Psychoacoustics of cochlear hearing impairment and the design of hearing aids

Abstract

Cochlear hearing impairment is usually associated with damage to the hair cells within the cochlea. When the damage is restricted to the outer hair cells (OHCs), the main consequence is disruption of the ‘‘active’’ mechanism which normally enhances the response of the basilar membrane to weak sounds and which sharpens the tuning (frequency selectivity) of the basilar membrane. Psychoacoustically, damage to OHCs results in loss of sensitivity (elevated absolute thresholds), loudness recruitment, and reduced frequency selectivity. Damage to the inner hair cells (IHCs) causes basilar membrane vibrations to be transduced less effectively, so absolute thresholds are elevated, but does not result in altered frequency selectivity or loudness recruitment. Sometimes, IHCs and/or neurons may be completely inoperative at certain places within the cochlea, giving rise to ‘‘dead regions.’’ Such regions can strongly influence the perception of pitch and loudness. Current hearing aids can partially compensate for the effects of loudness recruitment by using compression amplification, but there is much controversy about the ‘‘best’’ form of compression. The deleterious effects of reduced frequency selectivity on speech intelligibility in noise can be alleviated by various methods for improving the speech-to-noise ratio, although so far only directional microphones have given clear benefits.