Noise-cancelling headphone
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Noise-cancelling headphones reduce unwanted ambient sounds (i.e., acoustic 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 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.
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.
Retail noise cancelling headphones typically only 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 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.
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[edit] History
Although the theory behind noise reduction was not new, Dr. Amar Bose (founder of the BOSE Corporation) began work on the first noise-cancelling headphones[citation needed] in 1978 on board an airplane. During the international flight, he was provided with a set of headphones from the airline and almost immediately realized how dissatisfied he was with their quality and the loud engine noise. After nearly a decade of research, Bose would release the first noise-cancelling headphones. Since then, many other corporations (namely Sony) have contributed their own noise-cancelling headphones. Sennheiser Corporation was also a pioneer in noise-cancelling headphones, for an airline in that case.
In 1986, Bose applied their noise-cancellation technology to develop headphones to protect the hearing of pilots participating in the first non-stop around-the-world flight.[1]
[edit] Limitations
Noise-canceling headphones have several limitations (that vary from vendor to vendor):
- They consume power, usually supplied by a battery that must occasionally be replaced or recharged.
- They may not cancel all sound effectively. Some noise-canceling headphones can be overloaded by low-frequency pressure waves, distorting the desired signal. They work well for sounds that are continuous, such as the hum of a refrigerator, but are rather ineffective against speech or other rapidly changing audio signals.
- They may introduce additional noise, usually in the form of high-frequency hiss.
- They are generally more bulky than traditional headphones.
- They may pick up and convert stray electromagnetic fields, such as a mobile phone signal, into unwanted noise.
A number of airlines in their business and first class cabins supply noise-canceling headphones (the "continuous hum" of an airplane makes the noise-cancelling particularly effective). In these cases, the headphones are about the same size as normal headphones. The actual electronics to take the sound from the microphone behind the headphone, invert it, and add it back into the audio signal is done in the plane handrest.
These systems often have a special channel allocated with no underlying audio signal, thereby only using the noise cancellation. This can make sleeping easier, as the noise is significantly quieter than with no headphones on at all.
