Feb 1, 2005
This article describes the technology behind noise-canceling headphones. Whining jet engines pummel airline passengers with a mind-numbing 75 to 80 decibels of noise. Subways, trains and speeding cars also assail riders with a relentless howl. Putting on simple headphones and cranking up a compact-disc player to drown out the din just adds to the ear-pounding volume. Deep earplugs or earmuffs like those worn by factory workers typically reduce the racket by 15 to 25 decibels, but they are uncomfortable and do not allow wearers to hear the audio from an airplane movie, music channel or their own music player. Noise-reduction headphones can help. The most advanced models, priced around $300, are made from structural materials that passively block higher-frequency noise (above about 200 hertz). They employ electronics and a speaker to actively cancel lower-frequency sounds, which are otherwise difficult to stop. A microphone inside each muff senses sound waves that make it through the outer ear cup, and a speaker creates pressure waves that cancel them. If desired, music can then be piped in at a comfortable level. The best models passively reduce noise by 15 to 25 decibels; turning on the active circuitry can cut another 10 to 15 decibels of low-frequency tones. The interior microphone, circuitry and speaker--which constitute a feedback system--must create opposing waves fast and loud enough to almost match the sound in real time. Coming within 25 degrees of the needed 180-degree phase shift can cut noise by 20 decibels. Headphones that react more slowly provide less cancellation.