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Finding a better way to quiet noisy environments

Researchers at the University of California, San Diego (UCSD) report in the April 4 issue of the Journal of Sound and Vibration a new mathematical algorithm designed to dramatically improve noise-cancellation technologies that are used to quiet everything from airplane cabins to commercial air conditioning systems. The new technique improves the ability to achieve destructive interference, the generation of anti-noise signals that combine with and destroy unwanted sounds.

"Noise cancellation is a hidden technology that most consumers aren't aware of, but vehicles made by BMW, Mercedes, Honda, and other companies are now using it," said Raymond de Callafon, co-author of the paper and a professor of mechanical and aerospace engineering at UCSD's Jacobs School of Engineering. "Our new technique should greatly expand the potential of active noise-cancellation technologies."

Basic active noise-cancellation is composed of four inter-related parts: a microphone that measures incoming noise and feeds that information to a computer, a computer processor that converts the noise information into anti-noise instructions, and an audio speaker that is driven by the anti-noise signal to broadcasts sound waves that are exactly 180 degrees out of phase with the unwanted signal and of the same magnitude. In addition, a downstream microphone monitors residual noise and signals the computer as part of a process to optimize the anti-noise signal.

This "feedforward" active-noise control can reduce unwanted helicopter and cabin noise or the steady roar of industrial air handling systems by 40 decibels or more. However, most commercial systems suffer from acoustic feedback because the anti-noise signal produced by the noise-cancellation speakers can feed back into the microphone and become amplified repeatedly until the resulting sound becomes an ear-splitting squeal or whistle.

"Most people ignore this acoustic coupling but we took it into account and designed the feed
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Contact: Rex Graham
ragraham@ucsd.edu
858-822-3075
University of California - San Diego
4-Apr-2006


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