Method for calculating directivity index of a directional microphone in a hearing aid on a manikin

Abstract

A method for computing a directivity index (DI) on a manikin for directional microphones in hearing aids is proposed and investigated. Test devices included first‐ and second‐order directional microphones in hearing aids. Signal presentation involved a single noise source rotated completely around the directional microphone, in free field and on a manikin, at a defined radius. The area covered was equivalent to the approximate surface area of a sphere. It was anticipated that an equal angular resolution of 10 deg (elevation and azimuth) would effectively estimate the DI of first‐, second‐, and higher‐order directional microphone systems located in a hearing aid on a manikin. A total of 450 spatially varied presentation points was analyzed, each weighted in reference to direction of arrival on the directional microphone. The absolute difference between the Directivity Index derived from the modified method proposed in this investigation and the conventionally derived Directivity Index on a manikin were as large as 3.8 dB in the higher frequencies, depending on the device under test. The magnitude of these differences was dependent on microphone location. In other words, the further the microphone was placed into the ear of the manikin, the larger the absolute difference.