A comparison of four methods of implementing automatic gain control (AGC) in hearing aids.

Hearing impairment of cochlear origin is usually associated with loudness recruitment. As a consequence, the dynamic range between threshold and the highest comfortable level is smaller than normal. To ensure that low-level sounds can be heard, while avoiding discomfort at high levels, a hearing aid with automatic gain control (AGC) is required. This paper compares four different systems for implementing AGC, and compares each of them with unaided listening and with linear amplification. The systems were evaluated by measuring thresholds for understanding speech in quiet and in five types of background sound: speech-shaped noise, 12-talker babble, cafeteria noise, traffic noise and a single competing speaker. The first system used a new dual-action AGC (called dual front-end AGC) operating on the whole speech signal. A slow-acting control voltage (recovery time 5 s) held the average level of speech at the output constant, regardless of the input level. In response to sudden intense transients, a fast-acting control voltage (recovery time 150 ms) reduced the gain rapidly and then returned the gain to the value set by the slow-acting component. In the second system, referred to as the mark II aid, the output of the dual front-end AGC was split into two frequency bands, and fast-acting (syllabic) compression was applied in the high-frequency band only. The bands were then recombined. The third system resembled the mark II aid except that fast-acting compression was applied in both bands. The fourth system resembled the 2-channel aid evaluated in previous trials (Moore, 1987). It was similar to the third system, but had only single-action front-end AGC with a recovery time of 400 ms. Six subjects with moderate sensorineural hearing loss accompanied by recruitment were used. Best results overall were obtained using the mark II aid. Speech reception thresholds (SRTs) in noise were, on average, 4 dB better than for linear amplification and 2.4 dB lower than for the previous 2-channel aid. There was a significant advantage of having fast-acting AGC in the high-frequency band, but no advantage of having AGC in the low-frequency band.