Small Microphone Arrays with Postfilters for Noise and Acoustic Echo Reduction

Abstract

This chapter presents arrays with two microphones and a postfilter for noise reduction and acoustic echo cancellation. The postfilter algorithm exploits the spatial coherence of the microphone signals. In contrast to single-microphone enhancement algorithms, it does not need an explicit noise power spectral density estimate. An analysis of the mean square error reveals that the coherence properties of the microphone signals are of paramount importance for the performance of the postfilter. Coherence measurements of signals in various acoustic environments are presented. The influence of the directivity and orientation of the microphones on the measured coherence is discussed and rules for the design of the acoustic interface are given. Finally, applications of this approach are presented. The two-microphone algorithm is employed to reduce the non-stationary noise in the voice intercom of a computed tomography scanner. It is also combined with echo cancellers to be used in a robust desktop conferencing device.