Active noise control
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Active noise control (also known as noise cancellation, active noise reduction (ANR) or antinoise) is a method for preventing unwanted sound.
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 polarity 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 polarity reversed waveform to cancel it out by interference. It does not mean that all frequencies are rotated 180° out of phase (phase shifting). This waveform has identical or directly proportional amplitude to the waveform of the noise, but its polarity (not the 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).
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.
See also
External links
- [BYU physicists quiet fans in computers, office equipment]
- [Slashdot discussion of noise suppression]
- [Anti-Noise, Quieting the Environment with Active Noise Cancellation Technology], IEEE Potentials, April 1992
- [ANC commercialization status]
- [ANC FAQ]
- [Noise-Canceling Headphones construction]
- [Noise Cancelling versus Noise Reduction – The Difference When It Comes to Headphones]
- [It is not the phase - it is polarity reverse]
- [About Phase Shift and Polarity Reversal, part 1]
- [About Phase Shift and Polarity Reversal, part 2]
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