involved. Also this setting is not repeatable or consistent from hearing aid to hearing aid (Kasten and Lotterman, 1970). 2. Stock ear molds are generally used which many times do not form tight seals in the external ear canal, thereby, allowing acoustic feedback to occur, which prevents setting the hearing aid for optimal gain. 3. Most clinics do not have the equipment necessary for making precise measurements of the physical performance characteristics of the individual instruments; thus, the audiologist is not always certain the aids selected are meeting the manufacturer's specifications. 4. The time factor often necessitates bringing the patient back for a separate hearing aid evaluation apart from the initial hearing evaluation. Furthermore, the time constraint usually limits the trial to three or four hearing aids. 5. Finally, the assumption is made that the manufacturer's specifications present an accurate and valid representation of how the individual instruments will perform when worn by the hearing impaired person. With this "traditional" method of hearing aid selection many extraneous variables are left uncontrolled which lead to poor test- retest results. Better control of these variables would give a more accurate picture of the benefits of each hearing aid for the hearing impaired individual. Purpose of the Study The central focus of this research was to develop a new pro- cedure for hearing aid evaluations which would: 1. Allow for identical acoustical conditions when comparing several hearing aids via aided speech audiometry. 2. Eliminate or greatly reduce the need for using stock ear molds. 3. Decrease the possibility of a trial hearing aid not Operating according to specifica- tions. 4. Allow for hearing aid evaluations to be completed in a shorter period of time. To achieve these goals a new approach was designed for des- cribing the acoustical performance characteristics of hearing aids ‘which is different from that advocated by the Hearing Aid Industry Conference (HAIC) procedure (Lybarger, 1961). With this new approach the performance of the hearing aid is measured at an optimal rather than at maximal gain, since the hearing impaired person uses his hear- ing aid at an optimal gain setting. It is suggested that this new procedure for describing the performance characteristics of hearing aids offers a more accurate description of how the instrument should operate while worn by the hearing impaired individual. "The HAIC Standard Method of Expressing Hearing Aid Performance” (1961) notes only physical characteristics of hearing aids. In contrast the new method considers the hearing impaired individual as an essential part of the system, expressing numerical values for range of usable gain, frequency range, and maximum power output in a more realistic manner. Significance of the Study The significance of this study is two-fold: First, of primary importance is the description of hearing aid performance character- istics related to the hearing impaired individual's needs. Secondly, a method for hearing aid evaluations is proposed which will enable the audiologist to better serve hearing impaired children and adults who require amplification. CHAPTER II BACKGROUND INFORMATION AND LITERATURE REVIEW This chapter presents an overview of the present HAIC pro- cedure for expressing hearing aid performance characteristics. Further, two current viewpoints regarding how hearing aid evaluations should be handled by audiologists are summarized. Hearing Aid Response Characteristics Attempts to describe and categorize the acoustical parameters of electronic hearing aids date back to the late 1920's (Berger, 1968). In 1942, Romanow published one of the first extensive mono- graphs on measuring the performance characteristics of hearing aids. In 1944 Zenith standardized it's method for making measurements of acoustic gain, acoustic output and frequency response, and undoubtedly other manufacturers did likewise (Zenith publication). Later, under the auspices of the American Hearing Aid Association, Kranz (1945) published a "Tentative Code for Measurement of Performance of Hearing Aids". Although this method was proposed as a means for standardiz— ing the measurement and expression of hearing aid performance character- istics within the industry, the procedure was not universally put into practice. Until the early 1960's comparisons of the acoustical performance characteristics of hearing aids produced by various manufacturers were of doubtful value. Each manufacturer independently determined his own techniques for measuring hearing aid performance character- istics and for reporting these results to the consumer or the audiologist. Since a standardized method was not employed, manu- facturer specifications were of little value in the hearing aid evaluation procedure. Berger illustrated the nature of this problem by stating: Matters such as gain and frequency response were sometimes mentioned, with gain based on some average figures, or on the gain at 1000 Hz, or based on a peak frequency. Input, if mentioned, might be one of several levels (1970, p. 82). In 1960 HAIC established measurement procedures and standardized expressions for hearing aid responses pertaining to acoustic gain, output, and frequency range. The technical committee, which drafted the document, was composed of representatives from the manufacturing industry. This new standardized procedure was submitted to HAIC members for ballot in November of 1960, and was accepted and initiated in 1961 (Lybarger, 1961). The HAIC Method is primarily based on measurement procedures recommended by the United States of America Standards Institute (USASI) Standard 83.3-1960, entitled "USASI Standard Methods for Measurement of Electroacoustical Characteristics of Hearing Aids". In 1967 the HAIC Standard Method, with some modification, became USASI Standard 83.8-1967. The minor difference between the HAIC Standard Method and USASI Standard 83.8-1967 is that the latter calls for the frequency response graph to have a scale ratio of 15.05 dB per octave on the logarithmic frequency scale as opposed to the HAIC requirement of 13.5 to 15 dB per octave. To date the majority of hearing aid manufacturers in the United States are using the HAIC standard for expressing hearing aid per- formance characteristics. (The entire text of the HAIC Standard Method of Expressing Hearing Aid Performance may be found in Appendix A). In the following section details of the HAIC procedure are reviewed with discussion on acoustic gain, acoustic output, and fre- quency range as described by HAlC. Gain The term ”gain” as applied to a hearing aid shall mean the average of 500, 1000 and 2000 cps values of the full-on acoustic gain, as defined in Section 2.3 and as measured in accordance with Section 5.7 of American Standard 83.3 Unit: decibels (Lybarger, 1961, p. 17). For the above procedure the gain control of the hearing aid is set at it's maximum-on position, and the sound—field input level is adjusted for 50 dB SPL. A frequency response curve is then obtained with this input level held constant through a wide frequency range. The output sound pressure is measured for the three frequencies 500, 1000 and 2000 Hz. The 50 dB SPL input is then subtracted from the output measurements and the three selected frequencies are averaged. In this manner a single numerical values is obtained called the "HAIC Average Gain." In reviewing this procedure for determining gain three important facts should be mentioned: 1. Individuals do not wear hearing aids with the volume control turned full-on. 2. The normal input for conversational speech is not 50 dB SPL. 3. The HAIC formula for reporting gain as a single numerical value (i.e., average 500, 1000, 2000 Hz) tends to distort the picture if one is con- sidering high or low frequency emphasis hearing aids. In essence, the gain values as figured by the HAIC Method do not represent realistic gain values for the hearing impaired individual using the hearing aid. Davis and Silverman (1970) reported that average conversational speech at one yard averages between 65-70 dB SPL. Other investigators have reported slightly higher values for normal conversational speech ranging between 70-75 dB SPL. It would appear that if one were interested in portraying the actual beneficial gain for speech, that the hearing aid user would receive, two important deviations should be made from the HAIC Performance Standards. First, the input level to the hearing aid should be changed to a value more accurately representing normal conversational speech intensity levels. A more realistic value of 70 dB SPL appears appropriate. Secondly, the volume control of the 10 hearing aid should be varied from just-on to full-on in a systematic manner, thus giving a true representation of a Egggg of gain values at various volume control settings. It is proposed that a family of gain reaponse curves be obtained in 10 dB steps, by adjusting the hearing aid volume control while keeping the input stimulus constant. With this procedure one would generate a family of gain curves from just-on to full-on volume control. It is felt that the gain curve selected as representative for a particular hearing aid, should be a curve that would allow for one additional 10 dB increase and still remain linear with respect to the lower gain curves. This would allow for the hearing aid user to have a residual 10 dB of gain for listening situations with less than ideal input intensities (Carhart, 1946). The values reported would be output in dB SPL minus 70 dB SPL input for 500, 1000 and 2000 Hz. The three fre- quency gain average would also be reported. In this fashion one could report usable average gain over a 20 dB range as "X" dB i'lO dB. Figure l graphically shows an example of the pr0posed procedure described above applied to an individual hearing aid. Maximum Power Output The HAIC Standard defines output in the following manner: The term "output” shall mean the average of 500, 1000, 2000 cps values of the saturation sound-pressure level, as defined in Section 2.12, and as measured in accordance with Section 5.6 of American Standard 83.3 Unit: decibels re .0002 microbars (Lybarger, 1961, p. 17). ll Full-on response curve Ideal gain curve 120 g 110 C) 91 U H 21 g 100 O O o' [.11. 04 90 in Q 2: H Q 80 7o 200 500 1000 2000 5000 FREQUENCY IN HERTZ Figure 1. Family of gain curves for a specific hearing aid obtained with a different procedure than advocated by HAIC. A reducation in output is also shown for the full-on volume control adjustment. 12 A firm understanding of saturation output, sometimes referred to as Maximum Deliverable Pressure (MDP), Maximum Power Output (MPO) or Acoustic Output is essential for proper fitting of a hearing aid. Knowledge of this property of a hearing aid is useful and important in fitting for two reasons: first, and primarily, to minimize or prevent the amplified sound from becoming too loud or painful for the user. Secondly, to assure that the saturation output of the instrument is high enough to enable the input signal to be amplified at an adequately high sensation level above the hearing impaired individual's threshold. With the majority of hearing aids on the market, the second requirement is not a problem whereas, the first is of concern to the audiologist. According to the HAIC procedure for measuring saturation sound pressure level, the gain control on the hearing aid is adjusted to it's maximum full-on position. The sound-field input is then increased in a systematic manner of 60, 70 and 80 dB SPL or in 10 dB steps from the basic input of 50 dB SPL. At each step a frequency response curve is obtained through a wide frequency range. -In this manner a family of frequency response curves are obtained, which portray an input versus output relationship. When the output does not remain linear with respect to input, the hearing aid is considered to have reached it's saturation output level. In other words, a further increase in input does not provide a similar increase in output. When the maximum linear saturation output level is reached, the frequencies of 500, 1000 and 2000 Hz are averaged and reported as the saturation output in dB SPL. 13 Several basic difficulties are inherent in the HAIC pro- cedure for determining saturation output level, particularly as applied to the hearing impaired individual using the instrument. It is felt that the MPO should not be obtained with the hearing aid volume control adjusted to the full—on position but rather adjusted to the desirable gain volume control setting for each particular instrument as described earlier. At each setting a frequency res- ponse curve should be obtained through a wide frequency range. This procedure should be continued until the output does not remain linear with respect to the input for the frequency range of 500-2000 Hz. At this point the hearing aid would be considered to have reached it's saturation output. Figure 2 illustrates this procedure. It is believed that MPO values obtained at a "usable" volume control setting will give a more meaningful picture of maximum power output limits of the hearing aid, when worn by the hearing impaired individual. The rationale for this belief is based on the knowledge that often electroacoustic instruments lose efficiency when adjusted to a full-on position. For