Table Of Content

Hearing Aids Second Edition Harvey Dillon, Ph.D. Director of Research National Acoustic Laboratories of Australia Sydney, New South Wales BOOMERANG PRESS Sydney 2012 New York • Stuttgart hearing aids.indb 1 3/27/2012 9:48:13 AM Boomerang Press 17 Vernon Street Turramurra, 2074, Australia Distributed exclusively in Americas Europe, Asia, Africa Australia and New Zealand Thieme Publishers New York Thieme Publishers Stottgart Boomerang Press 333 Seventh Avenue Ruedigerstrasse 14 17 Vernon Street New York, NY 10001, USA D-70469 Stuttgart Turramurra, 2074, Australia Phone: +1 212 760 0888 Germany Phone: +61 (0)2 9440 2885 or 1 800 782 3488 Phone: +49 (0)711 893 0 Fax: +61 (0)2 94402583 Fax: +1 212 947 1112 Fax: +49 (0)711 893 1410 Email: [email protected] Email: [email protected] Email: [email protected] Internet: www.BoomerangPress.com.au Internet: www.thieme.com Internet: www.thieme.de Typesetting: Jean Banning Printing: Colorcraft, Hong Kong National Library of Australia Library of Congress Cataloging-in-Publication Data Cataloguing-in-Publication entry Author: Dillon, Harvey. Dillon, Harvey. Title: Hearing Aids Hearing aids / Harvey Dillon. -- 2nd ed. Edition: 2nd Edition p. ; cm. ISBN: 978-0-9578168-1-7 Includes bibliographical references and index. Notes: Includes Index, bibliography ISBN 978-1-60406-810-8 Subjects: Hearing aids I. Title. Hearing rehabilitation [DNLM: 1. Hearing Aids. 2. Hearing--physiology. 3. Rehabili- tation of Hearing Impaired--methods. WV 274] Dewey Number: / Harvey Dillon 617.8’9--dc23 2012011398 Copyright © 2012 by Boomerang Press. This book, including all parts thereof, is legally protected by copyright. Except for those pages so indicated, any use, exploitation, or commercialization outside the narrow limits set by copyright legislation, without the publisher’s consent, is illegal and liable to prosecution. This applies in particular to photostat reproduction, copying, photographic scanning, mimeographing, or duplication of any kind, translating, preparation of microfilms, and electronic data processing and storage. Important Note This book is designed to educate, and be a reference tool for, students and professionals engaged in hearing rehabilitation through the use of hearing aids. While the book aims to have a wide scope, it is not the intention of author or publisher to present all the information that is available on this topic. The reader should obtain and read all the material that the reader considers necessary to competently perform hearing rehabilitation. Audiology is an ever-changing science. Research and clinical experience are continually broadening and changing our knowledge, in particular our knowledge of proper treatment. Every effort has been made to make this book as accurate as possible. However, there may be mistakes both typographical and in content. This text should therefore be used only as a general guide and not as the ultimate source of information on hearing aids. For these reasons the author, publisher and distributor shall have neither liability nor responsibility to any person or entity with respect to any loss or damage caused, or alleged to be caused, directly or indirectly by the information in this book. If you do not wish to be bound by the above, you may return this book to the point of sale for a full refund. Some of the product names, patents and registered designs referred to in this book are in fact registered trademarks or proprietary names, even though specific reference to this fact is not always made in the text. Therefore, the appearance of a name without designation as proprietary is not to be construed as a representation by the publisher that it is in the public domain. Printed in Hong Kong 5 4 3 2 1 hearing aids.indb 2 3/27/2012 9:48:14 AM BRIEF CONTENTS PREFACE .......................................................................................................................................................xvi 1 INTRODUCTORY CONCEPTS ...............................................................................................................1 2 HEARING AID COMPONENTS ............................................................................................................20 3 HEARING AID SYSTEMS ....................................................................................................................50 4 ELECTROACOUSTIC PERFORMANCE AND MEASUREMENT ........................................................81 5 HEARING AID EARMOLDS, EAR SHELLS AND COUPLING SYSTEMS .......................................127 6 COMPRESSION SYSTEMS IN HEARING AIDS ...............................................................................170 7 DIRECTIONAL MICROPHONES AND ARRAYS ...............................................................................198 8 ADVANCED SIGNAL PROCESSING SCHEMES ..............................................................................226 9 ASSESSING CANDIDACY FOR HEARING AIDS .............................................................................255 10 PRESCRIBING HEARING AID AMPLIFICATION ..............................................................................286 11 SELECTING, ADJUSTING AND VERIFYING HEARING AIDS ..........................................................336 12 PROBLEM SOLVING AND FINE-TUNING .........................................................................................354 13 PATIENT EDUCATION AND COUNSELING FOR HEARING AID WEARERS .................................374 14 ASSESSING THE OUTCOMES OF HEARING REHABILITATION ...................................................403 15 BINAURAL AND BILATERAL CONSIDERATIONS IN HEARING AID FITTING ..............................430 16 SPECIAL HEARING AID ISSUES FOR CHILDREN ..........................................................................469 17 CROS, BONE-CONDUCTION, AND IMPLANTED HEARING AIDS .................................................513 REFERENCES ..............................................................................................................................................537 INDEX ............................................................................................................................................................597 iii hearing aids.indb 3 3/27/2012 9:48:14 AM DETAILED CONTENTS PREFACE .......................................................................................................................................................xvi 1 INTRODUCTORY CONCEPTS ...................................................................................................................1 1.1 Problems Faced by People with Hearing Impairment........................................................................2 1.1.1 Decreased audibility .......................................................................................................................2 1.1.2 Decreased dynamic range ..............................................................................................................2 1.1.3 Decreased frequency resolution .....................................................................................................3 1.1.4 Decreased temporal resolution .......................................................................................................5 1.1.5 Physiological origins of hearing loss ...............................................................................................5 1.1.6 Deficits in combination ....................................................................................................................6 1.2 Acoustic Measurements .....................................................................................................................8 1.2.1 Basic physical measures ................................................................................................................8 1.2.2 Linear amplifiers and gain .............................................................................................................9 1.2.3 Saturation sound pressure level ...................................................................................................10 1.2.4 Couplers and real ears ..................................................................................................................11 1.3 Types of Hearing Aids ......................................................................................................................11 1.4 Historical Perspective ......................................................................................................................14 1.4.1 The acoustic era ...........................................................................................................................15 1.4.2 The carbon era .............................................................................................................................15 1.4.3 The vacuum tube era ....................................................................................................................16 1.4.4 The transistor and integrated circuit era .......................................................................................17 1.4.5 The digital era ...............................................................................................................................18 1.4.6 The wireless era ...........................................................................................................................19 2 HEARING AID COMPONENTS ................................................................................................................20 2.1 Block Diagrams ................................................................................................................................21 2.2 Microphones ....................................................................................................................................22 2.2.1 Principle of operation ....................................................................................................................23 2.2.2 Frequency response of microphones ...........................................................................................23 2.2.3 Microphone imperfections ............................................................................................................24 2.2.4 Directional microphones ...............................................................................................................26 2.2.5 Microphone location .....................................................................................................................28 2.3 Amplifiers .........................................................................................................................................28 2.3.1 Amplifier technology .....................................................................................................................28 2.3.2 Peak clipping and distortion ..........................................................................................................29 2.3.3 Compression amplifiers ................................................................................................................30 2.4 Digital Circuits ..................................................................................................................................31 2.4.1 Analog-to-digital converters ..........................................................................................................31 2.4.2 Digital signal processors ...............................................................................................................33 2.4.3 Hard-wired digital processing .......................................................................................................33 2.4.4 General arithmetic digital processing ...........................................................................................34 2.4.5 Sequential processing, block processing, and hearing aid delay .................................................35 2.4.6 Digital-to-analog converters ..........................................................................................................36 iv hearing aids.indb 4 3/27/2012 9:48:14 AM Detailed Contents v 2.4.7 Specifications for digital hearing aids ...........................................................................................36 2.4.8 Digital versus analog hearing aids ................................................................................................38 2.5 Filters, Tone Controls and Filter Structures ..................................................................................38 2.5.1 Filters ...........................................................................................................................................38 2.5.2 Tone Controls................................................................................................................................39 2.5.3 Filter structures .............................................................................................................................39 2.6 Receivers ......................................................................................................................................41 2.6.1 Principle of operation ....................................................................................................................41 2.6.2 Frequency response of receivers .................................................................................................41 2.7 Acoustic Dampers ..........................................................................................................................42 2.8 Telecoils .........................................................................................................................................43 2.9 Audio (Electrical) Input ...................................................................................................................44 2.10 Remote Controls ............................................................................................................................44 2.11 Bone Conductors ...........................................................................................................................46 2.12 Batteries ........................................................................................................................................46 2.12.1 Principle of operation ..................................................................................................................46 2.12.2 Operating voltage .......................................................................................................................47 2.12.3 Capacity and physical size .........................................................................................................47 2.12.4 Rechargeable batteries ..............................................................................................................49 2.13 Concluding Comments ..................................................................................................................49 3 HEARING AID SYSTEMS .........................................................................................................................50 3.1 Custom and Modular Construction ..................................................................................................51 3.1.1 Custom hearing aids .....................................................................................................................51 3.1.2 Modular hearing aids ....................................................................................................................52 3.1.3 Semi-modular, semi-custom hearing aids ....................................................................................52 3.1.4 Hearing aid reliability ....................................................................................................................53 3.2 Linked Bilateral Hearing Aids ...........................................................................................................53 3.3 Programming the Hearing Aid ..........................................................................................................54 3.3.1 Programmers, interfaces, and software ........................................................................................54 3.3.2 Multi-memory or multi-program hearing aids ................................................................................55 3.3.3 Paired comparisons ......................................................................................................................55 3.4 Remote Sensing and Transmitting Hearing Aid Systems ...............................................................56 3.5 Induction Loops ...............................................................................................................................56 3.5.1 Field uniformity and direction ........................................................................................................57 3.5.2 Magnetic field strength .................................................................................................................58 3.5.3 Loop frequency response .............................................................................................................59 3.6 Radio-frequency Transmission ........................................................................................................62 3.6.1 Frequency modulation ..................................................................................................................62 3.6.2 Digital modulation techniques .......................................................................................................64 3.6.3 Coupling to the hearing aid ...........................................................................................................65 3.6.4 Combining wireless and local microphones ................................................................................66 3.7 Infra-red Transmission .....................................................................................................................69 3.8 Classroom Sound-field Amplification ...............................................................................................69 3.9 Comparative Strengths and Weaknesses of Magnetic Loops, Radio-frequency Wireless Systems, Infra-red, and Sound-field Amplification Systems ............................................................71 3.10 Assistive Listening Devices ............................................................................................................73 hearing aids.indb 5 3/27/2012 9:48:15 AM vi HEARING AIDS 3.11 Connectivity and Convergence ......................................................................................................75 3.11.1 Connecting electronic devices to hearing aids............................................................................75 3.11.2 Convergence...............................................................................................................................77 3.11.3 Interference between mobile phones and hearing aids ..............................................................78 3.12 Concluding Comments ..................................................................................................................80 4 ELECTROACOUSTIC PERFORMANCE AND MEASUREMENT ...........................................................81 4.1 Measuring Hearing Aids in Couplers and Ear Simulators ................................................................82 4.1.1 Couplers and ear simulators .........................................................................................................82 4.1.2 Test boxes.....................................................................................................................................85 4.1.3 Measurement signals ...................................................................................................................87 4.1.4 Gain-frequency response and OSPL90-frequency response ......................................................88 4.1.5 Input-output functions ...................................................................................................................90 4.1.6 Distortion ......................................................................................................................................92 4.1.7 Internal noise ................................................................................................................................93 4.1.8 Magnetic response .......................................................................................................................94 4.1.9 ANSI, ISO and IEC standards ......................................................................................................95 4.2 Real-Ear-to-Coupler-Difference (RECD) ........................................................................................97 4.2.1 Factors affecting RECD ................................................................................................................97 4.2.2 Measurement of RECD ................................................................................................................99 4.2.3 RECD and REDD .......................................................................................................................101 4.3 Real-Ear Aided Gain (REAG) ........................................................................................................101 4.3.1 Positioning the probe for REAG