Hearing Aids. von Harvey Dillon. 1. Auflage. Hearing Aids Harvey Dillon schnell und portofrei erhältlich bei beck-shop.de DIE FACHBUCHHANDLUNG

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1 Hearing Aids von Harvey Dillon 1. Auflage Hearing Aids Harvey Dillon schnell und portofrei erhältlich bei beck-shop.de DIE FACHBUCHHANDLUNG Thematische Gliederung: Medizintechnik, Medizinische Werkstoffe Thieme 2001 Verlag C.H. Beck im Internet: ISBN

2 BRIEF CONTENTS PREFACE... XV 1 INTRODUCTORY CONCEPTS HEARING AID COMPONENTS HEARING AID SYSTEMS ELECTROACOUSTIC PERFORMANCE AND MEASUREMENT HEARING AID EARMOLDS, EAR SHELLS AND COUPLING SYSTEMS COMPRESSION SYSTEMS IN HEARING AIDS ADVANCED SIGNAL PROCESSING SCHEMES FOR HEARING AIDS ASSESSING CANDIDACY FOR HEARING AIDS PRESCRIBING HEARING AID PERFORMANCE SELECTING AND ADJUSTING HEARING AIDS PROBLEM SOLVING AND FINE-TUNING OF HEARING AIDS COUNSELING THE NEW HEARING AID WEARER ASSESSING THE OUTCOMES OF HEARING REHABILITATION BINAURAL AND BILATERAL CONSIDERATIONS IN HEARING AID FITTING SPECIAL HEARING AID ISSUES FOR CHILDREN CROS, BONE-CONDUCTION, AND IMPLANTED HEARING AIDS REFERENCES INDEX v

3 DETAILED CONTENTS PREFACE... xv 1 INTRODUCTORY CONCEPTS Problems Faced by People with Hearing Impairment Decreased audibility Decreased dynamic range Decreased frequency resolution Decreased temporal resolution Deficits in combination Acoustic Measurements Basic physical measures Linear amplifiers and gain Saturation sound pressure level Couplers and real ears Types of Hearing Aids Historical Perspective The acoustic era The carbon era The vacuum tube era The transistor and integrated circuit era The digital era HEARING AID COMPONENTS Block Diagrams Microphones Principle of operation Frequency response of microphones Microphone imperfections Directional microphones Microphone location Amplifiers Amplifier technology Amplifier performance Peak clipping and distortion Output amplifiers Compression amplifiers Digital Circuits Analog-to-digital converters Digital signal processors Digital-to-analog converters Tone Controls and Filters Filter and tone control structures...37 vi

4 Detailed Contents vii Passive, active, and digital tone controls Receivers Principle of operation Frequency response of receivers Acoustic Dampers Telecoils Audio (Electrical) Input Remote Controls Bone Conductors Batteries Principle of operation Operating voltage Capacity and physical size Concluding Comments HEARING AID SYSTEMS Custom and Modular Construction Custom hearing aids Modular hearing aids Semi-modular, semi-custom hearing aids Analog Hearing Aids Digitally Programmable Analog Hearing Aid Systems Programmers, interfaces, and software Multi-memory programmable hearing aids Paired comparisons Digital Hearing Aids Digital hard-wired hearing aids Digital general arithmetic processor hearing aids Sequential processing versus block processing Specifications for digital hearing aids Advantages of digital hearing aids Remote Sensing and Transmitting Hearing Aid Systems Induction Loops Field uniformity and direction Magnetic field strength Loop frequency response Radio-frequency Transmission Principles and FM capture effect Coupling to the hearing aid Combined FM and local microphones Infrared Transmission Classroom Sound-field Amplification Strengths and Weaknesses of Remote Transmission Systems Assistive Listening Devices Concluding remarks...73

5 viii HEARING AIDS 4 ELECTROACOUSTIC PERFORMANCE AND MEASUREMENT Measuring Hearing Aids in Couplers and Ear Simulators Couplers and ear simulators Test boxes Measurement signals Gain-frequency response and OSPL90-frequency response Input-output functions Distortion Internal noise Magnetic response ANSI and IEC standards Real-Ear Aided Gain (REAG) Positioning the probe for REAG measurement Relationship between REAG, coupler gain and ear simulator gain Detecting incorrect aided measurements Insertion Gain Positioning the probe for insertion gain measurement Relationship between insertion gain, coupler gain and ear simulator gain Control microphones and insertion gain Detecting incorrect insertion gain measurements Accuracy of insertion gain measurements Practical issues in real-ear testing Probe calibration Effects of wax Contamination by background noise Hearing aid saturation Loudspeaker orientation Measurement signal characteristics Aided Threshold Testing and Functional Gain Feedback in Hearing Aids The feedback mechanism Effects of feedback on sound quality Probe-tube measurements and feedback Troubleshooting Faulty Hearing Aids Concluding Comments HEARING AID EARMOLDS, EARSHELLS AND COUPLING SYSTEMS Earmold and earshell physical styles BTE earmold styles ITE, ITC, and CIC earshell styles Overview of Earmold and Earshell Acoustics Venting Effects of vents on hearing aid gain and OSPL Venting and the occlusion effect Effects of vents and leaks on feedback oscillation Parallel versus Y (or diagonal) vents...136