example, Figure 1 demonstrates a decrease in output when the hearing aid is adjusted for full-on volume control. It is also felt that when reporting MPO, in addition to numerically expressing the three frequency average of 500, 1000 and 2000 Hz, the peak frequency saturation output level, and the MPO at 500, 1000 and 2000 Hz should also be given independently. l4 F—Peak frequency MPO MPO curve 130 Ideal gain curve 120 3 ed a 110 z N O O O O 100 :25 94 CG Q E 90 n—l 94 CO 80 rr-u- H 2’)“ 500 1000 2000 5300 FREQUENCY IN HERTZ Figure 2. Illustration of the new procedure for determining the maximum power output (MPO) of a hearing aid. 15 Frequency Ragga and Frequency Response Curve The HAIC Standard Method states the following with respect to frequency range: The frequency range of a hearing aid shall be expressed by two numbers, one representing the low-frequency limit of amplification in cps and the other the high frequency limit of amplification in cps . . . Determination of the frequency range shall be made using a basic frequency response curve as defined in Section 2.11 and measured per Section 5.5 of American Standard 83.3. The following procedure shall be employed to determine the lower and upper frequency limits. Determine the average of 500, 1000 and 2000 CpS ordinates on the frequency response curve and plot this values on the 1000 Hz ordinate. Plot a second point on the 1000 cps ordinate 15 dB below the first point. Through the second point, draw a straight line parallel to the frequency axis. The low-frequency limit of the hearing aid is defined as the frequency where this line first intersects the frequency response curve, moving in the direction of decreasing frequency from 1000 cps . . . The high-frequency limit of the hearing aid is defined as the frequency range where this line first intersects the response curve, moving in the direction of increasing frequency from 1000 cps . . . (Lybarger, 1961, p. 33). The HAIC Method further mentions a specific procedure to be employed, "where a single 'notch' of inconsequential effect on the 16 hearing aid's performance may exist". (See Appendix A) The method also mentions that the basic frequency response curve should be shown in addition to the numerical data. It should be pointed out that the bagig HAIC response curve is obtained with a 60 dB SPL input and the volume control adjusted for 40 dB gain or 100 dB SPL at 1000 Hz. (Figure 3) It is thought that the HAIC procedure expresses results in an unrealistic manner and without concern for the individual hearing aid user. The method is unrealistic primarily because one is not assured that the response curve obtained is within the linear operating range of the hearing aid. For example, for a "low” gain instrument, with a 60 dB input it may be necessary to adjust the volume control to the full—on position in order to achieve 100 dB SPL output at 1000 Hz. Furthermore, with many low gain instruments, this desired output cannot be attained. Thus, this non-ideal setting may introduce distortion changing the true representation of the hearing aid's frequency response. In a similar fashion with high gain aids, many times the just-on position produces more than 100 dB output (e.g. 110 or 115 dB SPL) and this extremely low volume control position may also introduce distortion with respect to the frequency response curve. A.more realistic representation of the frequency response of the hearing aid might be obtained from the "ideal" gain curve as discussed earlier in the section concerned with gain. It is proposed that the frequency range be determined by the 17 Average of 500, 1000 and /2000 Hz. 100‘ {k 5% PG 0 0!. U H 2 §§ 90.. O (5 15 dB r S a I\ Z ~ : : E I l l I I l 250 500 1000 2000 4000 FREQUENCY IN HERTZ Figure 3. HAIC Method for determining the basic frequency response curve and frequency range of a hearing aid. 18 following method: once the ideal gain curve has been obtained, the numerical three frequency average for 500, 1000 and 2000 Hz is marked on the 1000 Hz ordinate. Ten dB is subtracted from this average and a second mark is made. A line is then drawn parallel to the abscissa. The line intersections of the low and high fre- quency skirts of the "ideal” gain curve defines the frequency range. This procedure produces a range which is felt to correspond to the dynamic range of ongoing speech. Previous measurements by this author of ongoing speech were found to be within a 10 dB dynamic range. The frequency range is expressed in this way by two numbers, a low and high frequency. Figure 4 graphically expresses the prOposed new procedure for defining frequency range. Credit should be given to HAIC for taking the initiative in standardizing the measuring and reporting of hearing aid performance characteristics. However, it is felt by this writer that we are now capable of describing hearing aid characteristics in a manner which also takes into account the acoustic needs of the hearing impaired individual.' Thus, it is thought that hearing aid characteristics should be derived in a manner similar to how the instrument is actually used by the hearing impaired individual. The Clinical Hearing Aid Evaluation Evaluation Procedure In most speech and hearing centers in the United States some form of hearing aid evaluation procedure is employed in order to 19 Three frequency average of 500 Ideal gain curve I— 1000 and 2000 Hz. I .-w_ . i..4 -4 -.. . . __4.14.1 - . 4:4. 4... ..-.;wu.a a ad’s ..-...u.-.du....... :Ef .u-..--.-...-.i_.--...-...-_i. 1.... .i. .1“. 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[.41’IJ .to. v .. ... tell“. 4.3.... J i I v: - a A Jillwltnill “.11 I- F. . 3...... . . F4 . . . . ...._iti...._...i.---.........r.r-e._.-rrr. .. 1.....- .- ....... 1:..- . 3:. .. t........ . “he--.” ...--.-......-........._ .. -....:.....-.,..-¢..L.-.- .5... ._._-:_........i..w .- __ ._ . i filigl .11.- el.........r T . . . __ajm. .._...... ..?... . .inx...»1rt.b _. . .1..._....t...__._ I L. . PT. i. . {it .rli4+lfltfi _.J. . ..q. .._ jM-._... _. ... .1 TW._*mmnim.m.m_fl_._i_.m _”.wm...mw.:.w..flmw. .HW e _. . .W.I.. ._ ..L. .g- ._. . ._.4 4-4 ...- + “Tl”... _.fi_._.i~»..:n_:_.;..: t.__ __ ..._7 .1 _ __...“.......m_ .mm_.. i..” _ ._ . _ . n. _ _ ~ _ _.~__ .____.__A.__ _ ...\V ___”...__.__m ._ . _ ._....; . .. .3 wet, I" r HH+ .ulihlvi lllr.l-i_. . .. ..r.-/_ .f . H._... .5... _..:......._::.._ efi.n_.w___nfw*m__._ _; _._“m.q..___.“.fl._:_... 80 mmmomoHZ Nooo.o ”mm mm zH Amm FREQUENCY IN HERTZ g the Illustration of proposed method for determinin cy range of a hearing aid. Figure 4. frequen 20 select or recommend the most appropriate hearing aid for individuals who can benefit from wearable amplification. However, considerable differences of opinion exist as to the reliability and validity of current clinical methods and materials used in the hearing aid evaluation procedure. Most procedures that have been designed for hearing aid evaluation have sought a so called "objective" technique in an effort to determine if one particular instrument is better suited for a particular user. At the present time it appears that there are tow general viewpoints among audiologists concerning how the hearing aid evaluation should be handled (Berger and Millin, 1971; Ross, 1972). One view is that the audiologist should be responsible for selecting the particular instrument. This procedure frequently calls for the trial of several aids whose reported characteristics appear to be suitable for the individual's hearing impairment. The goal is to select an instrument that will provide sufficient benefit for the hearing aid user. During this clinical selection procedure various comparisons among instruments are made which include measurement of a speech reception threshold, speech discrimination both in quiet and in noise, threshold of discomfort, and even subjective evaluation of quality by the hearing impaired individual. The recommendation to the hearing aid dealer from this type of procedure may simply note the ear to be fitted, and manufacturer and model of hearing aid selected from the trial aids. The recommendation, 21 however, may also be more specific and include various internal adjustments for a particular hearing aid. A second viewpoint is that the hearing aid selection should be left to the hearing aid dealer. Several reasons have been given for this procedure: 1. the cost in man hours of professional time; 2. the administrative difficulties in main— taining an inventory of the hearing aids; 3. the opinion that present clinical materials do not distinguish between aids; 4. the cost to the dealer or manufacturer in assigning aids to clinics through— out the country; and 5. the belief that dealers are qualified to perform the service (1967, Conference on Hearing Aid Evaluation Procedures, p. 16). Advocates of this viewpoint feel that if an individual's hearing is examined, and if the proper recommendations are made regarding the acoustic characteristics of the instrument, then any hearing aid meeting the patient's amplification needs will be satis- factory. It is also believed that with this method more time may be spent in counseling and providing rehabilitative services. However, the main assumption is that "identical characteristics” to the audiologist's specification will be met by the hearing aid dealer. Perhaps the major point of controversy over the hearing aid evaluation procedure has been, "Can differences between hearing aids be demonstrated?” 22 Shore et.al. (1960) investigated the reliability of three measures obtained in speech audiometry that are commonly emphasized in the hearing aid evaluation. The three measures used were gain or residual hearing level for speech, speech discrimination in quiet and speech discrimination in noise. Fifteen clinical patients with mild to moderate impairments in three diagnostic groups, conductive, sensorineural and mixed, served as subjects. Four body-type hearing aids were used in the study with each aid being evaluated under a "good" and "bad” tone control setting. Tests of hearing aid performance with all hearing aids and tone control settings were repeated on four different days. The authors concluded that the reliability of the three measures obtained were not good enough to warrant the large investments of clinical time in obtaining them. The above cited study served as the impetus for those who argue that the hearing aid dealer is best equipped for selection of hearing aids. Jerger, Speaks and Malmnuist (1966) and Jerger and Thelin (1969) reported that differences between hearing aids can be demonstrated. However, they also pointed-out the ineffectiveness of contemporary clinical hearing aid procedures for showing differences between aids. The opinion of these researchers appears to be that monosyllabic word lists as they are commonly used in hearing aid evaluations are incapable of demonstrating differences between hearing aids. These authors suggested that a more meaningful and discriminating test is necessary, 23 such as a test which more closely approximates connected speech pre- sented with a competing message. With the above research in mind and the foregoing discussion concerning the HAIC Method for expressing hearing aid characteristics, the question might be asked: "If the hearing aid characteristics used in earlier experiments had been obtained under a different manner and if identical acoustic listening conditions were held constant for all instruments, would the same conclusions have been reached?" A hearing aid evaluation procedure emphasizing a new method for describing the acoustical characteristics of hearing aids is proposed. This procedure incorporates taped speech materials recorded through different hearing aids. The recordings are made after the hearing aid has been carefully adjusted for its "ideal" or linear gain curve. It is felt that with this new procedure initial hearing aid evaluations could be made quickly and accurately. This method would allow for: l. Standardized acoustical conditions. 2. Elimination of acoustic feed-back, due to ill-fitting stock earmolds. 3. Assurance that the hearing aid is delivering the proper gain. 4. Hearing aid evaluations to be made in a shorter time span. However, before this new procedure could be employed in a clinical setting this investigator sought to evaluate five stimuli conditions recorded through several hearing aids. This was done in order to find the most demanding and discriminatory listening condition 24 for showing differences between hearing aids with normal hearing subjects. For this reason a second investigation was conducted. Specific Questions Asked In summary two studies were performed. In the first the HAIC Method for determining and reporting hearing aid characteristics was compared with a new method designed by this author. The specific questions asked were: 1. Do both methods yield comparable gain values? 2. Do both methods yield comparable MPO values? 3. Do both methods yield comparable freruency response curves? 4. Do both methods yield comparable freeuency ranges? The second investigation sought to answer the following questions relative to the performance of normal hearing.subjects under the following listening conditions: 1. Can differences be shown between hearing aids when monosyllabic words (N.U. Auditory Test No. 6) are recorded through them under a variety of listening conditions, and played to normal hearing subjects at the same sensation level? 