measurement ..........................................................................103 4.3.2 Relationship between REAG, coupler gain and ear simulator gain ............................................105 4.3.3 Detecting incorrect aided measurements ...................................................................................106 4.4 Insertion Gain ................................................................................................................................107 4.4.1 Positioning the probe for insertion gain measurement ...............................................................108 4.4.2 Relationship between insertion gain, coupler gain and ear simulator gain .................................109 4.4.3 Detecting incorrect insertion gain measurements .......................................................................111 4.4.4 Accuracy of insertion gain measurements ...................................................................................111 4.5 Practical Issues in Real-Ear Testing ..............................................................................................111 4.5.1 Probe calibration .........................................................................................................................111 4.5.2 Control microphones ..................................................................................................................112 4.5.3 Effects of wax ..............................................................................................................................113 4.5.4 Contamination by background noise ..........................................................................................113 4.5.5 Hearing aid saturation .................................................................................................................114 4.5.6 Loudspeaker orientation ..............................................................................................................114 4.5.7 Measurement signal characteristics ............................................................................................115 4.6 Aided Threshold Testing and Functional Gain ...............................................................................116 4.7 Feedback in Hearing Aids ..............................................................................................................118 4.7.1 The feedback mechanism............................................................................................................118 4.7.2 Effects of feedback on sound quality ...........................................................................................119 4.7.3 Probe-tube measurements and feedback ..................................................................................120 4.8 Troubleshooting Faulty Hearing Aids .............................................................................................121 4.9 Concluding Comments ..................................................................................................................126 hearing aids.indb 6 3/27/2012 9:48:15 AM Detailed Contents vii 5 HEARING AID EARMOLDS, EARSHELLS AND COUPLING SYSTEMS ............................................127 5.1 Earmold, Earshell and Canal Fitting Physical Styles .....................................................................130 5.1.1 BTE earmold styles ....................................................................................................................130 5.1.2 ITE, ITC, and CIC earshell styles ...............................................................................................132 5.2 Overview of Earmold, Earshell and Canal Fitting Acoustics ..........................................................134 5.3 Venting ...........................................................................................................................................134 5.3.1 Effects of vents on hearing aid gain and OSPL90 ......................................................................136 5.3.2 Venting and the occlusion effect .................................................................................................140 5.3.3 Effects of vents and leaks on feedback oscillations....................................................................144 5.3.4 Interaction of vents with digital signal processing algorithms .....................................................146 5.3.5 Parallel versus Y (or diagonal) vents ..........................................................................................147 5.3.6 Open-canal fittings in summary ..................................................................................................148 5.4 The sound bore: tubing, horns and constrictions ...........................................................................149 5.4.1 Acoustic horns and constrictions ................................................................................................151 5.4.2 Tubing insertion depth ................................................................................................................154 5.5 Dampers ........................................................................................................................................155 5.6 Specific Tubing, Damping and Venting Configurations ..................................................................156 5.7 Procedure for Selecting Earmold and Earshell Acoustics..............................................................156 5.8 Ear Impressions .............................................................................................................................158 5.8.1 Standard ear impression techniques ..........................................................................................158 5.8.2 Ear impression techniques for CICs and high-gain hearing aids ................................................160 5.8.3 Ear impression materials ............................................................................................................162 5.9 Earmolds and Earshells ...............................................................................................................163 5.9.1 Earmold and earshell construction .............................................................................................163 5.9.2 Materials for earmolds and earshells ..........................................................................................164 5.9.3 Instant earmolds and hearing aids .............................................................................................167 5.9.4 Modifying and repairing earmolds and earshells ........................................................................167 5.10 Concluding Comments ................................................................................................................169 6 COMPRESSION SYSTEMS IN HEARING AIDS ....................................................................................170 6.1 Compression’s Major Role: Reducing the Signal’s Dynamic Range .............................................171 6.2 Basic Characteristics of a Compressor ..........................................................................................172 6.2.1 Dynamic compression characteristics: attack and release times ...............................................172 6.2.2 Static compression characteristics .............................................................................................175 6.2.3 Input and output control ..............................................................................................................178 6.2.4 Multichannel compression ..........................................................................................................179 6.3 Rationales for Use of Compression ...............................................................................................180 6.3.1 Avoiding discomfort, distortion and damage ...............................................................................180 6.3.2 Reducing inter-syllabic and inter-phonemic intensity differences ...............................................181 6.3.3 Reducing differences in long-term level......................................................................................182 6.3.4 Increasing sound comfort ...........................................................................................................183 6.3.5 Normalizing loudness .................................................................................................................184 6.3.6 Maximizing intelligibility ..............................................................................................................186 6.3.7 Reducing noise ...........................................................................................................................186 6.3.8 Empirical approaches .................................................................................................................189 6.4 Combinations of Compressors in Hearing Aids .............................................................................190 6.5 Benefits and Disadvantages of Different Compression Systems ..................................................190 hearing aids.indb 7 3/27/2012 9:48:15 AM viii HEARING AIDS 6.5.1 Compression relative to linear amplification ...............................................................................191 6.5.2 The benefits of multichannel relative to single-channel compression.........................................194 6.5.3 Slow versus fast compression ....................................................................................................196 6.6 Concluding Comments ..................................................................................................................197 7 DIRECTIONAL MICROPHONES AND ARRAYS ...................................................................................198 7.1 Directional Microphone Technology ...............................................................................................199 7.1.1 First-order subtractive directional microphones ..........................................................................199 7.1.2 Additive directional arrays ...........................................................................................................204 7.1.3 Complex directional arrays .........................................................................................................206 7.1.4 Bilateral directivity .......................................................................................................................213 7.2 Quantifying Directivity ....................................................................................................................215 7.2.1 