Hearing Aids

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
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
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
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
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
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
10.7 Prescribing OSPL90.....................................................................................................................323
10.7.1 General principles: avoiding discomfort, damage and distortion .............................................. 323
10.7.2 Type of limiting: compression or peak clipping.......................................................................... 324
10.7.3 OSPL90 prescription procedures.............................................................................................. 324
10.7.4 Prescribing OSPL90 at different frequencies............................................................................ 329
10.7.5 OSPL90 for non-linear hearing aids.......................................................................................... 331
10.7.6 OSPL90 for conductive and mixed losses................................................................................. 331
10.8 Excessive Amplification and Subsequent Hearing Loss...............................................................332
10.9 Concluding Comments.................................................................................................................334
11 SELECTING, ADJUSTING AND VERIFYING HEARING AIDS ..............................................................336
11.1 Selecting Hearing Aid Style: CIC, ITC, ITE, BTE, Spectacle Aid, or Body Aid..............................337
11.2 Selecting Hearing Aid Features....................................................................................................342
11.3 Hearing Aid Selection and Adjustment..........................................................................................346
11.4 Allowing for Individual Ear Size and Shape in the Coupler Prescription.......................................349
11.5 Verifying and Achieving the Prescribed Real-ear Response.........................................................350
11.6 Verifying Signal Processing Features...........................................................................................351
11.7 Evaluating and Fine-tuning OSPL90.............................................................................................352
11.8 Concluding Comments..................................................................................................................353
12 PROBLEM SOLVING AND FINE-TUNING ..............................................................................................354
12.1 Solving Common Problems..........................................................................................................355
12.1.1 Management difficulties............................................................................................................. 355
12.1.2 Earmold or earshell discomfort.................................................................................................. 356
12.1.3 Poor earmold or earshell retention............................................................................................ 357
12.1.4 Own voice quality and occlusion............................................................................................... 357
12.1.5 Feedback oscillation.................................................................................................................. 358
12.1.6 Tonal quality............................................................................................................................... 360
12.1.7 Noise, clarity, and loudness....................................................................................................... 362
12.2 Systematic Fine-tuning Procedures .............................................................................................365
12.2.1 Paired comparisons................................................................................................................... 365
12.2.2 Absolute rating of sound quality ................................................................................................ 366
12.2.3 Systematic selection by paired comparisons............................................................................. 367
12.2.4 Adaptive parameter adjustment by paired comparisons............................................................ 369
12.2.5 Adaptive fine-tuning by absolute rating of quality...................................................................... 370
12.2.6 Fine-tuning at home with multi-memory or trainable hearing aids............................................. 372
12.3 Concluding Comments: Fine-tuning in Perspective......................................................................372
13 PATIENT EDUCATION AND COUNSELING FOR HEARING AID WEARERS ......................................374
13.1 Understanding Hearing Loss........................................................................................................376
13.2 Acquiring a Hearing Aid................................................................................................................377
13.3 Using Hearing Aids.......................................................................................................................378
13.4 Adjusting to New Experiences with Sound and Hearing Aids.......................................................379
13.5 Care of Hearing Aids.....................................................................................................................383
13.6 Hearing Strategies........................................................................................................................384
13.6.1 Observing the talker and surroundings...................................................................................... 384
13.6.2 Manipulating social interactions................................................................................................. 385
13.6.3 Manipulating the environment................................................................................................... 387
13.6.4 Teaching hearing strategies....................................................................................................... 388
13.7 Involving Families and Friends. ...................................................................................................389
13.8 Auditory Training...........................................................................................................................390
13.9 Computer-Based Auditory Training at Home................................................................................392
13.10 Avoiding Hearing Aid-Induced Hearing Loss................................................................................392
13.11 Assistive Listening Devices..........................................................................................................393
13.12 Counseling Support......................................................................................................................393
13.13 Interacting with Different Personality Styles.................................................................................393
13.14 Structuring Appointments.............................................................................................................395
13.14.1 The assessment appointment(s)............................................................................................. 396
13.14.2 The fitting appointment(s)........................................................................................................ 396
13.14.3 The follow-up appointment(s).................................................................................................. 397