2. Which listening condition(s) is best for showing differences between hearing aids using normal listeners? 25 Can differences be shown between hearing aids with different frequency response characteristics when mono- syllabic words (N.U. Auditory Test No. 6) are recorded through them under a variety of listening con- ditions and played to normal hearing subjects at the same sensation level? Which listening condition(s) is best for normal listeners in showing differ- ences between hearing aids with different frequency response characteristics? CHAPTER III EXPERIMENTAL DESIGN This chapter has been divided into three sections. The first section describes the new procedure used for specifying and reporting hearing aid characteristics. The second section explains the speech stimuli and the procedure used for recording these materials through each instrument. The final section describes the testing design for evaluating the speech materials recorded through each hearing aid. Procedure for Describing Hearing Aid Characteristics Selection of Hearing_Aids The hearing aids used for this study were selected from the stock of new hearing aids in the Hearing Clinic at Michigan State University. Since a large selection of body style hearing aids were not available, only ear level instruments were chosen for inclusion in this research. Three categories of gain as determined by the HAIC Method. were selected as being representative of mild, moderate and high gain ear-level hearing aids. The numerical gain values were: 0-40 dB, 41-49 dB and 50-60 dB respectively. In order to have hearing aids which were eeually representative of this range, an eoual number of instruments were selected for each gain category. 26 27 To insure that the hearing aids selected were in good working condition and meeting the manufacturer's specifications, all hearing aids underwent spectral analysis according to the HAIC Standard Method of Expressing Hearing Aid Performance. Only those hearing aids that were within.i 6 dB of the manufacturer's specified HAIC average gain, within.:’70 Hz and i 300 Hz of the manufacturer's low and high frequency limits respectively, and whose maximum power outputs were within i_10 dB were included. Table 1 shows the manufacturer's HAIC specifications and the HAIC characteristics obtained at Michigan State University for the hearing aids used in this research. The above stated criteria for inclusion of hearing aids was made arbitrarily since specific information is not provided in the HAIC Standard as to allowable variations from the manufacturer's reported specifications. In fact, the HAIC Standard Method simply says: Sampling procedures should be adequate to insure that the published performance data will be, to the best of the manufacturer's knowledge, representative of the average pro- duct being offered for sale (Lybarger, 1961, p. 33). Using these criteria six hearing aids were selected for each gain category mentioned above. A total of eighteen hearing aids were chosen representing eight manufacturers. All of these aids were felt to be within the manufacturer's specifications according to HAIC. 28 oowdnonm ooamnood aNH NNH mm dm 0mm ummowvmm oomduOOd unusual: wNH 111 an om onuo ooaouoz oommuomd ooudnomd BNH NNH Hm md HH am 4 occuoHp5< oooduomm oomduomd mHH dNH om mm mwm mcoEmflm commuomm commuomm dNH dNH om wd mm mCOuocom oomduoOd oomdnomd HNH mNH om dm Nwm mawEmHm moouw dame fiwwm oedd1owd ooHd1omd mNH NNH ad dd uu0a3mz euaamn oomduowm oomduomd mHH mHH md om » mupuum>o mcouaom oomduowu commuomm HNH mNH ad ad omnd moouowps< oomduowm OOmduomm mHH oHH md wd mH1< odouOMpsd commuowd oommuowd mHH ONH dd ed M wuwuuo>o mcouamm oouduowm oomduowm dHH mHH Nd Nd w» oupunm>o occufiwm maouo dado oumumwoz oomdaomd oomduond dHH NHH 0d mm mm mcouocom oowdnoma oedduonm wHH HNH 0d Nd oooH “moowpmm oomdnomd oomdnoOd moH 00H mm md d woumumpoz cufiaow ooomnoom oomdnomd dHH oHH mm mm owm mcmEon oomdaoom ooHduomm mOH 00H mm on m voozummz zuwamm oowmnowa coeduoom NHH wHH 0d 0d xb coowuo macho ammo wHHE Aumv Aumv Amev Amwv Amev Amev and .s.m.z mmcmm .:.m.z omz .s.m.z ammo wannmmm mwcmm hocosvoum mucosvmum om: ammo .huoumuonmq nuummmom kmoHoHvs< hummuo>wab mumum unwanoflz um wmaflmuno mcowuwoqmaooam 0Hmq oatmmuw ”ANHHN mth .uommm paw Hopumv umumeouuummm xucmsvoam owns< ”ANOHN mmaH .ummnx paw Monumv umnhfimcd hoomsvmum “AmHNd mama .pmmnm paw Hosumv wonamso aflozooa< ”AdNHH make .uommm one Hmsumv "Eoumhw unmamuammoe cam wcwuwofi mo Emuwmww xooHn powmemem .m muswam noxmomm . nul 8:: IJ nonsmso oworomc< d a. wmume Hmvuouwm nouuoomm Hm>mA %uawsvoum umnhamad hoaosvoum noumumCmu Eopcmm smawm 31 Type 1124) and centered at 2000 Hz with a bandwidth of 100 cycles was intro- duced into the anechoic chamber at a level of 70 dB SPL. 3. A frequency response curve was then obtained at this volume control setting through the frequency range of 20-20,000 Hz. The input level of 70 dB SPL was held constant for all frequencies. 4. The sine-random generator was readjusted to produce the narrow band of noise as mentioned under step two. The volume control of the hearing aid was again rotated until a 10 dB increase in output, was obtained from the initial volume control setting. A new frequency response curve was obtained at this volume control adjustment. 5. Steps two through four were repeated until the volume control was rotated to the full- on or maximum-on position. With the procedure described above a family of gain versus frequency response curves were obtained with a constant input level 32 of 70 dB SPL for all frequencies. The frequency of 2000 Hz was arbitrarily selected for the adjustment input since many hearing aids have resonant peaks around this region, and also because this frequency is important for Speech intelligibility. A comparison between the narrow band of noise centered at 2000 Hz and a pure-tone of 2000 Hz was made for each instrument to determine if the gain values were the same. Identical gain values were found for both inputs for all hearing aids. New Frequency Response Curve and Gain Values Once the family of gain versus frequency response curves were obtained, the "ideal" gain versus frequency response curve, or volume control setting, was selected using the following criteria: a frequency response curve which allows for a volume control adjust- ment (gain setting) of an additional 10 dB and, still remains linear with respect to the lower curve within the speech frequency range of 500 to 2000 Hz. From the "ideal curve" the discrete gain values for 500, 1000 and 2000 Hz were obtained by subtracting the 70 dB input from the output. The average gain was derived by averaging the gain for the above three frequencies. New Frequency Range The frequency range of each instrument was also determined by using the "ideal” gain or frequency response curve. The frequency 33 range was obtained by plotting the numerical value of the three frequency average (500, 1000 and 2000 Hz) gain on the 1000 Hz ordinate; a line was then drawn parallel to the abscissa at a level 10 dB lower than the average gain. The points where the line crossed the lower and higher "ideal" frequency reaponse curve skirts defined the frequency range for the instrument. Figure 4 shows an example of this procedure. New Maximum Power Output The maximum power output for each instrument was also deter- mined by adjusting the hearing aid for it's "ideal" gain versus frequency response curve and then obtaining separate frequency response curves for each 10 dB increase in the input signal (i.e. 70, 80 and 90 dB SPL). With this procedure a new family of curves was obtained. Saturation output was defined as a frequency response curve where a further increase in input did not produce a further linear increase in output for the frequency range of 500-2000 Hz. From this curve the MPO for the discrete frequencies 500, 1000 and 2000 Hz was obtained in dB SPL. The three frequency average MPO for the above frequencies was also obtained. In addition the peak MPO for each aid was also noted in dB SPL from this curve. This new procedure for describing the performance character- istics of hearing aids was used on all eighteen hearing aids under investigation. 34 Speech Stimuli and Recording Procedure In order to evaluate different Speech stimuli conditions for future use in hearing aid evaluations, five Speech stimuli con- ditions were recorded through nine of the original eighteen hearing aids. The Speech conditions, recording procedures and hearing aids used are described within this section. Stimulus Materials Recorded Speech discrimination material under various Signal-to- noise and Signal-to-competing message ratios was recorded through each of the nine hearing aids. The speech material selected for use was the Northwestern University Auditory Test No. 6, Form B (Tillman and Carhart, 1966), which was locally recorded by a male talker (William F. Rintelmann, f0 95 Hz) with a predominately General American dialect. This test (Appendix B) has four lists of 50 monosyllabic consonant-nucleus- consonant (CNC)1 words derived from lists originally developed by Lehiste and Peterson (1959) and Peterson and Lehiste (1962). Each word was preceded by the carrier phrase, ”You will say _____," In a study at Michigan State University Rintelmann and Schumaier (1972) found the four lists of the locally recorded test to be highly equivalent, with the variability between lists no greater than the variability found within lists. 1CNC refers to monosyllabic words comprised of an initial and final consonant and a vowel nucleus. 35 Five speech stimuli conditions were used, recording one list of N.U. Auditory Test No. 6, for each condition. It was necessary to repeat one list for each hearing aid; however, care was taken in recording to assure three lists always intervened before any list was repeated. Primary and Secondary Siggals The Northwestern University Auditory Test No. 6 materials was introduced in the sound-field at a level of 70 dB SPL for all recording conditions. For the signal-to-noise conditions, continuous broad band white noise, generated by a Maico 24 clinical audiometer was used. For the signal-to-competing message conditions a disk recording of Fulton Lewis Jr. (fo 105 Hz) describing a housing development in Michigan was employed. This recording was selected as the competing message due to it's fairly constant intensity level.1 Both of the secondary signals were adjusted to give the appropriate Signal-to-noise or signal-to-competing message ratios. The clinical audiometer was used to mix and present the stimuli at the appropriate level through it's associated single Speaker in the sound-field. Listening7Conditions Following is a list of the stimulus conditions which were tape recorded through each hearing aid: lThis recording is commercially available from Technisonic Studios, St. Louis, Missouri. In. wit C0! .2 36 1. N.U. Auditory Test No. 6 in quiet 2. N.U. Auditory Test No. 6 signal-to-noise + 10 dB 3. N.U. Auditory Test No. 6 signal-to-noise 0 dB 4. N.U. Auditory Test No. 6 signal-to-competing message +'10 dB 5. N.U. Auditory Test No. 6 signal-to-competing message 0 dB In addition a calibration signal consisting of a narrow band of noise with a center frequency of 2000 Hz and a 100 Hz bandwidth was re- corded at a level of 70 dB SPL. The intention in selecting these primary-to-secondary ratios was to have listening conditions which were relatively easy (+ 10) and listening conditions which were relatively hard (0). Previous studies concerned with speech discrimination in noise or in a competing message context have used a variety of primary-to-secondary ratios, because the difficulty of the listening task is dependent upon a number of factors such as size of the response set, type of primary signal, type of subjects and method of presentation (Miller, Heise and Lichten, 1951). Selection of Hearing Aids Analysis of the "ideal" frequency response curves of the initial eighteen hearing aids was accomplished by referencing the intensities for the frequencies 500, 1000, 2000 and 4000 Hz to the intensity level of the center frequency of 2000 Hz. A visual in- spection of these curves revealed three categories of frequency 37 responses: (1) relatively flat response from 500-3000 Hz; (2) high frequency emphasis 500-3000 Hz; and (3) irregular frequency response 500-3000 Hz. Based on this classification system, and by visual inspection, three hearing aids with similar frequency responses were selected from each frequency response category. Figures 6, 7 and 8 show the frequency response curves for the three individual hearing aids in each frequency response category (flat, high frequency emphasis and irregular response). Figure 9 shows the mean frequency response for each of the three groupings. Only the five speech conditions as recorded through these nine hearing aids were evaluated in this phase of the research. Additionally, it should be pointed-out that gain was not a criterion for selecting the nine hearing aids employed in this phase of the study because all listening conditions were accomplished at a single sensation level. This will be explained further below. Recording Equipment Figure 10 shows a block diagram of the equipment used for recording. All recordings were made in a double walled pre-fabricated IAC sound-treated room (Series, 1200). The ambient noise in the room was less than 45 dB on the C scale of a sound level meter (Bruel and Kjaer, Type 2230). Before the recordings were made, the sound level meter was calibrated with a piston micrOphone (Bruel and Kjaer, Type 4220) ‘employing a 250 Hz tone at 124 dB SPL re 0.0002 microbar. A clinical audiometer (Maico, MA 24) with it's associated -12 .. DB RE: 2000 HZ ~161a -20 .