Individual Equalization of Hearing Aids with Integrated Ear Canal Microphones

摘要

In this paper, the estimation of frequency responses suitable to be employed in individual hearing aid equalization is investigated, using hearing aids which contain integrated commercial ear canal microphones. In order to be applicable in clinical settings, special attention is paid to minimize the on-site effort, both in terms of equipment and the number of measurements required. Firstly, methods to predict the sound pressure at the individual ear drum are reviewed and simplified for cases where the interaction of the individual ear with the source can be ignored. In these cases, the prediction can be based on just one measurement of the acciustic input impedance of the ear canal, which can be performed using the hearing aid on the patient. Secondly, the said measurement of the acoustic input impedance of the ear canal using the hearing aid was investigated. A coupling correction to account for differences between calibration and real ear measurements and the simplification of the calibration to just one on-site measurement on a closed volume are proposed. Thirdly, the methods to predict the sound pressure at the ear drum developed here were compared to probe tube measurements in a study with 19 human subjects wearing individual in-the-ear hearing aid shells equipped with a receiver and an ear canal microphone. It turned out that, compared to non-individual predictions using an ear simulator, the individual methods match probe-tube measurements better at low, medium and very high frequencies. It was furthermore found that inaccurate estimations of the lambda/4 minimum of the acoustic input impedance of the ear canal have surprisingly little effect on the individual prediction of the sound pressure at the ear drum, which can be explained by the transfer impedance not being a steep function of frequency in the relevant frequency range.