13.14.4 The power of groups................................................................................................................ 398
13.15 Concluding Comments.................................................................................................................401
14 ASSESSING THE OUTCOMES OF HEARING REHABILITATION ........................................................403
14.1 Outcome Domains........................................................................................................................404
14.2 Speech Understanding Tests........................................................................................................405
14.2.1 Limitations of speech tests to assess benefits.......................................................................... 405
14.2.2 Role of speech testing in evaluating benefit.............................................................................. 406
14.3 Self-report Questionnaires for Assessing Benefit ........................................................................407
14.3.1 Questionnaire methodology....................................................................................................... 407
14.3.2 Practical self-report measures ...................................................................................................411
14.4 Meeting Needs and Goals............................................................................................................414
14.5 Assessing Usage, Problems, and Satisfaction.............................................................................417
14.6 The International Outcomes Inventory for Hearing Aids (IOI-HA).................................................421
14.7 Changes in Outcomes with Time after Fitting...............................................................................423
14.8 Impact of Hearing Loss and Hearing Aids on Health-Related Quality of Life...............................425
14.8.1 Effect of hearing loss on health-related quality of life................................................................ 425
14.8.2 Effect of hearing aids on health-related quality of life................................................................ 425
14.9 Concluding Comments.................................................................................................................428
15 BINAURAL AND BILATERAL CONSIDERATIONS IN HEARING AID FITTING ...................................430
15.1 Binaural Effects in Localization ....................................................................................................432
15.1.1 Localization cues in normal hearing.......................................................................................... 432
15.1.2 Effects of hearing loss on localization....................................................................................... 435
15.2 Binaural Effects in Detection and Recognition..............................................................................436
15.2.1 Head diffraction effects.............................................................................................................. 437
15.2.2 Binaural squelch in noise........................................................................................................... 438
15.2.3 Binaural redundancy.................................................................................................................. 440
15.2.4 Binaural loudness summation................................................................................................... 441
15.3 Advantages of Bilateral Fittings....................................................................................................442
15.3.1 Speech intelligibility................................................................................................................... 442
15.3.2 Localization................................................................................................................................ 444
15.3.3 Sound quality............................................................................................................................. 448
15.3.4 Avoiding late-onset auditory deprivation.................................................................................... 448
15.3.5 Suppression of tinnitus.............................................................................................................. 450
15.3.6 Miscellaneous advantages........................................................................................................ 450
15.4 Disadvantages of Bilateral Fittings...............................................................................................450
15.4.1 Cost........................................................................................................................................... 450
15.4.2 Binaural interference................................................................................................................. 451
15.4.3 Self-image................................................................................................................................. 453
15.4.4 Miscellaneous disadvantages.................................................................................................... 453
15.5 Tests of Bilateral Advantage.........................................................................................................454
15.5.1 Bias in choosing the reference ear for the unilateral condition.................................................. 455
15.5.2 The sensitivity of speech tests for assessing bilateral advantage............................................. 455
15.5.3 Role for speech tests in assessing bilateral advantage............................................................. 456
15.5.4 Localization tests....................................................................................................................... 458
15.6 Fitting Asymmetrical Hearing Losses............................................................................................458
15.6.1 Bilateral versus unilateral fittings for asymmetrical losses........................................................ 458
15.6.2 Better ear versus poorer ear for unilateral fittings..................................................................... 460
15.6.3 Alternatives: FM and CROS...................................................................................................... 462
15.7 Deciding on Bilateral versus Unilateral Fittings............................................................................462
15.8 Effect of Bilateral versus Unilateral Fitting on Electroacoustic Prescriptions................................466
15.9 Concluding Comments.................................................................................................................467
16 SPECIAL HEARING AID ISSUES FOR CHILDREN ...............................................................................469
16.1 Sensory Experience, Sensory Deprivation, and Candidacy for Hearing Aids..............................470
16.1.1 Binaural stimulation................................................................................................................... 470
16.1.2 Unilateral loss............................................................................................................................ 471
16.1.3 Slight and mild hearing loss....................................................................................................... 473
16.1.4 Cochlear implantation................................................................................................................ 473
16.1.5 Auditory neuropathy spectrum disorder..................................................................................... 474
