Effect of Hearing-Aid Gain Control on Acoustic Gain

It is generally accepted that a hearing aid should not operate at or near the maximum setting of the gain control where distortion is usually highest. This investigation examined the effect that the physical movement of this gain control has on the acoustic gain of hearing aids. Using a large group of commercially available aids, the gain-control knob of each instrument was divided into eight increments of 1212% and acoustic gain was measured at 500, 1000, and 2000 Hz at each increment. The input signals to the hearing aids were maintained at 70 dB SPL (sound-pressure level), an intensity compatable with acoustic levels encountered in every day situations. It was found that, under the conditions employed in this investigation, a nonlinear relationship was most often observed. For the body aids, average gain exceeded 50% with a 25% rotation of the gain control and exceeded 78% at the 50% setting of the control. Corresponding control settings for ear-level aids yielded 47% and 72% acoustic gain.