2D and 3D directivity index .........................................................................................................215 7.2.2 AI-DI ............................................................................................................................................217 7.3 Directional Benefit ..........................................................................................................................218 7.3.1 Impact of listening environment ..................................................................................................219 7.3.2 Objective benefit in the clinic and self reported benefit in real life ..............................................221 7.3.3 Interaction of directivity with other technologies .........................................................................222 7.3.4 Disadvantages of directivity .......................................................................................................223 7.3.5 Candidates for directional microphones .....................................................................................223 7.3.6 Evaluation of directional microphones in the clinic .....................................................................224 7.4 Concluding comments ...................................................................................................................225 8 ADVANCED SIGNAL PROCESSING SCHEMES .................................................................................226 8.1 Adaptive Noise Reduction..............................................................................................................227 8.1.1 Adaptive noise reduction technology ..........................................................................................227 8.1.2 Adaptive noise reduction benefits ...............................................................................................232 8.1.3 Impulse noise reduction ..............................................................................................................233 8.2 Feedback Reduction ......................................................................................................................234 8.2.1 Feedback reduction by gain-frequency response control ...........................................................234 8.2.2 Feedback reduction by phase control .........................................................................................235 8.2.3 Feedback reduction by feedback path cancellation ....................................................................236 8.2.4 Feedback reduction by frequency shifting ..................................................................................238 8.2.5 Feedback reduction systems in combination ..............................................................................238 8.3 Frequency Lowering ......................................................................................................................239 8.3.1 Frequency lowering rules ...........................................................................................................239 8.3.2 Frequency lowering techniques ..................................................................................................240 8.3.3 Commercially available frequency lowering schemes ................................................................242 8.3.4 Frequency lowering, speech intelligibility and candidacy ...........................................................242 8.4 Speech Cue Enhancement ............................................................................................................245 8.5 Other Signal Processing Schemes ................................................................................................247 8.6 Concluding Comments ..................................................................................................................253 9 ASSESSING CANDIDACY FOR HEARING AIDS .................................................................................255 9.1 Factors Affecting the Lower Limit of Aidable Hearing Loss ............................................................258 9.1.1 Attitude and motivation ...............................................................................................................258 9.1.2 Pure tone loss and audiogram configuration ..............................................................................261 hearing aids.indb 8 3/27/2012 9:48:15 AM Detailed Contents ix 9.1.3 Speech identification ability ........................................................................................................263 9.1.4 Self reported disability ................................................................................................................263 9.1.5 Acceptance of noise ...................................................................................................................264 9.1.6 Listening environment, needs and expectations ........................................................................265 9.1.7 Stigma and cosmetic concerns ...................................................................................................268 9.1.8 Manipulation and management ..................................................................................................269 9.1.9 Age ...........................................................................................................................................270 9.1.10 Personality ................................................................................................................................271 9.1.11 Central auditory processing disorders ......................................................................................272 9.1.12 Tinnitus .....................................................................................................................................272 9.1.13 Factors in combination .............................................................................................................273 9.1.14 Counseling the unwilling patient: some examples ....................................................................274 9.2 The Upper Limit of Aidable Hearing Loss ......................................................................................278 9.2.1 Poor speech identification ability ................................................................................................278 9.2.2 Hearing aids or cochlear implants? ............................................................................................279 9.2.3 Hearing aids and cochlear implants: bimodal and hybrid / electroacoustic stimulation ..............280 9.2.4 Hearing aids or tactile aids? .......................................................................................................283 9.3 Medically Related Contra-indications to Hearing Aid Fitting ..........................................................284 9.4 Concluding Comments ..................................................................................................................284 10 PRESCRIBING HEARING AID AMPLIFICATION ..................................................................................286 10.1 General Concepts Behind a Prescriptive Approach and a Brief History ......................................287 10.2 Gain and Frequency Response Prescription for Linear Amplification..........................................290 10.2.1 POGO .......................................................................................................................................290 10.2.2 NAL ...........................................................................................................................................290 10.2.3 DSL ...........................................................................................................................................292 10.2.4 Examples and comparisons: POGO II, NAL-RP and DSL ........................................................294 10.3 Difficult Issues in Prescription ......................................................................................................297 10.3.1 Acclimatization and adaptation to gain and frequency response ..............................................297 10.3.2 Preferred loudness ...................................................................................................................298 10.3.3 Dead regions ............................................................................................................................298 10.3.4 Severe hearing loss, effective audibility and high-frequency amplification ...............................301 10.3.5 Prescribing compression thresholds .........................................................................................304 10.3.6 Need for accuracy in prescription .............................................................................................306 10.4 Gain, Frequency Response, and Input-output Functions for Non-linear Amplification ................307 10.4.1 LGOB ........................................................................................................................................308 10.4.2 IHAFF/Contour .........................................................................................................................308 10.4.3 ScalAdapt .................................................................................................................................309 10.4.4 FIG6 ..........................................................................................................................................310 10.4.5 DSL[i/o] and DSLm[i/o] ..............................................................................................................311 10.4.6 NAL-NL1 and NAL-NL2 ............................................................................................................313 10.4.7 CAMREST, CAMEQ and CAMEQ2-HF ....................................................................................315 10.4.8 Comparison of procedures .......................................................................................................315 10.5 Allowing for Conductive and Mixed Hearing Losses ....................................................................319 10.6 Selecting Options for Multi-memory Hearing Aids .......................................................................321 10.6.1 Music programs ........................................................................................................................322 10.6.2 Candidates for multi-memory hearing aids ...............................................................................322 hearing aids.indb 9 3/27/2012 9:48:16 AM

