Editing Active noise control (section)

== Explanation ==
Sound is a [[P-wave|pressure wave]], which consists of alternating periods of [[Compression (physical)|compression]] and [[rarefaction]]. A noise-cancellation speaker emits a sound wave with the same [[amplitude]] but with an inverted phase (also known as [[antiphase]]) relative to the original sound. The waves combine to form a new wave, in a process called [[Interference (wave propagation)|interference]], and effectively cancel each other out – an effect which is called [[destructive interference]].

Modern active [[noise control]] is generally achieved through the use of analog circuits or [[digital signal processing]]. Adaptive algorithms are designed to analyze the waveform of the background aural or nonaural noise, then based on the specific algorithm generate a signal that will either phase shift or invert the polarity of the original signal. This inverted signal (in antiphase) is then amplified and a transducer creates a sound wave directly [[Proportionality (mathematics)|proportional]] to the amplitude of the original waveform, creating destructive interference. This effectively reduces the volume of the perceivable noise.

A noise-cancellation speaker may be co-located with the sound source to be [[Attenuation|attenuated]]. In this case, it must have the same audio power level as the source of the unwanted sound in order to cancel the noise. Alternatively, the transducer emitting the cancellation signal may be located at the location where sound attenuation is wanted (e.g. the user's ear). This requires a much lower power level for cancellation but is effective only for a single user. Noise cancellation at other locations is more difficult as the three-dimensional wavefronts of the unwanted sound and the cancellation signal could match and create alternating zones of constructive and destructive interference, reducing noise in some spots while doubling noise in others. In small enclosed spaces (e.g. the passenger compartment of a car) global noise reduction can be achieved via multiple speakers and feedback [[microphone]]s, and measurement of the modal responses of the enclosure.