Gary S. Guthart
William C. Nowlin
Gregory K. Toth
Appl. Control & Signal Process. Group, SRI International, Menlo Park, CA 94025
Active noise control (ANC) is often discussed in the context of reducing noise in vehicles. The acoustic reverberation of typical vehicle cabins makes the application of ANC to quieting interior noise a stiff challenge. While some ANC approaches take advantage of the modal structure of reverberant enclosures to provide reduction throughout the enclosure, these approaches become infeasible for many frequency bands of practical interest because of increasing modal density with increasing frequency. For high-bandwidth, diffuse fields, noise reduction is achieved locally by specifying a quiet zone within the enclosure. Experimental results for broadband, local quieting in a diffuse field using a multichannel ANC system are presented. The reverberant enclosure is an ordinary room, measuring approximately 1.4 mx2.4 mx2.4 m and containing a seated operator with six microphones defining the quiet zone near the operator's ears. The control system uses a single reference signal and two error channels to drive four control speakers. An average of 20-dB reduction in sound pressure level is found at the quiet-zone microphones for the frequency range 50 to 1000 Hz. These results demonstrate the viability of real-time, multichannel ANC for locally attenuating random noise in diffuse acoustic fields.