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Hearing Aids Second Edition Harvey Dillon, Ph.D. Director of Research National Acoustic Laboratories of Australia Sydney, New South Wales BOOMERANG PRESS Sydney 2012 New York • Stuttgart hearing aids.indb 1 3/27/2012 9:48:13 AM Boomerang Press 17 Vernon Street Turramurra, 2074, Australia Distributed exclusively in Americas Europe, Asia, Africa Australia and New Zealand Thieme Publishers New York Thieme Publishers Stottgart Boomerang Press 333 Seventh Avenue Ruedigerstrasse 14 17 Vernon Street New York, NY 10001, USA D-70469 Stuttgart Turramurra, 2074, Australia Phone: +1 212 760 0888 Germany Phone: +61 (0)2 9440 2885 or 1 800 782 3488 Phone: +49 (0)711 893 0 Fax: +61 (0)2 94402583 Fax: +1 212 947 1112 Fax: +49 (0)711 893 1410 Email: [email protected] Email: [email protected] Email: [email protected] Internet: www.BoomerangPress.com.au Internet: www.thieme.com Internet: www.thieme.de Typesetting: Jean Banning Printing: Colorcraft, Hong Kong National Library of Australia Library of Congress Cataloging-in-Publication Data Cataloguing-in-Publication entry Author: Dillon, Harvey. Dillon, Harvey. Title: Hearing Aids Hearing aids / Harvey Dillon. -- 2nd ed. Edition: 2nd Edition p. ; cm. ISBN: 978-0-9578168-1-7 Includes bibliographical references and index. Notes: Includes Index, bibliography ISBN 978-1-60406-810-8 Subjects: Hearing aids I. Title. Hearing rehabilitation [DNLM: 1. Hearing Aids. 2. Hearing--physiology. 3. Rehabili- tation of Hearing Impaired--methods. WV 274] Dewey Number: / Harvey Dillon 617.8’9--dc23 2012011398 Copyright © 2012 by Boomerang Press. This book, including all parts thereof, is legally protected by copyright. Except for those pages so indicated, any use, exploitation, or commercialization outside the narrow limits set by copyright legislation, without the publisher’s consent, is illegal and liable to prosecution. This applies in particular to photostat reproduction, copying, photographic scanning, mimeographing, or duplication of any kind, translating, preparation of microfilms, and electronic data processing and storage. Important Note This book is designed to educate, and be a reference tool for, students and professionals engaged in hearing rehabilitation through the use of hearing aids. While the book aims to have a wide scope, it is not the intention of author or publisher to present all the information that is available on this topic. The reader should obtain and read all the material that the reader considers necessary to competently perform hearing rehabilitation. Audiology is an ever-changing science. Research and clinical experience are continually broadening and changing our knowledge, in particular our knowledge of proper treatment. Every effort has been made to make this book as accurate as possible. However, there may be mistakes both typographical and in content. This text should therefore be used only as a general guide and not as the ultimate source of information on hearing aids. For these reasons the author, publisher and distributor shall have neither liability nor responsibility to any person or entity with respect to any loss or damage caused, or alleged to be caused, directly or indirectly by the information in this book. If you do not wish to be bound by the above, you may return this book to the point of sale for a full refund. Some of the product names, patents and registered designs referred to in this book are in fact registered trademarks or proprietary names, even though specific reference to this fact is not always made in the text. Therefore, the appearance of a name without designation as proprietary is not to be construed as a representation by the publisher that it is in the public domain. Printed in Hong Kong 5 4 3 2 1 hearing aids.indb 2 3/27/2012 9:48:14 AM BRIEF CONTENTS PREFACE .......................................................................................................................................................xvi 1 INTRODUCTORY CONCEPTS ...............................................................................................................1 2 HEARING AID COMPONENTS ............................................................................................................20 3 HEARING AID SYSTEMS ....................................................................................................................50 4 ELECTROACOUSTIC PERFORMANCE AND MEASUREMENT ........................................................81 5 HEARING AID EARMOLDS, EAR SHELLS AND COUPLING SYSTEMS .......................................127 6 COMPRESSION SYSTEMS IN HEARING AIDS ...............................................................................170 7 DIRECTIONAL MICROPHONES AND ARRAYS ...............................................................................198 8 ADVANCED SIGNAL PROCESSING SCHEMES ..............................................................................226 9 ASSESSING CANDIDACY FOR HEARING AIDS .............................................................................255 10 PRESCRIBING HEARING AID AMPLIFICATION ..............................................................................286 11 SELECTING, ADJUSTING AND VERIFYING HEARING AIDS ..........................................................336 12 PROBLEM SOLVING AND FINE-TUNING .........................................................................................354 13 PATIENT EDUCATION AND COUNSELING FOR HEARING AID WEARERS .................................374 14 ASSESSING THE OUTCOMES OF HEARING REHABILITATION ...................................................403 15 BINAURAL AND BILATERAL CONSIDERATIONS IN HEARING AID FITTING ..............................430 16 SPECIAL HEARING AID ISSUES FOR CHILDREN ..........................................................................469 17 CROS, BONE-CONDUCTION, AND IMPLANTED HEARING AIDS .................................................513 REFERENCES ..............................................................................................................................................537 INDEX ............................................................................................................................................................597 iii hearing aids.indb 3 3/27/2012 9:48:14 AM DETAILED CONTENTS PREFACE .......................................................................................................................................................xvi 1 INTRODUCTORY CONCEPTS ...................................................................................................................1 1.1 Problems Faced by People with Hearing Impairment........................................................................2 1.1.1 Decreased audibility .......................................................................................................................2 1.1.2 Decreased dynamic range ..............................................................................................................2 1.1.3 Decreased frequency resolution .....................................................................................................3 1.1.4 Decreased temporal resolution .......................................................................................................5 1.1.5 Physiological origins of hearing loss ...............................................................................................5 1.1.6 Deficits in combination ....................................................................................................................6 1.2 Acoustic Measurements .....................................................................................................................8 1.2.1 Basic physical measures ................................................................................................................8 1.2.2 Linear amplifiers and gain .............................................................................................................9 1.2.3 Saturation sound pressure level ...................................................................................................10 1.2.4 Couplers and real ears ..................................................................................................................11 1.3 Types of Hearing Aids ......................................................................................................................11 1.4 Historical Perspective ......................................................................................................................14 1.4.1 The acoustic era ...........................................................................................................................15 1.4.2 The carbon era .............................................................................................................................15 1.4.3 The vacuum tube era ....................................................................................................................16 1.4.4 The transistor and integrated circuit era .......................................................................................17 1.4.5 The digital era ...............................................................................................................................18 1.4.6 The wireless era ...........................................................................................................................19 2 HEARING AID COMPONENTS ................................................................................................................20 2.1 Block Diagrams ................................................................................................................................21 2.2 Microphones ....................................................................................................................................22 2.2.1 Principle of operation ....................................................................................................................23 2.2.2 Frequency response of microphones ...........................................................................................23 2.2.3 Microphone imperfections ............................................................................................................24 2.2.4 Directional microphones ...............................................................................................................26 2.2.5 Microphone location .....................................................................................................................28 2.3 Amplifiers .........................................................................................................................................28 2.3.1 Amplifier technology .....................................................................................................................28 