Active noise control (also known as noise cancellation or 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 has the same amplitude and 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 has identical or directly proportional amplitude than the waveform of the noise, but its phase is the opposite. This creates destructive interference that reduces the amplitude of the perceived noise.

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, that is, to block noise (e.g. from an engine) but not useful signals (e.g. voice).

Applications

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, and the 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.

Antinoise is used to reduce noise at the working environment with ear plugs. Bigger noise cancellation systems are used for ship engines or tunnels.

The application of active noise reduction produced by engines has various goals:

• The operation of the engines is more convenient for personal.
• Noise reduction also eliminates vibrations that causes material wearout and increased fuel consumption.

See also

• Active vibration control

External Links

• Active Noise Reduction for fans in computers aimed (http://byunews.byu.edu/archive04-Mar-activenoisecontrol.aspx)
• Slashdot discussion of noise suppression (http://slashdot.org/article.pl?sid=04/05/27/1641254)