Microphone array for hearing aids taking into account the scattering of the head

This work describes the design of a robust microphone array with respect to the acoustic scattering from a head. In this sense, robust means that the major characteristics of the microphone array, like its directivity pattern or its directivity index (DI), vary only to a limited extent if the array is placed close to the head. Four different endfire arrays with a fixed filter and sum beamformer are simulated. For the simulations, the head is modeled as a sphere. The filters are designed to optimize the DI under a constraint for the white noise gain (WNG). Optimized filters for the free field as well as for the sphere mounted array are used. It is shown that a short array (4 cm) with three microphones yield almost the same performance in free field as mounted close to a head. An intelligibility weighted DI (DI/sub AI/) of 7.0 dB is achieved with a minimum WNG of -6 dB. This result is verified by measurement of the directivity pattern and the DI of the array in free field and an eyeglass mounted on an artificial head.