Acoustic feedback in hearing aids has received little attention in the literature. Feedback occurs when stability conditions of the open-loop transfer function of an in situ hearing aid are violated. Solving the feedback problem will first require knowledge of the open-loop transfer function. Included in the open-loop transfer function is the acoustical path by which sound emanating from the earmold vent returns to the microphone (i.e., the feedback path). Reported herein are two different mathematical procedures for simulating transfer functions of the feedback path of an eyeglass-type hearing aid. In one procedure the vent exit was modeled as a point source of sound located on a flat plane, while it was treated as a point source on a sphere in the other. Results of laboratory experiments indicate that the mathematical models accurately predict those acoustic phenomena for which they were intended: point sources on plane and spherical baffles. Results of manikin experiments showed both models to be less accurate for simulating the feedback path around the human head. The maximum difference between experiment and theory was 6 dB at one frequency. Surprisingly, the flat-baffle model produced better agreement with experimental results than did the sphere model.

The acoustic feedback which causes a hearing aid to become unstable at high gains is effectively reduced by the addition of negative electrical feedback. Electrical feedback is formed by a system estimator which closely matches the open-loop transfer function of an in situ hearing aid. The system estimator is produced by correlating the estimator signal with the hearing-aid signal. Because the estimator does not affect the characteristics of the hearing aid, speech intelligibility is not degraded by this feedback suppression circuit as it is by most other schemes. Thus 10 to 15 dB of additional stable gain have been achieved through the use of stabilizing negative electrical feedback.