Open‐loop transfer function simulation as a means for understanding acoustic feedback in hearing aids

Abstract

The “squeal” of unstable acoustic feedback in hearing aids occurs when Nyquist's stability criterion involving the system's open‐loop transfer function (OLTF) has been violated. To understand the causes and possible remedies of unstable feedback in hearing aids, one must first investigate the OLTF of such a system. Reported herein is a scheme for simulating, on the computer, the OLTF of an eyeglass‐type hearing aid. This scheme consists of taking the serial product of two‐port network models of the various components which transduce, amplify, and transmit sound to the eardrum. These components are (in order) the microphone, amplifier, receiver, sound tubes leading to the eardrum, earmold vent, and external pathway from the vent exit back to the microphone. Computer simulations were compared with laboratory measurements of the OLTF. These were obtained by applying an electrical signal to the amplifier input and simultaneously monitoring electrical output of the hearing‐aid microphone. Computer results and ...