Active noise control

From Wikipedia, the free encyclopedia

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

1 Explanation
2 Applications
3 See also
4 External links

[edit] Explanation

Sound is a pressure wave, which consists of a compression phase and a rarefaction phase. If a noise-cancellation speaker emits a sound wave with the same amplitude and the opposite polarity to the original sound, the waves combine to form a new wave, in a process called interference, and effectively cancel each other out, an effect which is called phase cancellation, the desired result being that there is no sound at all, though such efficiency has yet to be obtained. Depending on the circumstances and the method used, the resulting soundwave may be so faint as to be inaudible to human ears.

Modern active noise control is achieved through the use of a computer, which analyzes the waveform of the background aural or nonaural noise, then generates a polarisation reversed waveform to cancel it out by interference. This waveform has identical or directly proportional amplitude to the waveform of the original noise, but its polarity is reversed. This creates the 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 muffler.

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

more effective at low frequencies
less bulky
able to block noise selectively

The first patent for a noise control system was granted to inventor Paul Lueg in 1934 U.S. Patent 2,043,416 , describing how to cancel sinusoidal tones in ducts by phase-advancing the wave and canceling arbitrary sounds in the region around a loudspeaker by inverting the polarity. By the 1950s, systems were created to cancel the noise in airplane cockpits. In 1986, Dick Rutan and Jeana Yeager used prototype headsets built by Bose in their around-the-world flight.^[1]^[2]

[edit] 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 benefits:

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