6 Detailed Contents ix 5.4 The Sound Bore: Tubing, Horns and Constriction Acoustic horns and constrictions Special-purpose earhooks Dampers Specific Tubing, Damping and Venting Configurations Procedure for Selecting Earmold and Earshell Acoustics Ear Impressions Standard ear impression techniques Ear impression techniques for CICs and high gain hearing aids Ear impression materials Earmolds and Earshells Earmold and earshell construction Materials for earmolds and earshells Instant earmolds and hearing aids Modifying and repairing earmolds and earshells Concluding Comments COMPRESSION SYSTEMS IN HEARING AIDS Compression s Major Role: Reducing the Signal s Dynamic Range Basic characteristics of a compressor Dynamic compression characteristics: attack and release times Static compression characteristics Input and output control Multichannel compression Rationales for Use of Compression Avoiding discomfort, distortion and damage Reducing inter-syllabic and inter-phonemic intensity differences Reducing differences in long-term level Increasing sound comfort Normalizing loudness Maximizing intelligibility Reducing noise Empirical approaches Combinations of Compressors in Hearing Aids Benefits and Disadvantages of Different Compression Systems Compression relative to linear amplification Benefits of multichannel relative to single-channel compression Concluding Comments ADVANCED SIGNAL PROCESSING SCHEMES Multi-Microphone and Other Directional Hearing Aids Fixed directional arrays Adaptive arrays Single-Microphone Noise Reduction Feedback Reduction Feedback reduction by gain-frequency response control Feedback reduction by phase control Feedback reduction by feedback path cancellation...201

7 x HEARING AIDS Feedback reduction by frequency shifting Frequency Transposition Speech Cue Enhancement Concluding Comments ASSESSING CANDIDACY FOR HEARING AIDS The Lower Limit of Aidable Hearing Loss Pure tone loss and audiogram configuration Speech identification ability Self reported disability and handicap Listening environment, needs and expectations Cosmetic concerns Manipulation and management Age Personality Central auditory processing disorder Tinnitus Factors in combination Counseling the unwilling patient: some examples The Upper Limit of Aidable Hearing Loss Poor speech identification ability Hearing aids or cochlear implants? Hearing aids or tactile aids? Medically Related Contra-indications to Hearing Aid Fitting Concluding Comments PRESCRIBING HEARING AID PERFORMANCE General Concepts Behind Prescription and a Brief History Gain and Frequency Response Prescription for Linear Amplification POGO NAL DSL Examples and comparisons: POGO II, NAL-RP and DSL Gain, Frequency Response, and Input-Output Functions for Nonlinear Amplification LGOB IHAFF/Contour Madsen Aurical method ScalAdapt FIG DSL[i/o] NAL-NL Comparison of procedures Prescribing compression thresholds Allowing for Conductive and Mixed Hearing Losses Selecting Options for Multi-memory Hearing Aids Response alternatives for different environments and listening criteria Candidates for multi-memory hearing aids...266

