System identification of feedback in hearing aids

The feedback problems of behind the ear (BTE), in the ear (ITE), and in the ear canal (ITEC) hearing aid categories have been investigated. All possib le feedback paths (acoustical via vent, via tubing wall, mechanical, etc.) were converted to a single transfer function from the ear canal to the hear ing aid microphone, here called the acoustic feedback equivalent (AFE). The attenuation of the AFE represents the maximum gain that can be used withou t the hearing aid starting to howl. Magnitude and phase responses of the AF E were identified on ten human subjects and on a Knowles ear manikin (KEMAR ). The acoustic feedback via vent and leak between earmould and ear canal d ominated the AFE. The transfer function from a reference point under the ea r to the position of microphone of the different hearing aid categories was identified and used together with the AFE to calculate the maximum real ea r aided gain (REAG) for the hearing aid categories. A model of the AFE, con sisting of a fourth-order filter together with a delay, showed good agreeme nt with the measured data. (C) 1999 Acoustical Society of America. [S0001-4 966(99)02506-0].