- 0 I r 503' 1050 2000 3000 4000 ORadioear 1000 A Oticon UX UNorelco 6730 Figure 6. Individual frequency response curves of three hearing aids with relatively flat frequency response characteristics between 500 and 3000 Hz. 39 -16 .- DB RE: 2000 HZ '20 CI l I I I 500 1000 2000 3000 4000 ' FREQUENCY IN HERTZ D Siemens 382 0 Siemens 383 A Sonotone 37 Figure 7. Individual frequency response curves for three hearing aids with high frequency emphasis between 500 and 3000 Hz. 40 16 1 12 . 8- 3'... "o 40- ) ‘ EE 8 T 0 0d 1 N I 2‘3 -40- \'. pg I Q -g_ I :5 -12 u l I I I l + 500 1000 2000 3000 4000 FREQUENCY IN HERTZ O Radioear 990 El Beltone Overture R A Beltone Overture YY Figure 8. Individual frequency response curves for three hearing aids with irregular frequency reaponse between 500 and 3000 Hz. 41 12 - ‘. 8 - 4 a -4 _ ‘93 . ' c> 8 '3 ' I: o: I £5 m -12 - ea c: -16 - o -20 a l l l I I 500 1000 2000 3000 4000 FREQUENCY IN HERTZ 0 High D Flat A Irregular Figure 9. Mean frequency response curves from 500 to 4000 Hz for the three frequency response groups: high frequency emphasis, flat and irregular. 42 ”Aooo.o< .xomfioq venom nwsouzu mamwuouma noooam wcwvuooou pom pom: uaoEmHsuo mo adhwmww xoofin vofimwamawm woumEons¢ Haoaaaao .oHnmu auau was noxmomm .xoov mama woumHUOmmm m.u« aua3_A¢N oowmzv nouoaowpa< HmoHcHHo “Admoa make .uomnm was Hosumv wouwuocow eovamMuoaHm acumuoaou Eovamm:ocwm "mugs wafiumon .oH spawns Hovuooom ommH .4 nnumnnnunnumv, .Iiwr susEaN< oo 800m Hopscou wt LN HR‘ \\ V .HNHH Boom umoa aha nmumz noxmomm ‘ h. waaumom H964 venom 43 tape deck (Viking, Model 442) was used to present the stimulus material in the sound-field. Prior to recording, the output signal level for a narrow band of noise centered at 2000 Hz with a 100 Hz bandwidth was calibrated, for 70 dB SPL at the position of the hearing aid within the sound room. A sound level meter (Bruel and Kjaer, Type 2204) with it's associated octave band filter set (Bruel and Kjaer, Type 1613) employing a sound field condensor microphone (Bruel and Kjaer, Type 4145) was used for calibration. Procedure Used For Recording 3 artificial ear (Bruel Each hearing aid was coupled to the 2cm and Kjaer, Type DB 0138) using one inch of number 13 tubing. The hearing aid micrOphone was positioned for a zero degree azimuth in relation to the speaker. Using the sine-random generator (Bruel and Kjaer, Type 1024) a narrow band of noise 100 Hz wide, with a center frequency of 2000 Hz, was introduced in the sound-field at an intensity level of 70 dB SPL. The volume control on the hearing aid was then adjusted until it's "ideal” gain value for 2000 Hz was obtained on the sound level meter. In this manner one was assured the hearing aid was adjusted for the correct frequency response curve as ascertained earlier from the new calibration procedure. The sound level meter was then switched to the record mode and used as a micro- phone amplifier. With this instrumentation, the speech stimuli were introduced in the sound-field and recorded through each hearing aid. The Speech signal at the output of the hearing aid was recorded with an Ampex tape recorder (AG 600) using Scotch (201) magnetic tape. 44 Experimental Design This experiment was performed in order to evaluate the five speech conditions recorded through the nine hearing aids. An attempt was made to discover if certain recorded speech conditions using monosyllabic words were more discriminatory than others between hearing aids with different and similar frequency response character- istics. Since all recordings were made through each hearing aid at the same intensity level, this investigation was actually evaluating the frequency response of each aid and it's distortion against the five recorded speech conditions. Subjects The subjects selected were ninety normal hearing young adult untrained listeners. The subjects ranged in age from 18.6 to 30.0 years with a mean age of 23.7 years. All subjects had hearing threshold levels within 20 dB of audiometric zero re ANSI 1969 Standard for pure-tone audiometers. Only the better ear, as determined by the SRT and pure-tone thresholds, served as the test ear. Figure 11 shows the mean audiogram, speech reception threshold and pure-tone average for the ninety subjects. TestingiEquipment All testing was conducted in a two-room testing suite with the experimenter in the control room and the subject in a double- walled sound~treated test booth (IAC, Series 1200). The ambient noise level in the test room was 42 dB on the C scale of a sound level meter (Bruel and Kjaer, Type 2203). HEARING LEVEL IN DB RE: 1969 ANSI STANDARD 45 10 - 30 40 ‘ 50 J 60 - 80 ~ 90 q I I I 1 I l 250 500 1000 2000 4000 8000 FREQUENCY IN HERTZ Pure-tone Average 500-2000 Hz = 2.4 dB HL Speech Reception Threshold = 1.6 dB HL Figure 11. Mean audiogram, speech reception threshold, and pure-tone average for ninety normal listeners. 46 Pure-tone air-conduction thresholds were measured with a commercial audiometer (Beltone, Model 150) with TDH-39 earphones mounted in MX 41/AR cushions. For all speech testing, a commercially available speech audiometer (Grason-Stadler, Model 162) was used to amplify and attenuate the electrical output of the tape recorder (Ampex, Model 601) used to present the tape recorded material. The output from the speech audiometer drove a single TDH-39 earphone housed in a MX 4l/AR cushion. Calibration checks of the pure-tone and speech audiometric system were made prior to and after the completion of the experiment. Appendix C details the calibration procedures and findings for the speech audiometric system. Testing Procedures Normal hearing bilaterally was first ascertained for each subject prior to the experiment through pure-tone air-conduction threshold testing for the octave frequencies from 250-8000 Hz. All pure-tone thresholds were determined by the Hughson-Westlake technique as described by Carhart and Jerger (1959). Speech reception thresholds were then obtained bilaterally with tape-recorded spondee word lists, recorded by the same talker (William F. Rintelmann) who recorded the CNC monosyllables. The spondees were the same words used in CID Auditory Test Wel (Hirsh et. a1., 1952). Each subject was first familiarized with the spondee test vocabulary in a manner previously described by Tillman and Jerger (1959). 47 SRT's were then established in the following manner: The words were initially presented at a 20 dB HL by earphones. Two words were presented at this level and if both words were repeated correctly the level of the signal was attenuated 10 dB. This was continued until the subject missed both words. The level of the signal was then increased 10 dB and attenuated now in 2 dB steps. The criterion for starting was that five out of six words must be correctly repeated. If they were, the descent was continued. If not, the examiner increased the level by 10 dB and began again. The 2 dB descent, with presenting two words at each level, was continued until the subject missed five out of six words. The speech reception threshold was defined as the lowest level where the subject received both words correctly minus 1 dB for those words responded to correctly thereafter. Since the speech audiometric attenuator was calibrated in 2 dB steps, for all odd-integer spondee thresholds, the SRT was increased by 1 dB. Only the better ear, as determined by the SRT and the pure-tone threshold, served as the test ear. Each subject then listened to all five speech conditions as recorded through one of the nine hearing aids. The speech stimuli were presented at a 50 dB sensation level re the subject's SRT for the test ear. Since the group mean SRT was 1.6 dB HL, the average level for presenting the speech material was equal to a normal con- versational Speech level (71.6 dB SPL). Each subject responded to the speech material by writing his or her reSponses to each test word. 48 To avoid fatigue all subjects were given a five minute rest period between the second and third list of recorded material. All word responses were graded by one observer; thus, any bias in the acceptance of orthographic errors was systematic. Using the above procedure 90 normal hearing subjects par- ticipated in the experiment. Each subject was randomly assigned to one of the nine hearing aids; thus, 10 subjects responded to the material recorded through each hearing aid. The order of pre- sentation of the stimulus material (five listening conditions) was counterbalanced for each hearing aid thereby reducing the possibility of order effects. CHAPTER IV RESULTS AND DISCUSSION This chapter has been divided into three sections. The first section contains the data obtained from the first investiga- tion, and a discussion of its significance. This study compared the HAIC Standard Method for determining and reporting hearing aid char- acteristics versus a new method. The second section contains the data from the second investigation and a discussion of its signifi- cance. This experiment sought to evaluate five stimulus condi- tions, using CNC monosyllabic words, in an effort to find the best stimulus condition(s) for use in future hearing aid evaluations. The final section concerns the clinical implications of both investigations. Hearing Aid Response Characteristics The following four specific questions were asked regarding a comparison between the HAIC Method and the new method for deter- mining and expressing hearing aid performance characteristics: 1. Do both methods yield comparable gain values? 2. Do both methods yield comparable MPO values? 3. Do both methods yield comparable frequency ranges? 49 50 4. Do both methods yield comparable response curves? In this section each of the above questions is answered and discussed. 9313 Recall that the primary difference between the HAIC Method and the new method for determining gain dealt with input intensity level and volume control adjustment. The HAIC procedure required that each instrument be adjusted for full-on volume control and use a 50 dB SPL input, whereas the new method used a 70 dB SPL input and varied the volume control from just-on to full-on in 10 dB steps. Further, the new method selected a gain versus frequency response curve which allowed for a reserve linear gain of 10 dB. Table 2 shows, for all eighteen hearing aids, a comparison of the three frequency (500, 1000 and 2000 Hz) average gain obtained with both methods. In addition, the difference in gain between the two methods is shown for each hearing aid. The hearing aids have also been divided into the three original gain categories (mild, moderate and high) as determined from the HAIC characteristics. Inspection of this table reveals that for each hearing aid the average gain values obtained with the HAIC procedure were sub- stantially greater than the average gain values obtained with the new method. It can also be seen that the rank ordering of hearing aids, by gain categories, is not maintained with the new method. Table 3 shows for each gain category and the total sample, the mean differences, standard deviations (SDs) and the range of 51 Table 2. Comparison of gain values obtained with HAIC Method for determining gain and the new method for determining gain. The difference in gain between the two methods for each hearing aid is also shown. Hearing HAIC Gain New Method Difference in dB: Aid in dB* Gain in dB HAIC - New Method Mild Gain Group Oticon UX 40 18 22 Zenith Westwood B 35 11 24 Siemens 380 37 18 19 Zenith Moderator A 37 25 12 Radioear 1000 40 24 16 Sonotone 37 40 18 22 Moderate Gain Group Beltone Overture YY 42 19 23 Beltone Overture R 44 25 19 Audiotone A-19 45 22 23 Audiotone A-20 46 18 28 Beltone Overture Y 49 22 27 Zenith Newport 49 21 28 High Gain Group Siemens 382 50 23 27 Sonotone 77 50 36 14 Siemens 383 50 23 27 Audiotone A 21 II 51 38 13 Norelco 6730 51 36 15 53 36 17 Radioear 990 *The HAIC values were obtained on each hearing aid at Michigan State University Audiology Research Laboratory and do not represent manufacturers' published HAIC specifications. 