Active noise control

From Free net encyclopedia

Active noise control (also known as noise cancellation, active noise reduction (ANR) or antinoise) is a method for preventing unwanted sound.

1 Explanation
2 Applications
3 See also
4 External links

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Explanation

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 opposite phase to the original sound, the waves cancel out and the result is no sound at all. A computer analyzes 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 to 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 (soundproofing) 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 aircraft
they can be made to be selective, that is, to block noise (e.g. from an engine) but not useful signals (e.g. voice).

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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, but in these situations are limited to the cancellation of repetitive (or periodic) noise such as engine-, propeller- or rotor-induced noise.

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 personnel.
Noise reduction also eliminates vibrations that causes material wearout and increased fuel consumption.

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