Digital Hearing Aids

Hearing-Aid Technology

Types of Hearing Aids

Canal Aids
In-the-Ear Aids
Behind-the-Ear Aids
Open Fitting BTE Aids
Body-Worn Aids

From Analog to Digital

Moore’s Law
Digital Current Drain
Analog and Digital Power Consumption

Digital Circuit Components

Analog-to-Digital Converter
Digital Signal Processor
Memory
Clock

Batteries
Concluding Remarks
References

Signal Processing Basics

Signal and System Properties

Sequences
Linear Time Invariance
Convolution
Correlation
Transfer Functions
Feedback
Group Delay
Amplitude Modulation
Distortion

Discrete Fourier Transform
Filters and Filter Banks

Filter Definitions
Recursive Filters
Non-Recursive Filters
Frequency Resolution and Group Delay

Block Processing
Digital System Concerns

Integer Arithmetic
Quantization Noise
Aliasing Distortion
Temporal Aliasing
Algorithm Complexity
Algorithm Interaction

Concluding Remarks
References

The Electroacoustic System

Hearing Aid System
Head and Ear
Microphone and Receiver
Vent Acoustics

Acoustic Elements
Vent Size
Residual Ear-Canal Volume
Open BTE Fitting

Occlusion Effect
Concluding Remarks
References

Directional Microphones

Hearing-Aid Microphones

Microphone Construction
Directional Microphones and Spatial Signal Processing

Directional Response Patterns

Microphone Spacing and Time Delay
Microphone Mismatch

Frequency Response
Magnitude Frequency Response
Microphone Mismatch
Interaction with Vents
Microphone Noise

Microphones on the Head
Microphone Performance Indices

Front-to-Back Ratio
Directivity Index
Unidirectional Index The AI-DI

Rooms and Reverberation

Critical Distance
Early Reflections
Combining Noise and Reverberation

Benefit in the Real World
Concluding Remarks
References

Adaptive and Multi-Microphone Arrays

Two-Microphone Adaptive Array

Adaptive Delay
Adaptive Gain
Adaptive Filter

Delay-And-Sum Beamforming
Adaptive Arrays
Superdirective Arrays
Widely-Spaced Arrays
Array Benefits
Concluding Remarks
References

Wind Noise

Turbulence
Hearing-Aid Measurements

Air Flow and Turbulence
Sound Pressure

Signal Characteristics

Spectrum
Temporal Fluctuations
Correlation

Wind-Noise Reduction

Directional Microphones
Wind Screens
Spectrum Algorithms
Correlation Algorithms
Adaptive Arrays

Concluding Remarks
References

Feedback Cancellation

The Feedback System

System Equations
System Response

Gain-Reduction Solutions
Adaptive Feedback Cancellation

System Equations
LMS Adaptation
Constrained Adaptation

Adaptation Equations
Initialization
Simulation Results

Decorrelation Techniques

Interrupted Adaptation
Delay
Filtered-X Algorithm

Frequency-Domain Adaptation

Processing Limitations

Room Reflections

Measurement Procedure
Measurement Results
Maximum Stable Gain

Non-Linear Distortion

Concluding Remarks
References

Dynamic-Range Compression

Does Compression Help?
Algorithm Design Concerns

Frequency Resolution
Processing Delay

Single-Channel Compression

Input/Output Rules
Volume Control
Envelope Detection

Multi-Channel Compression

Temporal Response
Swept Frequency Response

Frequency-Domain Compression

Ideal FFT System
Practical FFT System
Side-Branch Structure

Frequency Warping

Digital Frequency Warping
Warped Compressor System
System Delay Comparison

Concluding Remarks
References

Single-Microphone Noise Suppression

Properties of Speech and Noise Signals
Low-Level Expansion
Envelope Valley Tracking
Bandwidth Reduction

Adaptive High-Pass Filter
Adaptive Low-Pass Filter
“Zeta Noise Blocker”

Envelope Modulation Filters
Concluding Remarks
References

Spectral Subtraction

Noise Estimation

Valley Detection
Minima Statistics
Histogram

Wiener Filter
Spectral Subtraction

Classical Approaches
General Equation
Nonlinear Expansion
Ephraim-Malah Algorithm
Auditory Masking
A Fundamental Compromise

Algorithm Effectiveness
Concluding Remarks
References

Spectral Contrast Enhancement

Auditory Filters in the Damaged Cochlea

Physiological Measurements

Tuning Curves
Neural Firing Patterns

Perceptual Measurements
Processing Strategies

Spectral Valley Suppression
Spectral Contrast Modification

Raise Spectrum to a Power
Spectral Filtering

Excess Upward Spread of Masking
F2/F1 Ratio
Processing Comparison
Combining Spectral Contrast Enhancement with Compression
Concluding Remarks
References

Sound Classification

The Rationale for Classification
Signal Features
Feature Selection

Mutual Information
Feature Selection Using Mutual Information

Classifier Algorithms

Training the Classifier
Types of Classifiers
Clustering
Linear Discriminant
Support Vector Machine
Neural Network
Hidden Markov Model
Bayes Classifier
Personalizing the Classifier

Classification Examples

Number of Features
Isolated Signals vs. Mixtures

Concluding Remarks
References

Binaural Signal Processing

The “Cocktail Party” Problem
Signal Transmission
Binaural Compression

Compression Using Binaural Loudness Summation
Auditory Efferents
Compression Using Auditory Efferents

Binaural Noise Suppression

Interaural Cross-Correlation
Binaural Wiener Filter
Binaural Signal Difference
Binaural Spectral Subtraction
Directional Cues
Localization Model

Dichotic Band Splitting
Concluding Remarks
References

Index

ABOUT THE AUTHOR
James M. Kates, E.E. holds the position of Research Fellow with hearing-aid manufacturer GN ReSound. He is also an Adjunct Professor in the Department of Speech Language and Hearing Sciences at University of Colorado in Boulder. Prior to moving to Colorado, he was Research Scientist in the laboratory of Harry Levitt at the City University of New York. He received the BSEE and MSEE degrees from the Massachusetts Institute of Technology in 1971, and the professional degree of Electrical Engineer from M.I.T. in 1972. He has developed many of the signal-processing algorithms implemented in the GN family of hearing aids, including the DFS feedback cancellation algorithm and the Warp-17 frequency-warped dynamic-range compression algorithm. He is a Fellow of the Acoustical Society of America and a Fellow of the Audio Engineering Society, and is the author or co-author of 47 technical papers and holds 17 US patents with seven patents pending. A widower, he resides in Niwot, Colorado.


AUDIENCE
Primary: Audiology

1. Modern Hearing Aids-Fundamentals
   Edited by Todd Ricketts, Ph.D. and H. Gustav Mueller, Ph.D.
2. Strategic Practice Management
   Edited by Robert G. Glaser and Robert M. Traynor Ed.D., MBA.