52 Table 3. Mean differences, standards deviations and range of differences in dB between the HAIC Method and the new method for determining gain. Values are given by the three original gain categories and for the total sample of eighteen hearing aids. Gain Mean Standard Range of Gain Grouping Difference Deviation Differences Mild 19.16 4.48 12-14 Moderate 24.66 3.61 19-28 High 18.83 6.46 13-27 Total Sample 20.88 5.44 13-28 53 differences between the HAIC Method and the new method for deter- mining average gain. This table indicates that the mean differences, SDs and the range of gain differences are similar for all three gain groups. Also, the mean difference between both methods varies from 19 to 25 dB. Differences between the two methods for the total sample were found to be statistically significant (t= 16.31; df= 17; p 0.01 = 2.567). In comparing the results from Tables 2 and 3, it is obvious that a constant correction factor may not be applied to the HAIC gain values in order to derive gain characteristics according to the new method. Therefore, it appears that the two measurement pro- cedures for gain do not yield directly comparable results. Thus, the average gain obtained by the HAIC Method cannot be corrected to accurately predict the average gain of the new method. The differences in average gain found between the two methods can perhaps account, at least partially, for the inability to achieve expected gain often encountered in routine hearing aid evaluations. To explain, a 50 dB HAIC "average" gain hearing aid will not improve a 50 dB HL SRT to 0 dB HL. Thus, it is thought that the new method for measuring and reporting hearing aid gain characteristics gives the audiologist a Inore accurate picture of how a particular hearing aid will operate on the hearing impaired individual with normal input intensities for speech. The new method also reports the gain for 500, 1000 and 2000 Hz independently (Appendix D). This information would be 54 helpful to the audiologist, especially for hearing aid evaluations on hypoacusic subjects with sloping audiograms. Maximum Power Output Several differences exist between the HAIC Standard Method and the new method for determining and reporting maximum power output. The HAIC procedure requires the hearing aid to be adjusted for full-on volume control and the input intensities are increased in 10 dB steps from the basic input of 50 dB SPL. At each input level a frequency response curve is obtained through a wide frequency range. When the output does not remain linear with respect to the input, the discrete frequencies 500, 1000 and 2000 Hz are averaged and reported as the saturation output level in dB SPL. In contrast, by the new method, each hearing aid's volume control is first carefully adjusted for it's "ideal" gain versus frequency response curve. Input intensities are then increased in 10 dB increments from the basic input level of 70 dB SPL. At each input level a frequency response curve is obtained through a wide frequency range. When the output does not remain linear with respect to the input, the hearing aid is considered to have reached it's saturation output level. The discrete speech frequencies (500, 1000 and 2000 Hz) are then averaged and reported as the average maximum power output. In addition, the new method also reports the MPO for 500, 1000 and 2000 Hz independently, and the frequency and intensity of the peak maximum power output in dB SPL. Table 4 shows a comparison between the average MPO values 55 mmH oom N NNH mNH com ammoHemm HmH oom m mNH wNH omso oonaoz mNH OOOH m «NH sNH HH Hm < mcouoHesa mNH comm o mHH mHH mmm mamEmHm «NH OOOH N NNH «NH as maouoaom oHH comm 0H HHH HNH Nwm mauson msouu cho :me HNH OOOH oH mHH mNH uuoezmz :uHawN mHH com a HHH mHH w magnum>o maOUme HNH oom H- «NH HNH om-< occuoHesa ONH com m on mHH mHua mucuoHesa mNH com o aHH aHH m masuumso meouHmm mNH oomN oH aoH qHH w» musuum>o acouHmm mdouw chu oumnmpoz NNH QONH H MHH eHH maouoaom HNH com N oHH wHH oooH ammoHemm mHH OOOH m- oHH HoH a noumnmeoz HUchN HNH ooom a OHH qHH owm mamsmHm moH comm a so moH sooaummz auHcmN NHH um com a wOH NHH x: :OUHuo asouo cho eHHz Hem me scamswoum eosumz 3mz Ham me cH Ham me eHa cH omz swam - oHam "me om: mmmuw>< cH wcHummm scenes 302 cH museumHHHa scenes 302 om: oHam xmmm man .OmH¢ .Nm ooom was oooH .oom mo owmnm>m hosesvmum mouse mnu ucmmouaou mmus> .c30fim ma woaumEVSoc ecu Sufi? vecHMuno omz %ocmswoum .pHm wcfiumos some now nw>Hw mH mvosume 03u mfiu ammSuon oodoummmwv are .vonume 3mm ozu was wosuoz 0Hflab oumum cmwwsowz um cww some now cocwwuno ouo3 mosHm> UHmms o cowq cowq cHHu owd cum com umoowcmm o oomd coma cm: cmq ccq omno ooHoHoz con ocmq comm coat omm omq HH am < odouoHcdd o coco coco OH own com mwm mcmEon o comm comm own coo cNm um occuocom o coma coma ocHu com oc¢ Nwm mcoEon macho aHmo smHm o oosq oosq caH- omo owe unoasmz auHamN 0 come cows 0 com owm % opauuo>o oaouHom com coco coma ONHu omo com oNu< ocouoflcom c coom ocom oNHu com cod oHu< ocouoHca< o comq ccmd coHu omq omm m oumuuo>o odouHom o coma coma omHn cmm com ww eunuuo>o ocouHom odouc Cwmo ouwwmcoz o oomq oomd coal cmm cmd mm occuocom com coco oowq cmH: com coH cooH HmoOHcom c comq coma cm: can one ¢ Houmpmcoz zuHcoN c ocom ooom oNH: coo cod cwm mcoEon o coma coma on: own com m cooBumoB :uHaoN o comm comm om oma omH xb Coowuo agape aHmo eHHz Mm CH modmsooum :sz mucosvoum mm CH mucosvoum 3oq shocosvowh cH< wocmuoHHHn vogue: zoz :me oHam summaaHHHm eozuuz amz 30H oHam wcHnao: .CBogm ome mw mcozuoe suoa dooaumn Nuke: CH muconMMHc mzH .owcmH mocmsvoum wchHEHouoc wow cornea 30: can can poxuoz cHnao uncommon hoaosvoum cognac osw .ooaouommwc umomumH onu £uw3.cflm weakens ofiu cam mucouommwv umoHHmEm osu wsfiuwnwsxo cam wow unmet can you monocooouo nuop nuHB coaflwuno mo>uso uncommon hocosvoum osu mo sowwumaaoo .MH owswfim Emma 5 55:3: Emma zH wozmsommm coco.“ onoN ”3.6.. oamN ... coon our: 8...... non :_ III!» N.U..“ : - «I ..., i. new? -.-- 1 : EH... _ a- _. I ....I A, . Di; H M W a or i -. I -.H.-_ T+!H N . ;.Li a I I i mHHmmm " m “A _ M m - 7“. ._ .- a kiwi; G -. - .. -l‘i. .- T m a .HH - - I. H, m. I , _ .. H “ ... ”Hzi 80 N1 NIVS 63 3. Can differences be shown between hearing aids with different frequency response characteristics when mono- syllabic words (N.U. Auditory Test No. 6) are recorded through them under a variety of listening con- ditions and played to normal hearing . subjects at the same sensation level? 4. Which listening condition(s) is best for normal listeners in showing differ- ences between hearing aids with different frequency response characteristics? Results of the analysis of variance for the nine hearing aids and five listening conditions are summarized in Table 6. A significant main effect at greater than the 0.0005 level was found for listening conditions (F of 1324.2) and hearing aids (F of 4.9). Also, a significant interaction was found between hearing aids and listening conditions at greater than the 0.0005 level (F of 10.5). Table 7 shows the mean discrimination scores (in percent correct) for each of the nine hearing aids for all five listening conditions and the mean for each listening condition across all nine hearing aids is also shown. In addition, the standard deviation, and range of scores obtained for each listening condition for all hearing aids is presented. Figure 14 presents a graphic display of the mean discrimination scores for all nine hearing aids under each of the five listening conditions. Listening Conditions Inspection of Table 7 and Figure 14 reveals that for all nine hearing aids the in quiet listening condition always gave the highest mean discrimination score. Also, this condition gave the smallest 64 Table 6. Summary of analysis of variance for hearing aids and listening conditions. Source of Probability Variance dF MS F'ratio of statistic W IT H IN Listening Conditions(A) 218871. 4 54717.9 1324.2 0.0005 A x Ear(B) 342. 4 85.5 2.1 0.085 A x Hearing Aid (C) 13823. 32 432 0 10.5 0.0005 A x B x C 1560. 32 48.8 1.2 0.238 Within Error 11900. 288 41.3 B ETW EEN A 94. 1 94.3 0.9 0.338 C 3993. 8 499.1 4.9. 0.0005 B x C 876. 8 109.6 1.1 0.385 Between Error 7288. 72 101.2 65 o.w~ e.sH o.a~ m.NH e.m mmuoom Ho «mama ~.oH c.¢ H.o N.d c.H om ¢.nm o.ow m.mm m.mn ¢.nm mcwm ocHz m0 use: ¢.om c.¢c o.ow o.mm w.mm o.no mm oGOuosom .o c.¢o o.om ¢.¢m ¢.om o.on w.om Nwm mcoEon .w H.Nn o.Nc ¢.mw «.mm o.mw N.wm mmm wcoEon .m uncommom hoaomwoum swflm c.Hn c.om m.mw N.om c.wn c.mm ww ownuuo>o odouHom .o m.mm ¢.¢o o.ow c.mq n.0m o.wo m ousuuo>o ocouHom .m n.¢o o.qd m.om w.HN c.mw o.wm com unoOHcmm .d owcomwom mucmswoum HustouHH m.cm m.mo w.ow o.oN o.¢w N.mm ommo OUHoHoz .m N.mo w.~o o.Hm 0.0m w.Hm N.mm noccoouo x: cooHuo .N m.mn c.Ho w.mn «.md N.wn N.mm oooH HmoOHcmm .H mmcoemom hucosvoum umHm 983380 m H0 new; 0 203 San? o 25 BE? 003a CH meowuwccoo mewamuqu cH< wcHumom .coucomoue one mcoHuwccoo wawcouwHH How Amomv mGOHumH>oc cumpcmum .COHuHccm cH .csosm cmHm mH mCHm wcfiumoa HHm mmouow coHuHccoo wchoumHH some now wcmmE onu can mcoHuHcaoo mchoumHH HHm mmouom UHm wawumoz 30mm How mama: .mcoHuHccoo wewaoumHH m>Hm you wows mcHumoa maH: mo Sumo How Auoouuoo unmoumm aHv mouoom cowumcHEHuoch cmoz .n maan 66 100 '1 NW 90 s 80 r 70 - Z Mn '5 é a e E3 50 ‘ I :14) :‘. . D H 40 '- E u h g: 30 ~ . I - E . A 20 .. 10 ' ‘ r 7 U I I I W U 1 2 3 4 5 6 7 ‘8 9 HEARING AIDS [P13 In quiet Cf<3 Signal-to-noise + 10 see? Signal-to-competing message + 10 Eli} Signal-to-noise 0 E}%3 Signal-to-competing message 0 Figure 14. Mean discrimination scores from each hearing aid for all five listening conditions. 67 standard deviation (1.0%), and range of scores (3.6%). In contrast, for all nine hearing aids, the less favorable signal-to-noise and signal-to-competing message listening conditions always gave lower discrimination scores when compared to the three more favorable listening conditions. These two listening conditions also have the largest standard deviations (9.1% and 10.2%) and range of scores (27.6% and 28.6%). When comparing the + 10 signal-to-competing message and + 10 signal-to-noise conditions across hearing aids, both conditions gave similar means (80.6% and 79.9%), standard deviations (4.0% and 4.2%) and range of scores (14.4% and 12.8%). Inspection of Table 7 and Figure 14 shows that for four hearing aids the signal-to-noise con- ditions gave the highest discrimination scores; with three aids the signal-to-competing message resulted in the best scores and for two aids the two conditions were equal in difficulty. Hence, for a sample of several hearing aids, both broad band white noise and the competing message appear to be essentially equal in difficulty at a + 10 signal-to-noise ratio. However, when comparing the 0 signal-to- competing message listening condition to the 0 signal-to-noise listening condition across hearing aids both conditions did not give similar means (57.4% and 33.8%). With eight hearing aids the signal-to-noise listening condition gave the lowest discrimination scores, and for the other (ninth) hearing aid both scores were equal. Thus, it appears that at less favorable signal—to-noise and signal-to-competing message listening situations the two secondary 68 signals have differential effects upon speech intelligibility, with the noise producing the poorest scores. One can also readily observe that as the listening conditions become more difficult, the variability in normal listener performance found between hearing aids is sub- stantially increased. Hearing_Aids When considering individual hearing aids across all five listening conditions Table 7 shows that hearing aids one and five achieved the highest and identical discrimination scores (72.9%), while hearing aids four and eight received the lowest scores (64.7% and 64.6% respectively). The five remaining hearing aids achieved very similar scores, within a small range of 2.9%. The range for the nine hearing aids across all conditions was also small being only 8.3%. It is also apparent from Table 7 that both hearing aids (one and five) producing the highest scores and both hearing aids (four and eight) giving the lowest scores across all conditions are not from the same frequency response groups. ListeningiConditions By Frequency Response Grouping Results of the analysis of variance between hearing aids grouped by frequency response and listening conditions are summarized in Table 8. This analysis shows a significant main effect beyond the 0.0005 level (F 723.5) for listening conditions. A significant inter- action between frequency response and listening conditions was also found (PA-0.01, F 2.5). However, a significant main effect was not 69 Table 8. Summary of analysis of variance for frequency response and listening conditions. Source of Probability Variance SS dF MS F ratio of Statistic WITHIN Listening Conditions(A) 218871.4 4 54717.9 723.5 0.0005 Frequency Response (B) x A 1516.1 8 189.5 2.5 0.012 Within Error 26318.1 348 75.6 BETWEEN B 262.6 2 ' 131.3 1.0 0.383 Between Error 11782.3 87 135.4 70 found for hearing aids grouped by frequency response. Table 9 presents the means for the three frequency response groupings (flat, irregular and high frequency) for all five listen- ing conditions. The means for all listening conditions across the three frequency response groups and the means for each frequency response group across listening conditions are also given. Figure 15 displays the mean discrimination scores achieved by each fre- quency reSponse group for all five listening conditions. Inspection of Table 9 and Figure 15 reveals that with one exception the three frequency response groups resulted in highly similar discrimination scores at each of the five listening con-~ ditions. However, for the 0 signal-to—competing message condition the flat frequency response group gave the highest mean discrimination soore by approximately 7%. With this one exception it appears that hearing aid frequency response per se did not affect the normal listeners performance under any of the listening tasks. Discussion In light of the above findings and in answer to the first experimental question, it appears that with CNC monosyllabic words differences can be shown between hearing aids with normal hearing listeners. However, in this study, the differences that were shown across all listening conditions for each hearing aid were rather small (range 8.3%). Therefore, since differences can be seen the important question appears to be: "What listening condition(s) shows the 71 H.mo d.