Is AGC beneficial in hearing aids?

Three different functions of Automatic Gain Control (AGC) circuits in hearing aids are distinguished and the evidence for their benefits is considered. The value of AGC's function as a relatively distortion-free means of limiting output has been well established. With regard to compression, the benefit of short-term or 'syllabic' compression has not been demonstrated convincingly. Most evaluations of this type of AGC have looked for increase in speech intelligibility, but theoretical predictions of its effect do not appear to take account of the acoustic cues to consonant contrasts actually used by hearing impaired people, and empirical studies have often used listening conditions which do not give a realistic test of benefit. Relatively little attention has been paid to long-term compression, or to the effect of AGC on comfort rather than intelligibility. Listening tests carried out at the RNID and reported here have shown that AGC can benefit hearing aid users by allowing them to listen to a wider range of sound levels without either strain or discomfort, and, if time constants are well chosen, without adverse effects on speech intelligibility in quiet or in noise.