16.2 Assessment of Hearing Loss........................................................................................................475
16.2.1 Frequency-specific and ear-specific assessment...................................................................... 475
16.2.2 Small ears and calibration issues.............................................................................................. 477
16.2.3 Auditory processing disorders................................................................................................... 478
16.2.4 Miscellaneous issues in assessment......................................................................................... 479
16.3 Hearing Aid and Earmold Styles...................................................................................................479
16.3.1 Hearing aid style........................................................................................................................ 479
16.3.2 Earmolds................................................................................................................................... 480
16.4 Prescribing Amplification for Children...........................................................................................481
16.4.1 Speech identification ability and amplification requirements..................................................... 481
16.4.2 Threshold-based versus loudness-based procedures............................................................... 485
16.4.3 Allowing for small ear canals..................................................................................................... 485
16.4.4 Signal processing features........................................................................................................ 490
16.4.5 Assistive listening devices (ALDs)............................................................................................. 493
16.5 Verifying Real-Ear Performance...................................................................................................494
16.6 Evaluating Aided Performance.....................................................................................................495
16.6.1 Speech tests.............................................................................................................................. 495
16.6.2 Paired-comparison tests............................................................................................................ 496
16.6.3 Evaluation of discomfort............................................................................................................ 497
16.6.4 Subjective report measures....................................................................................................... 498
16.6.5 Articulation Index or Speech Intelligibility Index (SII)................................................................. 500
16.6.6 Evoked cortical responses......................................................................................................... 502
16.6.7 Speech production and language acquisition............................................................................ 505
16.7 Helping Parents............................................................................................................................505
16.8 Hearing Habilitation Goals............................................................................................................507
16.8.1 Goals and strategies for infants................................................................................................. 508
16.8.2 Goals and strategies for toddlers............................................................................................... 509
16.8.3 Goals and strategies for pre-schoolers...................................................................................... 509
16.8.4 Goals and strategies for primary school children...................................................................... 510
16.9 Teenagers and Cosmetic Concerns............................................................................................510
16.10 Safety Issues.............................................................................................................................. 511
16.11 Concluding comments................................................................................................................512
17 CROS, BONE-CONDUCTION, AND IMPLANTED HEARING AIDS ......................................................513
17.1 CROS Hearing Aids......................................................................................................................514
17.1.1 Simple CROS aids..................................................................................................................... 514
17.1.2 Bilateral CROS (BICROS) aids................................................................................................. 518
17.1.3 Stereo CROS (CRIS-CROS) aids............................................................................................. 519
17.1.4 Transcranial CROS aids............................................................................................................ 519
17.2 Bone-conduction Hearing Aids.....................................................................................................520
17.2.1 Applications of bone-conduction hearing aids........................................................................... 520
17.2.2 Bone-conduction hearing aid output capabilities....................................................................... 521
17.2.3 Prescribing, adjusting and verifying electroacoustic characteristics for bone-conduction
hearing aids............................................................................................................................... 522
17.2.4 Disadvantages of bone-conduction hearing aids....................................................................... 525
17.3 Bone-Anchored Hearing Aids.......................................................................................................526
17.3.1 BAHAs for unilateral conductive or mixed hearing loss............................................................. 527
17.3.2 Bilateral BAHAs for bilateral conductive or mixed hearing loss................................................. 528
17.3.3 BAHAs for single-sided sensorineural deafness....................................................................... 528
17.3.4 Complications with BAHAs........................................................................................................ 529
17.4 Middle-ear Implantable Hearing Aids............................................................................................529
17.4.1 Output transducers.................................................................................................................... 530
17.4.2 Microphones.............................................................................................................................. 531
17.4.3 Complete systems..................................................................................................................... 531
17.4.4 Candidacy and benefits............................................................................................................. 534
17.4.5 Complications with middle-ear implants.................................................................................... 535
17.5 Concluding Comments.................................................................................................................535
REFERENCES ...............................................................................................................................................537
INDEX ............................................................................................................................................................597