8 Detailed Contents xi 9.6 Prescribing OSPL General principles: avoiding discomfort, damage and distortion Type of limiting: compression or peak clipping OSPL90 prescription Prescribing OSPL90 at different frequencies OSPL90 for nonlinear hearing aids OSPL90 for conductive and mixed losses Excessive Amplification and Subsequent Hearing Loss Concluding Comments SELECTING AND ADJUSTING HEARING AIDS Selecting Hearing Aid Style: CIC, ITC, ITE, BTE, Spectacle Aid, or Body Aid Selecting Hearing Aid Features Volume control Telecoil Direct audio input Directional microphones Selecting a Signal Processing Scheme and Prescription Method Selecting a signal processing scheme Choosing a prescription procedure Overview of Hearing Aid Selection and Adjustment Twelve Steps for Selecting and Adjusting Programmable Hearing Aids Eleven Steps for Selecting and Adjusting Non-programmable Aids Allowing for Individual Ear Size and Shape in the Coupler Prescription Allowing for Surgically Altered Ear Canals Verifying and Achieving the Prescribed Real-ear Response Verifying and Fine-tuning OSPL Concluding Comments PROBLEM SOLVING AND FINE-TUNING OF HEARING AIDS Solving Common Problems Management difficulties Earmold or earshell discomfort Poor earmold or earshell retention Own voice quality and occlusion Feedback oscillation Tonal quality Noise, clarity, and loudness Systematic Fine-tuning Procedures Paired comparisons Absolute rating of sound quality Systematic selection by paired comparisons Adaptive parameter adjustment by paired comparisons Adaptive fine-tuning by absolute rating of quality Fine-tuning at home with multi-memory hearing aids Concluding Comments: Fine-tuning in Perspective...320

9 xii HEARING AIDS 12 COUNSELING THE NEW HEARING AID WEARER Understanding Hearing Loss Acquiring a Hearing Aid Using Hearing Aids Adjusting to New Experiences with Sound and Hearing Aids Care of Hearing Aids Hearing Strategies Observing the talker and surroundings Manipulating social interactions Manipulating the environment Teaching hearing strategies Involving Families and Friends Communication Training Avoiding Hearing Aid-Induced Hearing Loss Assistive Listening Devices Ongoing Support Counseling Styles Structuring Appointments The assessment appointment(s) The fitting appointment(s) The follow-up appointment(s) The power of groups Concluding Comments ASSESSING THE OUTCOMES OF HEARING REHABILITATION Speech Identification Testing Limitations of speech tests to assess benefits Role of speech testing in evaluating benefit Self-report Questionnaires for Assessing Benefit Questionnaire methodology Practical self-report measures Meeting Needs and Goals Assessing Usage, Problems, and Satisfaction Changes in Outcomes with Time after Fitting Impact of Hearing Aids on General Health and Quality of Life Concluding Comments BINAURAL AND BILATERAL CONSIDERATIONS IN HEARING AID FITTING Binaural Effects in Localization Localization cues in normal hearing Effects of hearing loss on localization Binaural Effects in Detection and Recognition Head diffraction effects Binaural squelch in noise Binaural redundancy Binaural loudness summation Advantages of Bilateral Fittings...380

10 Detailed Contents xiii Speech intelligibility Sound quality Avoiding late-onset auditory deprivation Localization Suppression of tinnitus Miscellaneous advantages Disadvantages of Bilateral Fittings Cost Binaural interference Self-image Miscellaneous disadvantages Tests of Bilateral Advantage Bias in choosing the reference ear for the unilateral condition The sensitivity of speech tests for assessing bilateral advantage Role for speech tests in assessing bilateral advantage Localization tests Fitting Asymmetrical Hearing Losses Bilateral versus unilateral fittings for asymmetrical losses Better ear versus poorer ear for unilateral fittings Alternatives: FM and CROS Deciding on Bilateral versus Unilateral Fittings Effect of Bilateral vs Unilateral Fitting on Electroacoustic Prescriptions Concluding Comments SPECIAL HEARING AID ISSUES FOR CHILDREN Sensory Experience and Deprivation Assessment of Hearing Loss Frequency-specific and ear-specific assessment Small ears and calibration issues Hearing Aid and Earmold Features and Styles Hearing aids Earmolds FM wireless transmission systems Prescribing Amplification for Children Speech identification ability and amplification requirements Threshold-based versus loudness-based procedures Allowing for small ear canals Verifying Real-ear Performance Evaluating Aided Performance Helping Parents Hearing Habilitation Goals Goals and strategies for infants Goals and strategies for toddlers Goals and strategies for pre-schoolers Goals and strategies for primary school children Teenagers and Cosmetic Concerns Safety Issues Concluding Comments...433

11 xiv HEARING AIDS 16 CROS, BONE-CONDUCTION, AND IMPLANTED HEARING AIDS CROS Hearing Aids Simple CROS aids Bilateral CROS (BICROS) aids Stereo CROS (CRIS-CROS) aids Transcranial CROS aids Bone-conduction Hearing Aids Applications of bone-conduction hearing aids Bone-conduction hearing aid output capabilities Prescribing electroacoustic characteristics for bone-conduction aids Disadvantages of bone-conduction hearing aids Implanted and Semi-implanted Hearing Aids Bone-anchored hearing aids Middle-ear implants Concluding Comments REFERENCES INDEX