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At least half a dozen firms are selling antinoise systems in the U.S. and Europe. A pair of British firms, Racal Acoustics and Plessey, sell antinoise headphones that combat cockpit noise in military vehicles, such as the Sea King helicopter and the Warrior attack vehicle. Digisonix, a division of Nelson Industries in Stoughton, Wis., markets units that can be bolted to air ducts to mute the blast of industrial fans and heating and air-conditioning systems. Noise Cancellation Technologies of New York City just announced a joint venture with Tenneco to make electronic mufflers for automobiles and light trucks that can dampen engine boom without reducing engine performance or fuel efficiency.
The DSP Group in Emeryville, Calif., has developed computerized silencers that can cut through the line noise that makes cellular telephoning a chore. The same technology is being used by Government agencies involved in surveillance and intelligence gathering to improve the performance of eavesdropping devices. Active Noise and Vibration Technologies of Phoenix makes antinoise speakers for the headrests of helicopters, trucks and airplanes to surround passengers with zones of silence. Soon, lawn mowers and snow-blowers may be electronically muzzled to reduce suburban din. And, thanks to antinoise systems, submarines carrying nuclear warheads now run silent as well as deep. "Everywhere you hear noise, there's a business opportunity," says Gene Frantz, applications manager for digital signal processing at Texas Instruments. "We're at a stage of the technology where the first guy to the problem can be rich."
No antinoise system is perfect. The digital devices work well with repetitive noises, like the sounds of fans and turbines, but cannot stop random or unexpected noises. Analog systems fight low, random noises but do it by eliminating all low-frequency sounds, good or bad. And none of the antinoise devices currently on the market are very good at canceling high- pitched squeals and whistles. The problem: calculating antiwaves for sounds higher than middle C requires more computing power than today's chips can provide. For now, the most cost-effective way to block those tones is still to stick your fingers in your ears.