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Acoustic noise reduction for speech communication: (second-order gradient microphone)

Ambient acoustic noise is a significant factor for degrading speech quality of the telephone, particularly, of a digital telephone operating at low data rates. In extremely noisy environments (e.g., flight deck, high-performance aircraft, tanks, helicopters, stock exchange floor, to mention a few), speech communication would be difficult without using some form of noise reduction at the front-end of the telephone. Over the years, therefore, various noise reduction techniques have been developed to improve voice communication in noisy environments. Previous noise reduction methods focused primarily on digital preprocessing of the corrupted speech signal after it has been induced by the microphone. In this paper, we present an alternative that reduces noise while it is being mixed with speech to become corrupted speech. The noise reduction technique is based on the microphone array that is very insensitive to far-field sounds (i.e., noise), while it provides a flat frequency response for near-field sounds (i.e., speech). The microphone array is small enough to be installed within a handset or headset. The prototype we fabricated has shown excellent performance. In an extremely noisy environment where one cannot hear an adjacent person talking, the microphone array discussed in this paper provided highly intelligible speech.