2.3.2 Peak clipping and distortion ..........................................................................................................29 2.3.3 Compression amplifiers ................................................................................................................30 2.4 Digital Circuits ..................................................................................................................................31 2.4.1 Analog-to-digital converters ..........................................................................................................31 2.4.2 Digital signal processors ...............................................................................................................33 2.4.3 Hard-wired digital processing .......................................................................................................33 2.4.4 General arithmetic digital processing ...........................................................................................34 2.4.5 Sequential processing, block processing, and hearing aid delay .................................................35 2.4.6 Digital-to-analog converters ..........................................................................................................36 iv hearing aids.indb 4 3/27/2012 9:48:14 AM Detailed Contents v 2.4.7 Specifications for digital hearing aids ...........................................................................................36 2.4.8 Digital versus analog hearing aids ................................................................................................38 2.5 Filters, Tone Controls and Filter Structures ..................................................................................38 2.5.1 Filters ...........................................................................................................................................38 2.5.2 Tone Controls................................................................................................................................39 2.5.3 Filter structures .............................................................................................................................39 2.6 Receivers ......................................................................................................................................41 2.6.1 Principle of operation ....................................................................................................................41 2.6.2 Frequency response of receivers .................................................................................................41 2.7 Acoustic Dampers ..........................................................................................................................42 2.8 Telecoils .........................................................................................................................................43 2.9 Audio (Electrical) Input ...................................................................................................................44 2.10 Remote Controls ............................................................................................................................44 2.11 Bone Conductors ...........................................................................................................................46 2.12 Batteries ........................................................................................................................................46 2.12.1 Principle of operation ..................................................................................................................46 2.12.2 Operating voltage .......................................................................................................................47 2.12.3 Capacity and physical size .........................................................................................................47 2.12.4 Rechargeable batteries ..............................................................................................................49 2.13 Concluding Comments ..................................................................................................................49 3 HEARING AID SYSTEMS .........................................................................................................................50 3.1 Custom and Modular Construction ..................................................................................................51 3.1.1 Custom hearing aids .....................................................................................................................51 3.1.2 Modular hearing aids ....................................................................................................................52 3.1.3 Semi-modular, semi-custom hearing aids ....................................................................................52 3.1.4 Hearing aid reliability ....................................................................................................................53 3.2 Linked Bilateral Hearing Aids ...........................................................................................................53 3.3 Programming the Hearing Aid ..........................................................................................................54 3.3.1 Programmers, interfaces, and software ........................................................................................54 3.3.2 Multi-memory or multi-program hearing aids ................................................................................55 3.3.3 Paired comparisons ......................................................................................................................55 3.4 Remote Sensing and Transmitting Hearing Aid Systems ...............................................................56 3.5 Induction Loops ...............................................................................................................................56 3.5.1 Field uniformity and direction ........................................................................................................57 3.5.2 Magnetic field strength .................................................................................................................58 3.5.3 Loop frequency response .............................................................................................................59 3.6 Radio-frequency Transmission ........................................................................................................62 3.6.1 Frequency modulation ..................................................................................................................62 3.6.2 Digital modulation techniques .......................................................................................................64 3.6.3 Coupling to the hearing aid ...........................................................................................................65 3.6.4 Combining wireless and local microphones ................................................................................66 3.7 Infra-red Transmission .....................................................................................................................69 3.8 Classroom Sound-field Amplification ...............................................................................................69 3.9 Comparative Strengths and Weaknesses of Magnetic Loops, Radio-frequency Wireless Systems, Infra-red, and Sound-field Amplification Systems ............................................................71 3.10 Assistive Listening Devices ............................................................................................................73 hearing aids.indb 5 3/27/2012 9:48:15 AM vi HEARING AIDS 3.11 Connectivity and Convergence ......................................................................................................75 3.11.1 Connecting electronic devices to hearing aids............................................................................75 3.11.2 Convergence...............................................................................................................................77 3.11.3 Interference between mobile phones and hearing aids ..............................................................78 3.12 Concluding Comments ..................................................................................................................80 4 ELECTROACOUSTIC PERFORMANCE AND MEASUREMENT ...........................................................81 4.1 Measuring Hearing Aids in Couplers and Ear Simulators ................................................................82 4.1.1 Couplers and ear simulators .........................................................................................................82 4.1.2 Test boxes.....................................................................................................................................85 4.1.3 Measurement signals ...................................................................................................................87 4.1.4 Gain-frequency response and OSPL90-frequency response ......................................................88 4.1.5 Input-output functions ...................................................................................................................90 4.1.6 Distortion ......................................................................................................................................92 4.1.7 Internal noise ................................................................................................................................93 4.1.8 Magnetic response .......................................................................................................................94 4.1.9 ANSI, ISO and IEC standards ......................................................................................................95 4.2 Real-Ear-to-Coupler-Difference (RECD) ........................................................................................97 4.2.1 Factors affecting RECD ................................................................................................................97 4.2.2 Measurement of RECD ................................................................................................................99 4.2.3 RECD and REDD .......................................................................................................................101 4.3 Real-Ear Aided Gain (REAG) ........................................................................................................101 4.3.1 Positioning the probe for REAG measurement ..........................................................................103 4.3.2 Relationship between REAG, coupler gain and ear simulator gain ............................................105 4.3.3 Detecting incorrect aided measurements ...................................................................................106 4.4 Insertion Gain ................................................................................................................................107 4.4.1 Positioning the probe for insertion gain measurement ...............................................................108 4.4.2 Relationship between insertion gain, coupler gain and ear simulator gain .................................109 4.4.3 Detecting incorrect insertion gain measurements .......................................................................111 4.4.4 Accuracy of insertion gain measurements ...................................................................................111 4.5 Practical Issues in Real-Ear Testing ..............................................................................................111 4.5.1 Probe calibration .........................................................................................................................111 4.5.2 Control microphones ..................................................................................................................112 4.5.3 Effects of wax ..............................................................................................................................113 4.5.4 Contamination by background noise ..........................................................................................113 4.5.5 Hearing aid saturation .................................................................................................................114 4.5.6 Loudspeaker orientation ..............................................................................................................114 4.5.7 Measurement signal characteristics ............................................................................................115 4.6 Aided Threshold Testing and Functional Gain ...............................................................................116 4.7 Feedback in Hearing Aids ..............................................................................................................118 4.7.1 The feedback mechanism............................................................................................................118 4.7.2 Effects of feedback on sound quality ...........................................................................................119 4.7.3 Probe-tube measurements and feedback ..................................................................................120 4.8 Troubleshooting Faulty Hearing Aids .............................................................................................121 4.9 Concluding Comments ..................................................................................................................126 hearing aids.indb 6 3/27/2012 9:48:15 AM Detailed Contents vii 5 HEARING AID EARMOLDS, EARSHELLS AND COUPLING SYSTEMS ............................................127 5.1 Earmold, Earshell and Canal Fitting Physical Styles .....................................................................130 5.1.1 BTE earmold styles ....................................................................................................................130 5.1.2 ITE, ITC, and CIC earshell styles ...............................................................................................132 5.2 Overview of Earmold, Earshell and Canal Fitting Acoustics ..........................................................134 5.3 Venting ...........................................................................................................................................134 5.3.1 Effects of vents on hearing aid gain and OSPL90 ......................................................................136 5.3.2 Venting and the occlusion effect .................................................................................................140 5.3.3 Effects of vents and leaks on feedback oscillations....................................................................144 5.3.4 Interaction of vents with digital signal processing algorithms .....................................................146 5.3.5 Parallel versus Y (or diagonal) vents ..........................................................................................147 5.3.6 Open-canal fittings in summary ..................................................................................................148 5.4 The sound bore: tubing, horns and constrictions ...........................................................................149 5.4.1 Acoustic horns and constrictions ................................................................................................151 5.4.2 Tubing insertion depth ................................................................................................................154 5.5 Dampers ........................................................................................................................................155 5.6 Specific Tubing, Damping and Venting Configurations ..................................................................156 5.7 Procedure for Selecting Earmold and Earshell Acoustics..............................................................156 5.8 Ear Impressions .............................................................................................................................158 5.8.1 Standard ear impression techniques ..........................................................................................158 5.8.2 Ear impression techniques for CICs and high-gain hearing aids ................................................160 5.8.3 Ear impression materials ............................................................................................................162 5.9 Earmolds and Earshells ...............................................................................................................163 5.9.1 Earmold and earshell construction .............................................................................................163 5.9.2 Materials for earmolds and earshells ..........................................................................................164 5.9.3 Instant earmolds and hearing aids .............................................................................................167 5.9.4 Modifying and repairing earmolds and earshells ........................................................................167 5.10 Concluding Comments ................................................................................................................169 6 COMPRESSION SYSTEMS IN HEARING AIDS ....................................................................................170 6.1 Compression’s Major Role: Reducing the Signal’s Dynamic Range .............................................171 6.2 Basic Characteristics of a Compressor ..........................................................................................172 6.2.1 Dynamic compression characteristics: attack and release times ...............................................172 6.2.2 Static compression characteristics .............................................................................................175 6.2.3 Input and output control ..............................................................................................................178 6.2.4 Multichannel compression ..........................................................................................................179 6.3 Rationales for Use of Compression ...............................................................................................180 6.3.1 Avoiding discomfort, distortion and damage ...............................................................................180 6.3.2 Reducing inter-syllabic and inter-phonemic intensity differences ...............................................181 6.3.3 Reducing differences in long-term level......................................................................................182 6.3.4 Increasing sound comfort ...........................................................................................................183 6.3.5 Normalizing loudness .................................................................................................................184 6.3.6 Maximizing intelligibility ..............................................................................................................186 6.3.7 Reducing noise ...........................................................................................................................186 6.3.8 Empirical approaches .................................................................................................................189 6.4 Combinations of Compressors in Hearing Aids .............................................................................190 6.5 Benefits and Disadvantages of Different Compression Systems ..................................................190 hearing aids.indb 7 3/27/2012 9:48:15 AM viii HEARING AIDS 6.5.1 Compression relative to linear amplification ...............................................................................191 6.5.2 The benefits of multichannel relative to single-channel compression.........................................194 6.5.3 Slow versus fast compression ....................................................................................................196 6.6 Concluding Comments ..................................................................................................................197 7 DIRECTIONAL MICROPHONES AND ARRAYS ...................................................................................198 7.1 Directional Microphone Technology ...............................................................................................199 7.1.1 First-order subtractive directional microphones ..........................................................................199 7.1.2 Additive directional arrays ...........................................................................................................204 7.1.3 Complex directional arrays .........................................................................................................206 7.1.4 Bilateral directivity .......................................................................................................................213 7.2 Quantifying Directivity ....................................................................................................................215 7.2.1 2D and 3D