In recent years, vocal fatigue research has been increasingly studied particularly with application to the reduction of its impact on schoolteachers and other occupational voice users. However, the concept of vocal fatigue is complex and neither well defined or well understood. Vocal fatigue seems to be highly individualized and dependent on several underlying factors or concepts. The purpose of this dissertation is to propose and support through experimentation a framework that can identify the factors contributing to vocal fatigue. The main hypothesis is that the change in vocal effort, vocal performance, and/or their interaction through a vocal demand (load) will implicate vocal fatigue. To test this hypothesis, three primary research questions and experiments were developed. For all three experiments vocal effort was rated using the Borg CR-100 scale and vocal performance was evaluated with five speech acoustic parameters (fundamental frequency mean and standard deviation, speech level mean and standard deviation, and smoothed cepstral peak prominence).The first research question tests whether perceived vocal effort can be measured reliably and if so, how vocal performance in terms of vocal intensity changes with a vocal effort goal. Participants performed various speech tasks at cued effort levels from the Borg CR-100 scale. Speech acoustic parameters were calculated and compared across the specific vocal effort levels. Additionally, the test-retest reliability across the effort levels for speech level was measured. Building from that experiment, the second research question was to what degree are vocal performance and vocal effort related given talker exposure to three equivalent vocal load levels. This experiment had participants performing speech tasks when presented with three different equivalent vocal load scenarios (communication distance, loudness goal, and background noise); for a given load scenario, participants rated their vocal effort associated with these tasks. Vocal effort ratings and measures of vocal performance were compared across the vocal load levels. The last research question built on the previous two and asked to what degree do vocal performance, vocal effort, and/or their interaction change given a vocal load of excess background noise (noise load) over a prolonged speaking task (temporal load). To test this, participants described routes on maps for thirty minutes in the presence of loud (75 dBA) background noise. Vocal effort ratings and measures of vocal performance were compared throughout the vocal loading task.The results indicate that elicited vocal effort levels from the BORG CR-100 scale are distinct in vocal performance and reliable across the participants. Additionally, a relationship between changes in vocal effort and vocal performance across the various vocal load levels was quantified. Finally, these findings support the individual nature of the complex relationship between vocal fatigue, vocal effort, and vocal performance due to vocal loads (via cluster and subgroup analysis); the theoretical framework captures this complexity and provides insights into these relationships. Future vocal fatigue research should benefit from using the framework as an underlying model of these relationships.
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