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Noise-cancelling headphones reduce unwanted ambient sounds (i.e., acoustic noise) by means of active noise control (ANC). Essentially, this involves using a microphone, placed near the ear, and electronic circuitry which generates an "antinoise" sound wave with the opposite polarity of the sound wave arriving at the microphone. This results in destructive interference, which cancels out the noise within the enclosed volume of the headphone.

Noise cancellation makes it possible to enjoy music without raising the volume excessively. It can also help a passenger sleep in a noisy vehicle such as an airliner.

Retail noise cancelling headphones typically use ANC to cancel the lower-frequency portions of the noise; they depend on more traditional methods (like the materials from which the earcups are made) to prevent higher-frequency noise from reaching the ear. This approach is preferred because:

  • it reduces the demand for complicated electronic circuitry.
  • at higher frequencies, active cancellation is less effective. 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 users eardrum, which is not technically feasible.
  • due to the shorter wavelength of higher frequency sounds, passive isolation becomes more effective.

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