Chapter 7 — Hearing aids
DSP, WDRC, directional processing, feedback cancellation.
A modern hearing aid is a small but extraordinarily sophisticated computer attached to the ear. Two omnidirectional microphones sample the acoustic environment 20,000–48,000 times per second. The samples flow through a digital signal-processing pipeline that performs spectral analysis, frequency-dependent gain shaping, dynamic-range compression, directional beamforming, noise reduction, feedback cancellation, and frequency lowering — all at audio latencies under 6 milliseconds, all in a few cubic centimetres of plastic, all running on a battery that lasts several days. The DSP chip in a 2026 hearing aid runs at 10s of GFLOPS and consumes about 1 mW.
This chapter develops the modern hearing aid algorithmically. We start with the DSP pipeline as a whole — the block diagram from microphone to receiver — then drill into three of its most important components: wide-dynamic-range compression (the technique that fits the 80 dB range of real-world acoustic levels into the patient’s narrowed residual dynamic range), directional microphones (the two-mic beamformer that gives 3–6 dB of SNR improvement against background noise), and feedback cancellation (the adaptive-filter algorithm that lets a small open-fit hearing aid apply 40 dB of gain without howling).
The chapter is the longest in this book; hearing aids are the workhorse intervention that the rest of the toolkit ultimately exists to support. Four lessons:
- 7.1 The hearing-aid DSP pipeline — block diagram, latencies, the role of each stage, the form-factor tradeoffs (BTE, RIC, ITE, IIC, CROS).
- 7.2 Wide-dynamic-range compression — the kneepoint/ratio/MPO triplet, multichannel compression, attack/release, NAL-NL2 and DSL v5 prescriptions.
- 7.3 Directional microphones and noise reduction — first-order patterns, adaptive directionality, single-channel statistical noise reduction.
- 7.4 Feedback cancellation and frequency lowering — closed-loop oscillation criterion, adaptive feedback cancellation, frequency lowering for dead high-frequency regions.