¢m m.mn n.Hm H.Nw m.nm anm c.mo «.mm H.Nw m.qm «.mn w.om anawowuH m.on m.mo ¢.ow m.mm N.wm m.nm umHm 203380 3: 28m OH + 5% o Em oH + Em 03:0 3:80 mo cmmz CH omcoammm kocwsvoum .caosm OmHm we wsowuwccoo o>Hm HHm wmouom maaouw woman oflu mo some now save use .maoHuHc Ices wcwnwuwHH o>Hm ago you Ahocoavoum smHs can umHnwouuH .umHmV mmchaonm omsommou honoavonm mp Auoouuoo unmouom aHv mmuoom GOHumaHeHuomHv use: .m oHan 72 100 I b aw ‘5 90 . 80 - o——— M 2: £3 70 i 91 .¢ 2: H E3 60 ' ad :3 23 H Cl 50 ‘ E4 :z [fl ‘2 a, 40 - o. a D“ L E: 30 - 20 d 10 - l I I Flat Irregular High FREQUENCY RESPONSE é-J-A In quiet Cr*3 Signal-to-noise + 10 area Signal-to-competing message + 10 EI‘CI Signal—to-noise 0 5}»?1 Signal-to-competing message 0 Figure 15. Mean discrimination scores for the three frequency response groupings for all five listening conditions. 73 greatest difference(s) between hearing aids?" It seems reasonable to assume that the listening condition which exhibits the largest standard deviation in discrimination scores across hearing aids would be the most likely condition for showing differences between hearing aids. As was seen in Table 7 and Figure 14, the quiet listening condition gave almost identical Speech discrimination scores for all hearing aids. This was exhibited by the small standard deviation (1.0%) and the narrow range of scores (3.6%). It was also found that the less favorable, (0), signal-to-competing message and signal-to-noise conditions gave the largest standard deviations (10.2% and 9.1%) and range of Speech discrimination scores (28.6% and 23.9%).' Thus, it is felt that these listening conditions tax the listener and hearing aid the most and, therefore, are best for Showing differences between hearing aids. From Table 7 it was also observed that the more favorable (+ 10) signal-to-noise and signal-to-competing message ratios gave similar means across hearing aids (79.9% and 80.6% respectively). However, the same close agreement between means (38.8% and 57.4%) across hearing aids was not found for the less favorable (0) signal- to-noise and Signal-to-competing message conditions. A possible explanation for this discrepancy is that at the more favorable conditions (+ 10) neither of the competing signals are sufficiently interfering with the primary message so as to demonstrate which is a more effective competing Signal. However, under the less favorable listening situations, the competing signals are sufficiently high so 74 as to interfere substantially with the speech signal. The higher scores found for the Signal-to-competing message condition may be due to the small fluctuations in intensity or the short pauses in the secondary signal; whereas, the broad band white noise has a flat continuous spectrum. A finding Similar to that reported above was obtained by Carhart et a1. (1968) when comparing the masking effect of an unmodulated white noise with a white noise which was modulated four times per second to a "depth” of 10 dB with a 50% duty cycle. Spondee thresholds were improved by approximately 4 dB with the modulated masker. Related to their findings the authors stated: . . . it is reasonable to expect that the presence of modulation in a masker, whether it be artificially induced or be the normal modulation of connected speech, should furnish acoustic "windows," which the listener utilizes to advan- tage (Carhart, Tillman and Greetis, 1968, p. 695). Thus, in the present investigation the better speech discrimination scores obtained by the normal listener for the 0 signal-to-competing message listening condition are perhaps attributable to what Carhart et al. describe as the ”window effect." The important finding appears to be that all hearing aids are not equally affected by the same type of secondary signal (noise or competing message). If they were equally affected, all hearing aids would give similar depressions in speech discrimination scores, Since without a secondary signal (quiet condition) they appear very similar. 75 In view of these findings, hearing aid evaluations employing monosyllabic Speech discrimination material in "noise-free" environ- ments or under favorable listening conditions, may not accurately assess how the instrument will operate under the less favorable Signal-to-noise condition in which we live. These findings appear to be in direct conflict with the results of Shore et a1. (1960). The study found no significant differences attributable to hearing aids for speech discrimination in noise at a signal-to-noise ratio of zero dB. They also found larger differences between hearing aids for discrimination in quiet. However, in their study they used only four hearing aids, each with two tone control settings. In an appendix to the study the authors remarked that the possibility remained that since they only used four good hearing aids, the restricted sample eliminated any bad aids. The interaction found between hearing aids and listening conditions for the favorable (+ 10) signal-to—noise and Signal-to- competing message ratio's are not surprising since both listening conditions gave approximately the same results. Also, when comparing the less favorable (0) Signal-to-noise and signal-to-competing message conditions (Figure 14) an interaction was found for only one of the nine hearing aids. The data from this experiment also tends to indicate that when hearing aids are grouped by frequency response (flat, irregular and high frequency) and monosyllabic speech materials are presented under five different listening conditions, differences in speech 76 discrimination scores among normal listeners are typically not seen due to frequency response. The results also showed that each listening condition equally affected the three frequency response groups (See Figure 15). This would tend to suggest that differences seen between individual hearing aids are due to something other than frequency response characteristics. It is felt that at the present time these results should be interpreted with caution, however, since only the scores for three hearing aids were averaged for each frequency response group and there was some variability within groups under the least favorable (0) signal-to—noise and signal-to-competing message listening con- dition. Thus, perhaps a larger sample of hearing aids under each category are necessary before group differences between types of frequency response can be shown. The above findings await further verification. Other acoustical parameters of hearing aids such as harmonic distortion and rise and decay times should be investigated. In this experiment a retrospective comparison between the average harmonic distortion (500, 700 and 900 Hz) measured according to A.S.A. 83.3-1960 and the discrimination scores obtained under each listening con- dition and across all listening conditions was attempted. No trends were noted. In fact, under the input and gain conditions employed the average distortion for all nine hearing aids was small (2.9%) and within a range of 3.9%. 77 Clinical Implications The results from the first study have Shown that large differ- ences exist between the HAIC Method versus the new method for deter- mining gain. Both methods give similar results for average maximum power output; however, the new method also indicates a hearing aid's peak maximum power output intensity which may be considerably higher than the average. Similar frequency ranges are obtained with both methods, with the new method tending to increase (make higher) the low frequency cut-off by approximately 100 Hz. Both methods yield very similar frequency response curves. The rationale was developed earlier that the new method attempts to measure a hearing aid's acoustical parameter in a manner similar to how the hearing impaired patient wears the instru- ment. The important finding from this study appears to be the discrepancy found between gain with the two methods. With the traditional clinical hearing aid evaluation procedure several hearing aids are generally selected for trial whose char- acteristics are dependent upon the results from the hearing evaluation test battery. These hearing aids are individually tried by the patient. The volume control of each instrument is first adjusted to a level where the patient "feels" it is most comfortable. Generally stock ear molds are used with this procedure which often do not form tight seals of the external ear canal, causing feedback, which limits adjustment of the instrument for higher gain. Once the hearing aid is adjusted, speech discrimination material is introduced into the sound-field at a level of normal conversational speech (approximately 70 dB SPL). 78 With this procedure each trial hearing aid is evaluated. The instrmnent giving the best speech discrimination score is usually recommended for purchase. Critiques of this procedure (Shore, Bilger and Hirsh, 1960; Zerlin, 1962; and Jerger, Malmquist and Speaks, 1966) generally contend that it is not sufficiently reliable for Showing differences between hearing aids. This criticism is understandable since many variables are left uncon- trolled in the typical hearing aid evaluation. The second study demonstrated that by attempting to control all variables except frequency response and associated hearing aid distortion, differences could be observed between instruments. This experiment also showed that monosyllabic Speech materials pre- sented in quiet were not as effective in demonstrating differences between hearing aids. However, the same material in a difficult signal-to-noise or signal-to-competing message context showed wide differences between the same group of aids. In view of the results from both studies it appears that accurate and reliable clinical hearing aid evaluations may be performed with the use of tape recorded materials. This would be accomplished by first ascertaining the acoustical characteristics of hearing aids according to the new procedure advocated in the present investigation. Speech discrimination material in an un- favorable signal-to-noise or signal-to-competing message context would then be recorded through each hearing aid while adjusted for it's "ideal” frequency reSponse curve as described in chapter three. 79 These recordings would be classified according to the frequency response and gain characteristics of the instruments. Thus, one tape could contain high frequency emphasis, mild gain instruments etc. After the initial hearing evaluation is completed, the audiologist would select a tape containing discrimination material recorded through instruments with the generally desired acoustical characteristics. The speech material recorded through each hearing aid would be played to the individual at a level equal to the amount of gain the instrument would deliver. Thus, for a hearing aid with 30 dB of gain the discrimination material would be pre- sented at 80 dB HL. With this procedure each subject could respond to a number of hearing aids, and the hearing aid giving the highest discrimination score would normally be recommended for trial or purchase. Since this procedure presents all materials through earphones the problem of feedback would be eliminated. Gain would also be held constant. Further, individual hearing aid adjustments would not be necessary thus saving a considerable amount of time, thereby allowing for a larger number of hearing aids to be evaluated. If a procedure similar to this would be adopted in a large number of clinics, it is conceivable that hearing aid manufacturers could furnish clinics with individual recordings of their instruments rather than several actual hearing aids. This has obvious advantages over a clinic "library" of hearing aids for both the manufacturer and the audiologist. CHAPTER V SUMMARY, CONCLUSIONS AND RECOMMENDATIONS Summary Two procedures for hearing aid evaluations are currently being employed in the majority of hearing clinics. A traditional procedure seeks to evaluate individual hearing aids with the hearing impaired subject. Measures such as aided speech reception thresholds and aided speech discrimination scores are obtained in an effort to recommend a specific instrument for purchase or trial. The other procedure is founded on the basis that reliable differences cannot be shown between hearing aids with the usual measures in speech audiometry. Using this procedure, evaluations of specific hearing aids are completely eliminated and no particular hearing aid is recommended. Instead, the hearing impaired patient is given recommendations as to the type of hearing aid which should be purchased. With both of these procedures the audiologist must rely on the manufacturer's HAIC specifications for accurately representing the acoustical characteristics of the different hearing aids. 80 81 This thesis consisted of two investigations. The first sought to evaluate the HAIC procedure for measuring and reporting the acoustical characteristics of hearing aids, as opposed to a new procedure developed by this writer. The argument was made that the HAIC procedure does not accurately represent the performance of a hearing aid under normal listening conditions. The new method for evaluating and reporting hearing aid characteristics was designed to overcome the basic criticism of the HAIC procedure. The results of both methods were compared on the following specific acoustical parameters: (1) gain, (2) maximum power output, (3) frequency range, and (4) frequency response curve. The second investigation was performed in order to determine if differences could be observed between hearing aids, using CNC monosyllabic speech discrimination materials. Normal hearing subjects responded to these speech materials after they were recorded under five different listening conditions through nine different instruments. The listening conditions employed were: (1) CNCS in quiet, (2) + 10 Signal-to-noise ratio, (3) + 10 Signal-to-competing message ratio, (4) 0 signal-to-noise ratio, and (5) 0 signal-to- competing message ratio. The hearing aids employed were selected from three types of frequency response groups: flat, irregular and high frequency emphasis. All subjects listened to the recorded materials at the same sensation level. It was reasoned that if differences were seen between hearing aids, these differences could be attributed to the hearing aid's frequency response and associated acoustical distortion. 