Fighting Noise with Antinoise

By Philip Elmer-DeWitt

The oversize, matte-black headsets look like the kind of industrial-strength ear protection worn by airport baggage handlers. But these are no ordinary earmuffs. They are high-tech earphones designed for pilots of small jets and other light (and noisy) aircraft. Rather than soften the drumming engine noise with thick layers of plastic foam, the earphones eliminate it electronically. A tiny microphone samples sound waves at the wearer's ear, processes them through special circuitry and broadcasts countertones that cancel the offending sounds in midair. Result: silence, or something close to it.

The $965 aviation headset, made by Bose, a Framingham, Mass., manufacturer of hi-fi speakers, is one of the latest applications of antinoise, a surprising new technology that is changing the way people block unwanted sounds -- from the whine of electrical transformers to the rumble of internal- combustion engines -- while leaving human voices, alarm bells and other useful sounds untouched. The technology should have many uses: the American Medical Association estimates that more than 9 million U.S. workers are exposed to hazardous noise levels on the job. In some professions -- notably mining, shipbuilding, food processing and printing -- it is not unusual for young workers to begin employment with perfect hearing and end up, 25 years later, nearly deaf.

The principle behind all antinoise devices is the same. Noise is basically a pressure wave traveling through the air. Antinoise is the mirror image of that wave, an equal and opposite vibration exactly 180 degrees out of phase with the noise to be blocked. When noise and antinoise collide, they interact with what is called destructive interference, canceling each other out. The idea is not new; generations of high-school physics students have seen destructive interference demonstrated with undulating Slinkies or jump ropes. But it is only recently -- with the advent of small, high-speed signal processors -- that scientists have had the computer power to make practical antinoise devices.

There are two ways to generate an antinoise wave. The analog approach, first developed in the 1930s using vacuum-tube technology, works something like a seesaw. A mechanism drives a loud speaker that pushes the air when incoming sound waves rise and pulls it back when the sound waves fall. Alternatively, antinoise waves can be created digitally, using a signal processor to convert incoming sound waves into a stream of numbers. Given those numbers, computers can quickly calculate the frequency and amplitude of the mirror-image waves. Those specifications are then fed to a conventional speaker and broadcast into the air. Sounds that the system wants to preserve, like human voices, can be subtracted out in the beginning of the process and added back in at the end.

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.