`Anti-Noise' Cuts Down Sound

Companies use new high-tech strategies to quiet engines, compressors, and fans

ALTHOUGH he calls it a fad, "anti-noise" is something that Amar Bose won't board an airplane without.

When he travels, the chairman of Bose Corporation brings a $1,000 headset that keeps out much of the excess noise in the plane. Partly this is done by traditional muffling - the ergonomically designed headset.

But inside the earphones, developed by Dr. Bose and his company's engineers, something more high-tech is happening: The system distinguishes the pattern of extraneous noise in the cabin and creates sound waves that are the mirror image of that unwanted noise. By adding more sound waves - mathematically correct ones - sound is canceled.

The result, he says, is not only a more comfortable flight, but also less of a jet-lag effect. After returning from Japan, "I was far more alert" than on flights where the noise was unchecked.

Bose is not the only one declaring war on noise pollution. Makers of industrial equipment, automobiles, and home appliances are interested in using anti-noise technology to quiet engines, compressors, and fans.

The best solution, experts say, is to make such systems quieter in the first place. And Bose says noise-canceling technology like his headset may not fulfill public expectations, which have been growing in recent years.

Noise Cancellation Technologies Inc. of Stamford, Conn., is trying to prove that applications can be more widespread.

In the last two years, NCT has entered joint ventures to develop automotive mufflers with Detroit Diesel Corporation and Walker Manufacturing Company, and will develop quieter household appliances with Sweden's Electrolux and France's Boet S.A.

Last month NCT patented an automotive muffler that uses "active" noise-cancellation technology rather than the "passive" approach of traditional mufflers. Spokeswoman Irene Lebovics says this can improve car's fuel efficiency by 2 percent on the highway and 5 percent in the city, because traditional mufflers make the engine work harder.

The company is not yet profitable, but investors pushed its stock price up 352 percent in 1991 to $4 a share. Most of its $6 million revenue last year came from licenses and partnerships rather than from product sales.

NCT has sold industrial systems to muffle the noise of CSX Corporation's large vacuum machines that are used to unload shipments of grain, sugar, and plastic pellets. Bringing the cost down is a key to building a market for consumer applications.

Bose, for example, says the use of anti-noise is too expensive to replace traditional car mufflers, but NCT is hopeful that its system can become competitive in this area. It is already economical for large engines such as those in trucks and industrial equipment, Ms. Lebovics says.

In March, NCT entered a joint venture with Foster Electric Company Ltd. of Tokyo to produce headsets that cut noise for construction and factory workers.

The Bose headset, meanwhile, is finding a market with private pilots and some commercial pilots. It allows radio communications to enter, while constantly filtering out and adjusting to background noise. The headset reached the market in 1989, after testing in 1986 on the Voyager airplane's first-ever nonstop flight around the world. The Voyager's light weight made it a particularly noisy craft.

Anti-noise technology works well with low-frequency sounds that have traditionally been the hardest to get rid of. For example, it takes a thick, heavy wall to keep out the bass sound of a stereo system next door. High-frequency noise, in contrast, can be dealt with relatively easily using acoustical insulation.

Since noise is hard to control once it is bouncing around in a room, the best anti-noise approach is either source-point cancellation or headsets.

European industry currently has stricter noise regulations than the United States does, Lebovics says. She says technology like NCT's may prompt the US Occupational Safety and Health Administration to tighten its rules. WHAT IS ANTI-NOISE? `Anti-noise' devices work to cancel unwanted noise by generating a signal that is the mirror image of the sound-wave pattern of the target noise (such as an engine).

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