82 Conclusions Several conclusions appear warranted from the first in- vestigation: 1. The HAIC procedure for measuring and des- cribing the acoustical parameters of hear- ing aids does not give an accurate descrip— tion of how these instruments function under normal listening conditions. When comparing the HAIC Method and the new method for determining gain, the HAIC Method tends to over-estimate actual usable gain by approximately 21 dB; however, because considerable variability exists between in- struments, a constant correction factor can- not be subtracted from the HAIC gain in order to predict the average gain which would be derived by the new method. Both methods tend to give approximately the same average (500, 1000 and 2000 Hz) max- imum power output; however, the peak MPO can be considerably higher than the average. The frequency range found with both methods are similar; however, the new method tends to raise the low frequency cut-off by 83 approximately 100 Hz, thereby reducing slightly the reported frequency range. Both methods give identical frequency res- ponse curves. The following conclusions appear warranted from the second experiment employing normal listeners: 1. Differences between hearing aids cannot be adequately demonstrated when using CNC mono- syllabic speech discrimination material in a quiet listening condition. When noise or a competing message was simul- taneously presented with the primary speech signal at a + 10 signal to noise (competing message) ratio, speech discrimination scores of normal listeners were essentially equally depressed with both secondary signals. However, this condition did not demonstrate differences between hearing aids. In order to show differences between hearing aids, the discrimination material must be in a difficult listening situation. The 0 signal-to-noise and 0 signal-to-competing message listening conditions showed the 84 largest differences between hearing aids. 4. In general, when the hearing aids were grouped by frequency response, (flat, irregular and high frequency emphasis) differences were not seen between groupings for any of the five listening conditions. Recommendations In the present investigation differences observed between hearing aids were obtained with normal hearing listeners. It is clinically important to determine whether these or other differences can be observed with hypoacusic subjects. The second eXperiment also revealed that the undistorted monosyllabic words when presented in quiet were incapable of Showing differences between hearing aids. It would be of interest to see if these same findings hold true for hypoacusic subjects under conditions of the present study. When the hearing aids in the second investigation were grouped by frequency reSponse, differences were not observed. It was suggested that the small sample of hearing aids in each group accounted for this finding. However, this assumption needs to be verified. Since differences can exist between competing messages employed as secondary signals, future work should consider the use of modulated white noise. The employment of this type of secondary signal would allow for exact duplication of stimuli used in different research and clinical settings. REFERENCES 85 BIBLIOGRAPHY A conference on hearing aid evaluation procedures. Amer. Speech and Hearing Assoc. Reports, Number 2, September (1967). American Standard methods for measurement of electro- acoustical characteristics of hearing aids, 83.3-1960. New York: American National Standards Institute (1960). Berger, K. W., Hearing aid evaluative procedures. National Hearinngid Journal, 21; 6, 33-36 (1968). Berger, K. W., The Hearinngid: Its Operation and Development. National Hearing Aid Society (1970). ' Berger, K. W., and Millin, J. P., Chapter 14, "Hearing Aids." In Audiological Assess- mppp, Rose, D. E. (Editor), Englewood Cliffs, New Jersey: Prentice-Hall, Inc. (1971). Carhart, R., Tests for selection of hearing aids. Laryngoscope, 56, 780-794 (1946). Carhart, R., and Jerger, J. F., Preferred method for clinical determination of pure-tone thresholds. J. Speech and Hearing Dis., 24, 330-345 (1959). Carhart, R., Tillman, T. W., and Greetis, E. S., Perceptual masking in multiple sound backgrounds. J. acoust. Soc. Amer., 45, 694-703 (1968). Davis, H., and Silverman, S. R., Hearing and Deafness. New York: Holt, Rinehart and Winston (1970). Hirsh, I. J., Davis, H., Silverman, S. R., Reynolds, E. G., Eldert, E., and Benson, R. W., Development of materials for speech audiometry. J. Speech and Hearing Dis., 17, 321-337 (1952). 86 87 Jerger, J., Speaks, C., and Malmquist, C., Hearing aid performance and hearing aid selection. J. Speech and Hearing Res., 9, 136-149 (1966). Jerger, J., and Thelin, J., Effects of electroacoustic characteristics of hearing aids on speech understanding. Bull. Prosthetic Res., Fall, 159-197 (1968). Kasten, R. N., and Lotterman, S. H., The influence of hearing aid gain control rotation on Acoustic gain. Audecibel, Summer, 97-102 (1970). Kranz, F. W., Tentative code for measurement of per- formance of hearing aids. J. acoust. Soc. Amer., 17, 144-150 (1945). Lehiste, 1., and Peterson, C., Linguistic considerations in the study of speech intelligibility. J, acoust. Soc. Amer., 31, 280-286 (1959). Lybarger, S. F., A new HAIC Standard method of express- ing hearing aid performance. Hearinngealer, February; 11, 16-17, 33 (1961). Miller, G. A., Heise, G. A., and Lichten, W., The intelligibility of Speech as a function of the content of the test materials. J. Exp. Psychol., 41, 329-335 (1951). Peterson, C., and Lehiste, 1., Revised CNC lists for auditory tests. J. Speech and Hearing Dis., 27, 62-70 (1962). Rintelmann, W. F., and Associates, Five experiments on speech discrimination utilizing CNC monosyllabic words. Laboratory Report SHSLR 2-72, Michigan State University (1972). Romanow, F. F., Methods for measuring the performance of hearing aids. J. acoust. Soc. Amer., 13, 294-304 (1942). Also appeared as Bell Tele- phone Monograph B-l3l4. Ross, M., Chapter 33, "hearing Aid Evaluation." In Handbook of Clinical Audiology, Katz, J. (Editor), Baltimore: Williams and Wilkins Co. (1972). Shore, 1., Bilger, R. C., and Hirsh, 1., Hearing aid evaluations: Reliability of repeated measures. J. Speech and Hearing Dis., 25, 106-113 (1960). 88 Tillman, T. W., and Carhart, R., An expanded test for Speech discrimination utilizing CNC mono- syllabic words. USAF School of Aerospace Medicine, Brooks AFB, Texas, SAM-TR-66-55, 1-11. Tillman, T. W., and Jerger, J. F., Some factors affecting the spondee threshold in normal hearing subjects, J. Sppech and HearingARes., 2, 141-146 (1959). Tillman, T. W., Johnson, K. R., and Olsen, W., Earphone versus sound-field threshold sound pressure levels for spondee words. J. acoust. Soc. Amer., 39, 125-133 (1966). Zerlin, S., A new approach to hearing-aid selection. J. Speech and Hearing Res., 5, 370-376 (1962). APPENDICES 89 APPENDIX A HAIC STANDARD METHOD OF EXPRESSING HEARING AID PERFORMANCE 90 HAIC Standard 1. General: The purpose of this Standard is to provide a uniform method of numerically and graphically expressing certain fundamental performance characteristics of hearing aids in a Simple manner, so that those using such data can be assured of its meaning. 2. Method of Measurement: All quantities to be Specified in this Standard Shall be based on measure- ments made in accordance with American Standard 33.3, entitled "American Standard Methods for Measurement of the Electro- Acoustical Characteristics of Hearing Aids," or a succeeding Standard approved by the American Standards Assn. The above Standard is published by the American Standards Assn., Inc., 10 East 40 St., New York 16, N.Y. 3. Definitions: - Simple numerical expression of the terms gain, output and frequency range shall be defined as follows: 3.1 Gain The term "gain" as applied to a hear- ing aid shall mean the average of the 500, 1000 and 2000 cps values of the full-on acoustic gain, as defined in Section 2.3, and as measured in accordance with Section 5.7, of American Standard 83.3. Unit: decibels. 3.2 Output The term "output” shall mean the average of the 500, 1000 and 2000 cps values of the saturation sound-pressure level, as defined in Section 2.12, and as measured in accordance with Section 5.6, of American Standard 83.3. Unit: decibels re .0002 microbars. 3.3 Frequency Range 3.3.1 The frequency range of a hear- ing aid Shall be expressed by two numbers, one 91 3.3.2 3.3.3 92 representing the low-frequency limit of amplification in cps, and the other the high-frequency limit of amplifica- tion in cps, both as defined below. (Note: The frequency range of a hearing aid Shall not be expressed as the number of cycles per second between the low and high-frequency limits, because of the distorted impression this method can give.) Determination of the frequency range shall be made using a basic frequency-response curve as defined in Section 2.11, and as measured per Section 5.5 of American Standard 83.3. The following procedure shall be employed to determine the lower and upper-frequency limits. 3.3.3.1 Determine the average of the 500, 1000 and 2000 cps ordinates on the frequency- response curve and plot this value on the 1000 cps or- dinate. 3.3.3.2 Plot a second point on the 1000 eps ordinate 15 dB below the first point. 3.3.3.3 Through the second point draw a straight line parallel to the frequency axis. 3.3.3.4 The low-frequency limit of the hearing aid is defined as the frequency where this line first intersects the response curve, moving in the direction of decreasing frequency from 1000 cps. (Note: In the event the curve dips below the 15 dB line and returns above it, the second downward crossing of the line may be considered the low-frequency limit provided: (a) that the band width of the dip does not exceed 15 percent of the frequency of the first downward crossing, and (b) that the band width of 93 the following rise above the 15 dB line is 15 perdent or more of the frequency of the first upward crossing. The purpose of this exception is to avoid penalty where a single "notch" of inconse- quential effect on the hear- ing aid's performance may exist.) 3.3.3.5 The high-frequency limit of the hearing aid is defined as the frequency where this line first intersects the response curve, moving in the direction of increasing frequency from 1000 cps. (Note: In the event the curve dips below the 15 dB line and returns above it, the second downward crossing of the line may be considered the high-frequency limit provided: (a) that the band width of the dip does not exceed 15 percent of the frequency of the first downward crossing and (b) that the band width of the following rise above the 15 dB line is 15 percent or more of the frequency of the first upward crossing. The purpose of this exception is to avoid penalty where a single "notch" of inconse- quential effect on the hear- ing aid's performance may exist.) 4. Frequency Response: A frequency-response curve of the hearing aid shall be known in addition to specifying numerical data. This curve shall be the ”basic frequency response” as defined in Section 2.11, and as measured in Section 5.5, of American Standard 33.3. The curve Shall be plotted on a grid having a linear decibel ordinate scale and a 94 logarithmic frequency scale. One octave's length on the logarithmic scale shall equal between 13.5 and 15 decibels' length on the decibel scale. 5. Supplementary Information: At least the following data shall be presented with the corresponding numerical and graphical data: 5 1 Manufacturer's model number. 5 2 External earphone type (if applicable) 5.3 Control settings. 5 4 Nominal battery voltage. 