Noise-cancelling headphone
Noise-cancelling headphones reduce unwanted ambient sounds (in common parlance, noise) by means of active noise control. Essentially, this involves using a microphone, placed near the ear, and electronic circuitry which generates an "antinoise" sound wave of the opposite polarity from the sound wave arriving at the microphone. This results in destructive interference, which cancels out the noise within the enclosed volume of the headphone. Keeping noise low at the ear makes it possible to enjoy music without raising the volume unnecessarily. It can also help a passenger sleep in a noisy vehicle such as an airliner. A popular example is the Bose QuietComfort series of headphones, although other manufacturers including Sony/Aiwa also make noise-cancelling headphones. These headphones only try to actively cancel the lower-frequency portions of the noise; they depend upon traditional noise suppression techniques (such as their earcups) to prevent higher-frequency noise from ever reaching the interior of the headphone. This approach has three benefits: The demands on the electronic circuitry are vastly simplified. Because of the shorter wavelength of the high-frequency sound, active cancellation is much less effective at higher frequencies. To truly cancel high frequency components (coming at the ear from all directions), the sensor and emitter for the cancelling waveform would have to be adjacent to the user's eardrum, an impractical situation. For the same reason (shorter wavelength), effective passive isolation becomes easier with rising frequency. There are also noise-reducing headphones produced by Shure and Etymotic Research, among others. These remove sound purely by using passive techniques (sound-absorbing insulation) instead of by producing anti-noise sound.
Disadvantages of noise-cancelling headphones
Noise-cancelling headphones have several disadvantages (that vary from vendor to vendor): They consume power, usually supplied by a battery that must occasionally be replaced. They may not cancel all sound effectively. Some noise-cancelling headphones can be overloaded by low-frequency pressure waves, distorting the desired signal. They may introduce additional noise, usually in the form of high-frequency hiss.
Some models of noise-cancelling headphones Around-ear style:* Bose QuietComfort* Bose QuietComfort II* Sony NC-10 In-ear style:* Sony NC-20
See also Active vibration control Active noise control
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