Hearing Aids: What Works Well and What Can Be Improved.

This paper evaluates the current performance of hearing aids, based on research findings and my experiences with hearing aids. The type of acoustic coupling to the ear is important. The fitting can be "closed" (sealing the ear canal), but this can lead to the occlusion effect; the user's own voice sounds too loud or too boomy. Alternatively, the fitting can be open (the eartip has a vent). This alleviates the occlusion effect, but it introduces comb-filtering (heard as perceptual coloration) and leads to little or no gain at low frequencies. Also, the highest frequency at which useful gain can be achieved is often about 5 kHz, which is lower than optimal. While acoustic feedback cancellation systems have improved markedly, they can still introduce artifacts and impair sound quality, especially for music. Hearing aids use multi-channel amplitude compression to compensate for the reduced dynamic range of hearing-impaired people, but they often fail to restore the audibility of soft sounds, especially at high frequencies, and the amount of compression is often limited (and less than indicated by the manufacturers' fitting software), leading to loudness discomfort (and sometimes reduced speech intelligibility) at high sound levels. Also, compression systems introduce cross-modulation, impairing sound quality. Most hearing aids incorporate directional processing and noise-reduction systems intended to improve the ability to understand speech in noisy situations. These systems can be effective with a closed fitting, but much of the benefit is lost with an open fitting because of leakage of background sounds through the vent.