5 5 If applicable, earphone-tubing dimensions L and D, per Figs. 2 or 4 of American Standard 83.3 (For conventional insert earphones, it is assumed that the HA-2 coupler, shown in Fig 3 of A.S.A. 83.3, will be used.) 6. Sampling: Sampling procedures Should be adequate to insure that the published performance data will be, to the best of the manufacturer's knowledge, representa- tive of the average product being offered for sale. 7. Identification: It is recommended that data pre- sented in conformity with this standard method carry the statement: "Data are expressed using Standard HAIC method." (Lybarger, 1961, p. 17, p. 33) APPENDIX B NORTHWESTERN UNIVERSITY AUDITORY TEST NUMBER SIX, FORM B, LISTS I-IV 95 96 FORM B List I burn kite king rag whip lot sell size mode met sub nag pool tip home take vine page dime fall chalk raid which week laud raise keen death goose bean yes love shout hash boat tough fat limb sure gap puff third hurl moon jar jail door choice reach knock List II live dab white gaze lower voice loaf hush young (lore) ton goal dead keep south learn shack pad tool match far mill soap chair witch (which) merge hate deep rot juice turn pike pick keg rain room fail gin Shawl read (reed) said nice bought calm wag numb thought book haze chief bite List III sheep cause rat bar mouse talk hire search luck cab rush five 97 team pearl soup half chat road pole phone life pain base mop mess germ thin name ditch tell cool seize dodge youth hit light jug lid wire good walk date when ring check* note gun beg void shall * In recording N.U. Auditory Test No. 6 at Michigan State University Audiology Research Laboratory, the word check was substituted for cheek accidently. List IV rose dog time such have mob bone sail rough dip join check wheat thumb near lease yearn kick get lose kill fit judge Should pass back hall bath tire P98 perch chain make long wash food mood sour neat wife tape ripe hole gas came vote lean red doll shirt APPENDIX C CALIBRATION OF EQUIPMENT 98 Prior to and after the experiment the total speech audiometric system was checked for calibration. Periodically during the experiment the equipment was monitored for correct intensity calibration. The method used for monitoring and checking the calibration of the equipment is described below. Acoustic Oupput of the Grason-Stadler 162 Speech Audiometer The acoustic output of the speech audiometer was measured before the experiment, weekly during the experiment and after the termination of the experiment. The speech audiometer was calibrated so that audiometric zero was defined as being 20 dB above 0.0002 dynes/cmz. For all speech materials the level of the narrow band calibration noise recorded on the tape was adjusted to O VU. The speech audiometer system including the left earphone (TDH-39) with the associated cushion (MX-4l/AR) was calibrated with an artificial ear assembly (Bruel and Kjaer, Type 4152) using a condenser microphone (Bruel and Kjaer, Type 4144), a sound level meter (Bruel and Kjaer, Type 2203) with it's associated octave band filter network (Bruel and Kjaer, Type 1613). "Speech Spectrum Noise" was used for calibration of the earphone system of the speech audiometer according to a procedure described by Tillman, Johnson and Olsen (1966). The input level of the noise, at a given attenuator setting, was adjusted until it produced a deflection to zero on the speech audiometer VU meter. The resultant acoustic output of the 99 100 system was then measured. This value was accepted as the intensity of the spondee words at the same attenuator setting under the con- dition in which the peaks of the words produced a deflection to zero on the VU meter. For example, with the speech audiometer attenuator set to 60 dB HL, the output of the artificial ear would be 80 dB SPL re 0.0002 dynes/cmz. All measurements made during the course of the investigation were within i 1 dB. Tape Recorder The tape recorder (Ampex, Model 601) heads and contacts were cleaned daily during the course of the investigation. Earphone Frequency Response Prior to and after the experiment the frequency response of the left earphone (TDH-39) with it's associated cushion (MX-4l/AR) was independently obtained with an artificial ear assembly (Bruel and Kjaer, Type 4152) and a condenser micrOphone (Bruel and Kjaer, Type 4144). A sine-random generator (Bruel and Kjaer, Type 1024) was used to drive the earphone. The output from the artificial ear assembly was connected to a microphone amplifier (Bruel and Kjaer, Type 2603) which in turn was coupled to a power level recorder (Bruel and Kjaer, Type 2305). No changes in the frequency response of the earphone were noted for these two measurements. Figure 16 shows the final frequency response of the earphone obtained at the end of the experiment. 101 .uouoaoHoam :oooem Noa uoHvoumIGOmmuo no ozonepoo umoH mo oncogene hocoavoum as 5 3 on SOS 88 oow\EE ldqfifiooem a2, .Hmowuaocw ones oo>uao uncommon hoaoavmum Hmunoswuomxo umom cam ohm Nsmmm zH wozmoommm u}. we: on ”.33 oeoN coca coon NI linemen...— ocm com cc .ED 526.. Jfiquocnoom 3 endow xocoovoi 39qu I. on NI on c— uj A L LO 414] 1 Pl mp iquomcam toooEoneoHon cm ow cm ocH cHH ONH .sH unste HH 'IdS NI SCI HVSOHOIW ZOOO'O 102 Frequency Response of the Entire System The frequency response of the entire system (tape recorder, speech audiometer and earphone) was also checked prior to and after the completion of the experiment. This was accomplished by using a frequency test tape (Ampex, No. 01-31321-01). The output from the earphone was measured using an artificial ear and sound level meter. The descrete frequencies 12KHz, lOKHz, 7.5KHz, SKHz, 2.5KHz, 1KHz, 500 Hz, 250 Hz, 100 Hz, 50 Hz and 30 Hz were measured. The results showed that the frequency response of the system was identical to the frequency response of the earphone. Thus, the frequency response of the entire system was limited by the frequency response of the test earphone (TDH-39) and cushion (MX-4l/AR). This system did not Ichange in it's characteristics during the experiment. APPENDIX D NEW METHOD AND HAIC DATA FOR ALL EIGHTEEN HEARING AIDS 103 Hearing Aid: Oticon UX Model: Greendot Serial Number: 684107 New lethod Data HAIC Data Gain Gain Average 18 dB i;10 dB 40 dB 500 Hz 15 dB 1000 Hz 19 dB 2000 Hz 20 dB Maximum Power Output Maximum Power Output Average 108 dB SPL 112 dB SPL 500 Hz 106 dB SPL 1000 Hz 109 dB SPL 2000 Hz 109 dB SPL Peak 112 dB, 800 Hz Frequency Range Frequency Range - Low Frequency 150 Hz Low Frequency 180 H High Frequency 3800 Hz High Frequency 3800 Hz 104 105 Hearing Aid: Zenith Model: Westwood B Serial Number: B 21768 New Method Data HAIC Data Gain Gain Average 11 dB 1'10 dB 35 dB 500 Hz 1 dB 1000 Hz 17 dB 2000 Hz 16 dB Maximum Power Output Maximum Power Output Average 96 dB SPL 103 dB SPL 500 Hz 85 dB SPL 1000 Hz 103 dB SPL 2000 Hz 100 dB SPL Peak 103 dB, 2500 Hz Frequency Range Frequency Range _ Low Frequency 550 Hz Low Frequency 500 Hz High Frequency 4500 Hz High Frequency 4500 Hz 106 Hearing Aid: Siemens Model: 380 Serial Number: 511 New Method Data HAIC Data Gain Gain Average 18 dB i,10 dB 37 dB 500 Hz 4 dB 1000 Hz 20 dB 2000 Hz 31 dB Maximum Power Output Maximum Power Output Average 110 dB SPL 114 dB SPL 500 Hz 96 dB SPL 1000 Hz 113 dB SPL 2000 Hz 121 dB SPL Peak 121 dB, 2000 Hz Frequency Range Frequency Range Low Frequency 600 Hz Low Frequency 480 Hz High Frequency 5000 Hz High Frequency 5000 Hz Hearing Aid: Zenith Model: Moderator A Serial Number: DW 681 New Method Data Gain Average 14 dB :;10 dB 500 Hz 14 dB 1000 Hz 30 dB 2000 Hz 37 dB Maximum Power Output Average 110 dB SPL 500 Hz 96 dB SPL 1000 Hz 119 dB SPL 2000 Hz 116 dB SPL Peak 119 dB, 1000 Hz Frequency Range Low Frequency 540 Hz High Frequency 4300 Hz HAIC Data Gain 37 dB Maximum Power Output 107 dB SPL Frequency Range . Low Frequency 450 Hz High Frequency 4300 Hz Hearing Aid: Radioear Model: 1000 Serial Number: PH 083 New Method Data Gain Average 24 dB i_10 dB 500 Hz 18 dB 1000 Hz 25 dB 2000 Hz 28 dB Maximum Power Output Average 116 dB SPL 500 Hz 110 dB SPL 1000 Hz 120 dB SPL 2000 Hz 117 dB SPL Peak 121 dB, 900 Hz Frequency Range Low Frequency 340 Hz High Frequency 4600 Hz W Gain 40 dB Maximum Power Output 118 dB SPL Frequency Range Low Frequency 190 Hz High Frequency 4800 Hz Hearing Aid: Sonotone Model: 37 Serial Number: 62367 New Method Data Gain Average 18 dB :.10 dB 500 Hz 6 dB 1000 Hz 23 dB 2000 Hz 25 dB Maximum Power Output Average 113 dB SPL 500 Hz 105 dB SPL 1000 Hz 119 dB SPL 2000 Hz 115 dB SPL Peak 122 dB, 1200 Hz Frequency Range Low Frequency 550 Hz High Frequency 4500 Hz 109 HAIC Data Gain 40 dB Maximum Power Output 114 dB SPL Frequency Range Low Frequency 450 Hz High Frequency 4500 Hz 110 Hearing Aid: Beltone Model: Overture YY Serial Number: W 25604 New Method Data HAIC Data Gain Gain Average 19 dB i;10 dB 42 dB 500 Hz 17 dB 1000 Hz 24 dB 2000 Hz 17 dB Maximum Power Output Maximum Power Output Average 104 dB SPL . 114 dB SPL 500 Hz 91 dB SPL 1000 Hz 117 dB SPL 2000 Hz 103 dB SPL Peak 125 dB, 2500 Hz Frequency Range Frequency Range Low Frequency 550 Hz Low Frequency 380 Hz High Frequency 4200 Hz High Frequency 4200 Hz Hearing Aid: Beltone Model: Overture R Serial Number: W 27153 New Method Data Gain Average 25 dB 1,10 dB 500 Hz 17 dB 1000 Hz 33 dB 2000 Hz 25 dB Maximum Power Output Average 119 dB SPL 500 Hz 116 dB SPL 1000 Hz 122 dB SPL 2000 Hz 118 dB SPL Peak 125 dB, 900 Hz Frequency Range Low Frequency 450 Hz High Frequency 4300 Hz HAIC Data Gain 44 dB Maximum.Power Output 119 dB SPL Frequency Range . Low Frequency 350 H High Frequency 4300 Hz 112 Hearing Aid: Audiotone Model: A-l9 Serial Number: 1641 New Method Data HAIC Data Gain Gain Average 22 dB‘i 10 dB 45 dB 500 Hz 5 dB 1000 Hz 27 dB 2000 Hz 33 dB Maximum Power Output Maximum Power Output Average 108 dB SPL 113 dB SPL 500 Hz 90 dB SPL 1000 Hz 115 dB SPL 2000 Hz 120 dB SPL Peak 120 dB, 2000 Hz Frequency Range Frequency Range . Low Frequency 580 Hz Low Frequency 460 H High Frequency 3600 Hz High Frequency 3600 Hz Hearing Aid: Audiotone Model: A-20 Serial Number: 5142 New Method Data Gain Average 18 dB i_10 dB 500 Hz 11 dB 1000 Hz 23 dB 2000 Hz 20 dB Maximum Power Output Average 122 dB SPL 500 Hz 120 dB SPL 1000 Hz 126 dB SPL 2000 Hz 120 dB SPL Peak 127 dB, 900 Hz Frequency Range Low Frequency 450 Hz High Frequency 4600 Hz HAIC Data Gain 46 dB Maximum Power Output 121 dB SPL Frequency Range , Low Frequency 280 Hz High Frequency 4900 Hz Hearing Aid: Beltone Model: Overture Yellow Serial Number: W 34395 New Method Data Gain Average 22 dB i.10 dB 500 Hz 13 dB 1000 Hz 29 dB 2000 Hz 25 dB Maximum Power Output Average 111 dB SPL 500 Hz 105 dB SPL 1000 Hz 116 dB SPL 2000 Hz 113 dB SPL Peak 119 dB, 900 Hz Frequency Range Low Frequency 380 Hz High Frequency 4200 Hz HAIC Data Gain 49 dB Maximum Power Output 115 dB SPL Frequency Range . Low Frequency 380 H High Frequency 4200 Hz Hearing Aid: Zenith Model: Newport Serial Number: NB 667 New Method Data Gain Average 21 dB i_10 dB 500 Hz 5 dB 1000 Hz 29 dB 2000 Hz 28 dB Maximum Power Output Average 113 dB SPL 500 Hz 100 dB SPL 1000 Hz 121 dB SPL 2000 Hz 191 dB SPL Peak 121 dB, 1000 Hz Frequency Range Low Frequency 650 Hz High Frequency 4400 Hz W Gain 49 dB Maximum.Power Output 123 dB SPL Frequency Range Low Frequency 480 Hz High Frequency 4400 Hz 116 Hearing Aid: Siemens Model: 382 Serial Number: 10649 New Method Data HAIC Data Gain Gain Average 23 dB :,10 dB 50 dB 500 Hz 14 dB 1000 Hz 24 dB 2000 Hz 30 dB Maximum Power Output Maximum Power Output Average 111 dB SPL 121 dB SPL 500 Hz 105 dB SPL 1000 Hz 113 dB SPL 2000 Hz 115 dB SPL Peak 116 dB, 3500 Hz Frequency Range Frequency Range , Low Frequency 500 Hz Low Frequency 400 Hz High Frequency 4900 Hz High Frequency 4900 Hz Hearing Aid: Sonotone Model: 778 Serial Number: 25633 New Method Data Gain Average 36 dB :1; 10 dB 500 Hz 29 dB 1000 Hz 38 dB 2000 Hz 40 dB Maximum Power Output Average 122 dB SPL 500 Hz 118 dB SPL 1000 Hz 124 dB SPL 2000 Hz 124 dB SPL Peak 124 dB, 1000 Hz Frequency Range Low Frequency 400 Hz High Frequency 3800 Hz HAIC Data Gain 50 dB Maximum Power Output 124 dB SPL Frequency Range Low Frequency 320 Hz High Frequency 3800 Hz Hearing Aid: Siemens Model: 383 Serial Number: 9610 New Method Data Gain Average 23 dB $.10 dB 500 Hz 10 dB 1000 Hz 26 dB 2000 Hz 33 dB Maximum Power Output Average 115 dB SPL 500 Hz 104 dB SPL 1000 Hz 119 dB SPL 2000 Hz 121 dB SPL Peak 123 dB, 3500 Hz Frequency Range Low Frequency 380 Hz High Frequency 4600 Hz 118 HAIC Data Gain 50 dB Maximum Power Output 115 dB SPL Frequency Range . Low Frequency 390 H High Frequency 4600 Hz 119 Hearing Aid: Audiotone Model: A 21 11 Serial Number: 1913 New Method Data HAIC Data Gain Gain Average 38 dB :.10 dB 51 dB 500 Hz 27 dB 1000 Hz 42 dB 2000 Hz 46 dB Maximum Power Output Maximum Power Output Average 124 dB SPL 127 dB SPL 500 Hz 118 dB SPL 1000 Hz 129 dB SPL 2000 Hz 124 dB SPL Peak 129 dB, 1000 Hz Frequency Range Frequency Range Low Frequency 550 Hz Low Frequency 450 Hz High Frequency 4500 Hz High Frequency 5200 Hz Hearing Aid: Norelco Model: 6730 Serial Number: 85132 New Method Data Gain Average 35 dB :.10 dB 500 Hz 29 dB 1000 Hz 39 dB 2000 Hz 39 dB Maximum Power Output Average 123 dB SPL 500 Hz 122 dB SPL 1000 Hz 126 dB SPL 2000 Hz 122 dB SPL Peak 131 dB, 800 Hz Frequency Range Low Frequency 450 Hz High Frequency 4500 Hz HAIC Data Gain 51 dB Maximum Power Output 128 dB SPL Frequency Range - Low Frequency 400 H High Frequency 4500 Hz 121 Hearing Aid: Radioear Model: 990 Serial Number: 2L6 906 New Method Data HAIC Data Gain Gain Average 36 dB i'10 dB 53 dB 500 Hz 28 dB 1000 Hz 43 dB 2000 Hz 37 dB Maximum Power Output ' Maximum Power Output Average 127 dB SPL 129 dB SPL 500 Hz 125 dB SPL 1000 Hz 129 dB SPL 2000 Hz 126 dB SPL Peak 133 dB, 900 Hz Frequency Range Frequency Range Low Frequency 480 Hz Low Frequency 370 Hz High Frequency 4800 Hz High Frequency 4800 Hz