A directional adaptive least-mean-square acoustic array for hearing aid enhancement.

This paper introduces directional microphones to adaptive array processing for hearing aid applications. Acoustic fixed arrays are designed to match a focused array gain pattern, while acoustic adaptive arrays are designed to attenuate interference noises with changing characteristics. However, as currently constructed, acoustic adaptive arrays cannot stay focused with a limited number of microphones available in a cosmetically acceptable hearing aid. In this paper, a technique is discussed that combines fixed and adaptive arrays in a system which enhances the desired signal while effectively attenuating interference speech and background noise. In particular, the design and performance of a directional adaptive least-mean-square (LMS) acoustic four-element array with a restricted geometry, where the array microphones are directional microphones, are examined. Simulations show that the directivity index of the directional adaptive array using four hypercardioid microphones is improved to between 8.6 and 11 dB. The array reduces the interference noises by 29.7 to 42.3 dB and provides a signal-to-noise ratio improvement of 11.5 to 12.2 dB over a single omnidirectional microphone. The sensitivity analysis is also discussed. It is concluded that the small size (four-element) microphone array can spatially filter interference noise effectively and so improve SNR performance significantly.