Signal Processing for Hearing Aids

Abstract

This chapter deals with signal processing for hearing aids. The primary goal of a hearing aid is to improve the understanding of speech by an individual with a hearing impairment, although the perception of music and environmental sounds is also a concern. The basic signal-processing system consists of linear filtering followed by amplification, with more sophisticated techniques used to try to compensate for the nature of the hearing impairment and to improve speech intelligibility in noise.20

The chapter starts with a review of auditory physiology and the nature of hearing loss. Linear amplification systems are then discussed along with hearing-aid design objectives and the limitations of conventional technology. Feedback cancellation, which can improve hearing-aid system stability, is presented next. Dynamic-range compression is an important signal-processing approach since the impaired ear has a reduced dynamic range in comparison with the normal ear, and single-channel and multi-channel compression algorithms are described. Noise suppression is also a very important area of research, and several single-microphone approaches are described, including adaptive analog filters, spectral subtraction, and spectral enhancement. Multi-microphone noise-suppression techniques, such as adaptive noise cancellation, are discussed next. Noise can be more effectively suppressed using spatial filtering, and directional microphones and multi-microphone arrays are described. The chapter concludes with a brief summary of the work being done in cochlear implants.