Optimized second-order gradient microphone for hands-free speech recordings in cars

Hands-free telephony and automatic speech recognition in adverse acoustic conditions require noise reduction and speech enhancement methods to achieve acceptable quality and speech recognition rates. This contribution is primarily focussed on the optimization of the recording microphone. The directivity of the microphone can be used to reduce sound components outside the microphone's main axis which suppresses noise and room reverberation effectively. Considering the outer dimensions, gradient microphones possess a high directivity and good noise suppression properties. In this study, the feasibility and the noise-cancelling characteristics of an optimized second-order gradient microphone are investigated. The development of a simple sensitivity-matching principle allows the use of low-priced microphone capsules. The directivity index, characterizing the diffuse noise suppression, is nearly frequency-independent in the telephone bandwidth and is about 3 dB higher than that of conventional directional microphones used in telecommunication. The microphone is evaluated in an automobile environment with the help of a speaker-dependent single-word recognizer and is compared with lower-order gradient microphones. The noise robustness of the speech recognizer is improved by at least 4.7 dB, provided the microphone is in an optimum position. Conclusions are then drawn regarding such an optimum microphone positioning. (C) 2001 Elsevier Science B.V. All rights reserved.