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Noise-cancelling headphones are headphones that reduce unwanted ambient sounds using active noise control engineering. This is distinct from passive headphones which, if they reduce ambient sounds at all, use techniques such as soundproofing. 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. In the aviation environment, noise-cancelling headphones increase the signal-to-noise ratio significantly more than passive noise attenuating headphones or no headphones, making hearing important information such as safety announcements easier. Noise-cancelling headphones can improve listening enough to completely offset the effect of a distracting concurrent activity.〔. Recognition and recall of audio information improved when 65 dB(A) background noise was blocked by noise-cancelling headphones, and performance was "no different" when listening with the headphones while at the same time working a mathematical puzzle, than while trying to listen to the same audio with neither puzzle nor headphones.〕 ==Theory== To cancel the lower-frequency portions of the noise, noise-cancelling headphones use active noise control. They incorporate a microphone that measures ambient sound, generate a waveform that is the exact negative of the ambient sound, and mix it with any audio signal the listener desires. Most noise-cancelling headsets in the consumer market generate the noise-cancelling waveform with analogue technology. In contrast, other active noise and vibration control products use digital processing. To prevent higher-frequency noise from reaching the ear, most noise-cancelling headphones depend on soundproofing. Higher-frequency sound has a shorter wavelength, and cancelling this sound would require locating devices to detect and counteract it closer to the listener's eardrum than is currently technically feasible, or would require digital algorithms that would complicate the headphone's electronics.〔Reinhard Lerch, Gerhard Sessler, Dietrich Wolf: ''Technische Akustik: Grundlagen und Anwendungen'', Kapitel 14.7.3 ''Kopfhörer - Hörertypen'', Verlag Springer (2008), ISBN 9783540234302, Seite 431〕 Noise-cancelling headphones specify the amount of noise they can cancel in terms of decibels. This number may be useful to compare products, but does not tell the whole story, as it does not specify noise reduction at various frequencies. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Noise-cancelling headphones」の詳細全文を読む スポンサード リンク
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