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.