Active noise control (also known as antinoise) is a method for preventing unwanted sound. Sound consists of vibrations in the air, which can be represented as a wave. If a speaker emits a sound whose wave is the exact opposite phase to the original sound, the waves cancel out and the result is no sound at all. A computer analyses the waveform of the background aural or nonaural noise, then generates a similar waveform rotated 180° out of phase to cancel it out by interference. This waveform is identical or directly proportional to the waveform of the noise, except for its opposing amplitude.
This method differs from passive noise control methods (sound proofing) in that a powered system is involved, rather than unpowered methods such as insulation, sound-absorbing ceiling tiles or automobile mufflers.
Practical implementation is complex, due to the way that sound waves can interact, particularly at high frequencies or in geometrically complex areas. They are most easily used with simple, low-frequency sounds, such as the sound of air moving through a duct. Antinoise systems have seen some use for quiet headphones and for reducing engine noise in vehicles.