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Noise-cancelling_headphone.
Noise-cancelling headphone
A noise-cancelling headphone reduces unwanted ambient sounds (i.e., noise) via a microphone placed near the ear and electronic circuitry which generates an "antinoise" signal of the opposite phase. This causes destructive interference to occur, that is to say, a sound wave is generated with opposite amplitude. The sound with the reversed phase is emitted by the driver and cancels out the noise as it enters the ear.
Keeping noise low at the ear makes it possible to enjoy music without raising the volume unnecessarily. In this case the headphone must be open (not insulating sound), or the headphone can produce the sum of the music and the anti-noise sound.
It can also help a passenger sleep in a noisy vehicle such as an airliner.
The above article is adapted from from Wikipedia All Wikipedia article text is available under the terms of the GNU Free Documentation License
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