Effect of phase compensation and damping on the open loop transfer function of a vented hearing aid worn in situ

Abstract

An in‐the‐ear hearing aid having 120‐dB peak SPL, 50‐dB peak gain, and a 3‐mm‐diam vent mounted on Kemar was treated as a feedback control system. The feedback loop was broken between microphone and amplifier to obtain the magnitude and phase of the open loop transfer function and hence the proximity of the fitting to onset of acoustical feedback oscillation. With the loop closed, as the gain control was adjusted, the hearing aid generally oscillated if the open loop magnitude exceeded 0 dB. The frequency of acoustic feedback oscillation correlated fairly well to the frequency at which the open loop magnitude crossed 0 dB with gain decreasing. The same measurements were repeated with a phase compensator added between the hearing aid microphone and amplifier and with an acoustic resistance damper added in the tubing of the hearing aid receiver. Both the phase compensator and damper improved phase margin and hence rendered the hearing aid fitting less likely to oscillate.