INFLUENCE OF EAR CANAL OCCLUSION AND AIR-CONDUCTION FEEDBACK ON SPEECH PRODUCTION IN NOISE by David C. Byrne BA in Physics, University of Pittsburgh, 1984 MS in Communication Disorders, The Pennsylvania State University, 1987 Submitted to the Graduate Faculty of the School of Health and Rehabilitation Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2013 i UNIVERSITY OF PITTSBURGH School of Health and Rehabilitation Sciences This dissertation was presented by David C. Byrne It was defended on September 25, 2013 and approved by John D. Durrant, PhD, Professor Emeritus, Department of Communication Science and Disorders Susan Shaiman, PhD, Associate Professor, Communication Science and Disorders Jeffrey S. Vipperman, PhD, Associate Professor of Mechanical Engineering / Associate Professor of BioEngineering, Department of Mechanical Engineering & Materials Science William J. Murphy, PhD, Research Physicist, National Institute for Occupational Safety and Health, Cincinnati, Ohio Dissertation Advisor: Catherine V. Palmer, PhD, Associate Professor, Communication Science and Disorders ii Copyright © by David C. Byrne 2013 iii INFLUENCE OF EAR CANAL OCCLUSION AND AIR-CONDUCTION FEEDBACK ON SPEECH PRODUCTION IN NOISE David C. Byrne, PhD University of Pittsburgh, 2013 Millions of workers are exposed to high noise levels on a daily basis. The primary concern for these individuals is the prevention of noise-induced hearing loss, which is typically accomplished by wearing of some type of personal hearing protector. However, many workers complain they cannot adequately hear their co-workers when hearing protectors are worn. There are many aspects related to fully understanding verbal communication between noise-exposed workers that are wearing hearing protection. One topic that has received limited attention is the overall voice level a person uses to communicate in a noisy environment. Quantifying this component provides a starting point for understanding how communication may be improved in such situations. While blocking out external sounds, hearing protectors also induce changes in the wearer’s self-perception of his/her own voice, which is known as the occlusion effect. The occlusion effect and attenuation provided by hearing protectors generally produce opposite effects on that individual’s vocal output. A controlled laboratory study was devised to systematically examine the effect on a talker’s voice level caused by wearing a hearing protector and while being subjected to high noise levels. To test whether differences between occluded and unoccluded vocal characteristics are due solely to the occlusion effect, speech produced while subjects’ ear canals were occluded was measured without the subject effectively receiving any attenuation from the hearing protectors. To test whether vocal output differences are due to iv the reduction in the talker’s self-perceived voice level, the amount of occlusion was held constant while varying the effective hearing protector attenuation. Results show the occlusion effect, hearing protector attenuation, and ambient noise level all to have an effect on the talker’s voice output level, and all three must be known to fully understand and/or predict the effect in a particular situation. The results of this study may be used to begin an effort to quantify metrics in addition to the basic noise reduction rating that may be used to evaluate a hearing protector’s practical usability/wearability. By developing such performance metrics, workers will have information to make informed decisions about which hearing protector they should use for their particular work environment. v TABLE OF CONTENTS 1.0 INTRODUCTION ........................................................................................................ 1 2.0 BACKGROUND .......................................................................................................... 3 2.1 SPEECH ACOUSTICS ....................................................................................... 3 2.1.1 Basics of speech production ............................................................................ 3 2.1.2 Long-term average speech spectrum ............................................................. 5 2.2 SPEECH PRODUCTION IN NOISE ................................................................ 7 2.2.1 The Lombard effect ......................................................................................... 7 2.2.2 Other vocal characteristics ........................................................................... 10 2.2.3 Summary ........................................................................................................ 12 2.3 EFFECT OF “LOUD SPEECH” AND NOISE ON INTELLIGIBILITY ... 15 2.4 MEASUREMENT OF SPEECH INTELLIGIBILITY ................................. 23 2.4.1 Word tests ....................................................................................................... 23 2.4.2 Sentence tests.................................................................................................. 29 2.5 EFFECTS OF HEARING PROTECTION ON INTELLIGIBILITY ......... 34 2.5.1 Mechanisms of performance ......................................................................... 34 2.5.2 Listener effects ............................................................................................... 37 2.5.3 Talker effects .................................................................................................. 44 vi 2.6 ADDITIONAL CONSIDERATIONS FOR SPEECH INTELLIGIBILITY 51 2.6.1 Self-hearing .................................................................................................... 51 2.6.2 Prediction techniques .................................................................................... 55 2.6.3 Clear speech/Speech training ....................................................................... 56 2.7 ANALYSIS AND SUMMARY OF THE EXISTING RESEARCH ............. 59 3.0 RESEARCH QUESTION AND METHODS .......................................................... 65 3.1 OBJECTIVE ...................................................................................................... 65 3.1.1 Hypotheses ...................................................................................................... 66 3.2 STUDY PLAN .................................................................................................... 67 3.2.1 Approach ........................................................................................................ 67 3.2.1.1 Voice feedback system instrumentation ............................................ 68 3.2.1.2 Evaluation of voice feedback instrumentation ................................. 69 3.2.2 Ear canal occlusion conditions ..................................................................... 73 3.2.3 Hearing protector attenuation conditions ................................................... 74 3.2.3.1 Attenuation measurements ................................................................. 74 3.2.3.2 Setting the voice feedback system ...................................................... 76 3.2.4 Background noise levels ................................................................................ 77 3.2.5 Speech measurements.................................................................................... 77 3.3 STUDY PARTICIPANTS ................................................................................. 80 3.3.1 Protection of human subjects ....................................................................... 80 3.3.2 Qualification procedures ............................................................................... 82 3.4 TEST PROCEDURE ......................................................................................... 83 3.5 EXPERIMENTAL DESIGN ............................................................................ 84 vii 4.0 RESULTS ................................................................................................................... 89 4.1 PRELIMINARY MEASURES ......................................................................... 89 4.1.1 Occlusion effect measurements .................................................................... 89 4.1.2 Hearing protector attenuation measurements ............................................ 90 4.2 EFFECT OF OCCLUSION TYPE, ATTENUATION, AND NOISE .......... 92 4.2.1 WAV file analysis........................................................................................... 92 4.2.2 Linear mixed model ....................................................................................... 94 4.2.3 Occlusion type ................................................................................................ 96 4.2.4 Attenuation level ............................................................................................ 98 4.2.5 Interaction effects ........................................................................................ 100 5.0 DISCUSSION ........................................................................................................... 105 5.1 EFFECT OF OCCLUSION TYPE, ATTENUATION, AND NOISE ........ 105 5.2 PRACTICAL IMPLICATIONS .................................................................... 108 5.3 LIMITATIONS OF THE STUDY ................................................................. 110 5.4 CONCLUSIONS .............................................................................................