Structural feedback analysis of a hearing aid using finite element and lumped parameters methods

Abstract

Hearing aids are designed to overcome hearing losses suffered by some individuals and operated by measuring and amplifying the acoustic field at certain frequency depending on the user hearing loss. In many cases, its transducers (microphone and loudspeaker) are near located, and a feedback signal phenomenon may occur, either through structural or airborne transmission. The structural feedback is characterized by a vibrational response at the microphone position produced by the loudspeaker, also called receiver, that result in a unwanted signal due to the microphone vibration sensitivity. If not properly controlled, the feedback signal builds up, resulting in a loud whistle at the user ear causing great discomfort. The objective of this work is to analyse the structural feedback mechanism at a behind-the-ear hearing aid, and proposed a numerical approach that allows the consideration of the phenomena during the design phase of such systems. A numerical model, based on the finite element method (FEM) and the lumped parameters method, was used to analyse and quantify the problem. While the hearing aid case was modelled using FEM, the receiver and its suspensions were represented by means of a single degree-of-freedom or a multiple degree-of-freedom systems. The procedures used to define material properties and dynamic parameters are presented and discussed. Experimental tests are used to validate the models separately and to determine vibration levels imposed at the microphone location during the hearing aid operation, which are used to verify the overall accuracy of the numerical model. Finally, a sensitivity analysis is performed, indicating some design options that can be used to minimize the structural feedback problem in hearing aids.