By Philip Elmer-Dewitt

The oversize, black headsets look like the kind of 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 layer of plastic foams, the earphones eliminate it electronically. A tiny microphone samples sound waves at the wearer’s ear, processes them through special circuity and broadcasts countertones that cancel the offending sounds in midair. Result: silence, or something close to it.

The 1965 aviation headset, made by Bose, a 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 US 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 travelling through the air. Antinoise is the mirror image of that wave, an equal and opposite vibration exactly 180% out of phase with the noise to be blocked. When noise and antinoise collide, they interact with what is called destructive interference, cancelling each other out. The idea is not new; generations of high-school physics students have seen destructive interference demonstrated with 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 1930’s using vacuum tube technology, works something like a seesaw. A mechanism drives a loud speaker that pushes the air when incoming 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.

But no antinoise system is perfect. The digital devices work well with repetitive noise, like the sound of fans and turbines, but cannot stop random or unexpected noises. Analog systems fight low, random noises but do it by eliminating nil low-frequency sounds, good or bad. And none of the antinoise devices currently on the market are very good at cancelling high-pitched squeals and whistles.

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