Antinoise

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.

The advantages of active noise control methods compared to passive ones are:

• they are more effective at low frequencies
• they are less bulky, something valuable on aircrafts
• they can be made to be selective, i.e. to block noise (e.g. from an engine) but not useful signals (e.g. voice).

Applications can be 1-dimensional or 3-dimensional, depending on the type of zone to protect.

Protection of a 1-dimension zone is easier and requires only a couple of microphones and speakers to be effective. Several commercial applications have been successful: noise-cancelling headphones, active mufflers, control of noise in air conditioning pipes.

Protection of a 3-dimension zone requires many microphones and speakers, making it less cost-effective. Commercial applications include the protection of aircraft cabins and car interiors.