directivity index .........................................................................................................215 7.2.2 AI-DI ............................................................................................................................................217 7.3 Directional Benefit ..........................................................................................................................218 7.3.1 Impact of listening environment ..................................................................................................219 7.3.2 Objective benefit in the clinic and self reported benefit in real life ..............................................221 7.3.3 Interaction of directivity with other technologies .........................................................................222 7.3.4 Disadvantages of directivity .......................................................................................................223 7.3.5 Candidates for directional microphones .....................................................................................223 7.3.6 Evaluation of directional microphones in the clinic .....................................................................224 7.4 Concluding comments ...................................................................................................................225 8 ADVANCED SIGNAL PROCESSING SCHEMES .................................................................................226 8.1 Adaptive Noise Reduction..............................................................................................................227 8.1.1 Adaptive noise reduction technology ..........................................................................................227 8.1.2 Adaptive noise reduction benefits ...............................................................................................232 8.1.3 Impulse noise reduction ..............................................................................................................233 8.2 Feedback Reduction ......................................................................................................................234 8.2.1 Feedback reduction by gain-frequency response control ...........................................................234 8.2.2 Feedback reduction by phase control .........................................................................................235 8.2.3 Feedback reduction by feedback path cancellation ....................................................................236 8.2.4 Feedback reduction by frequency shifting ..................................................................................238 8.2.5 Feedback reduction systems in combination ..............................................................................238 8.3 Frequency Lowering ......................................................................................................................239 8.3.1 Frequency lowering rules ...........................................................................................................239 8.3.2 Frequency lowering techniques ..................................................................................................240 8.3.3 Commercially available frequency lowering schemes ................................................................242 8.3.4 Frequency lowering, speech intelligibility and candidacy ...........................................................242 8.4 Speech Cue Enhancement ............................................................................................................245 8.5 Other Signal Processing Schemes ................................................................................................247 8.6 Concluding Comments ..................................................................................................................253 9 ASSESSING CANDIDACY FOR HEARING AIDS .................................................................................255 9.1 Factors Affecting the Lower Limit of Aidable Hearing Loss ............................................................258 9.1.1 Attitude and motivation ...............................................................................................................258 9.1.2 Pure tone loss and audiogram configuration ..............................................................................261 hearing aids.indb 8 3/27/2012 9:48:15 AM Detailed Contents ix 9.1.3 Speech identification ability ........................................................................................................263 9.1.4 Self reported disability ................................................................................................................263 9.1.5 Acceptance of noise ...................................................................................................................264 9.1.6 Listening environment, needs and expectations ........................................................................265 9.1.7 Stigma and cosmetic concerns ...................................................................................................268 9.1.8 Manipulation and management ..................................................................................................269 9.1.9 Age ...........................................................................................................................................270 9.1.10 Personality ................................................................................................................................271 9.1.11 Central auditory processing disorders ......................................................................................272 9.1.12 Tinnitus .....................................................................................................................................272 9.1.13 Factors in combination .............................................................................................................273 9.1.14 Counseling the unwilling patient: some examples ....................................................................274 9.2 The Upper Limit of Aidable Hearing Loss ......................................................................................278 9.2.1 Poor speech identification ability ................................................................................................278 9.2.2 Hearing aids or cochlear implants? ............................................................................................279 9.2.3 Hearing aids and cochlear implants: bimodal and hybrid / electroacoustic stimulation ..............280 9.2.4 Hearing aids or tactile aids? .......................................................................................................283 9.3 Medically Related Contra-indications to Hearing Aid Fitting ..........................................................284 9.4 Concluding Comments ..................................................................................................................284 10 PRESCRIBING HEARING AID AMPLIFICATION ..................................................................................286 10.1 General Concepts Behind a Prescriptive Approach and a Brief History ......................................287 10.2 Gain and Frequency Response Prescription for Linear Amplification..........................................290 10.2.1 POGO .......................................................................................................................................290 10.2.2 NAL ...........................................................................................................................................290 10.2.3 DSL ...........................................................................................................................................292 10.2.4 Examples and comparisons: POGO II, NAL-RP and DSL ........................................................294 10.3 Difficult Issues in Prescription ......................................................................................................297 10.3.1 Acclimatization and adaptation to gain and frequency response ..............................................297 10.3.2 Preferred loudness ...................................................................................................................298 10.3.3 Dead regions ............................................................................................................................298 10.3.4 Severe hearing loss, effective audibility and high-frequency amplification ...............................301 10.3.5 Prescribing compression thresholds .........................................................................................304 10.3.6 Need for accuracy in prescription .............................................................................................306 10.4 Gain, Frequency Response, and Input-output Functions for Non-linear Amplification ................307 10.4.1 LGOB ........................................................................................................................................308 10.4.2 IHAFF/Contour .........................................................................................................................308 10.4.3 ScalAdapt .................................................................................................................................309 10.4.4 FIG6 ..........................................................................................................................................310 10.4.5 DSL[i/o] and DSLm[i/o] ..............................................................................................................311 10.4.6 NAL-NL1 and NAL-NL2 ............................................................................................................313 10.4.7 CAMREST, CAMEQ and CAMEQ2-HF ....................................................................................315 10.4.8 Comparison of procedures .......................................................................................................315 10.5 Allowing for Conductive and Mixed Hearing Losses ....................................................................319 10.6 Selecting Options for Multi-memory Hearing Aids .......................................................................321 10.6.1 Music programs ........................................................................................................................322 10.6.2 Candidates for multi-memory hearing aids ...............................................................................322 hearing aids.indb 9 3/27/2012 9:48:16 AM

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