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 reducing unwanted sound.
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[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 (in antiphase) 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. Cyclic sounds, even complex ones, are easier to cancel than random sounds due to the repetition in the wave form.
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 ducts.
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, protection is 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. An engines' cyclic nature makes FFT analysis and the noise canceling easier to apply.
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 cause material wearout and increased fuel consumption.
- Quieting of submarines.
[edit] See also
[edit] 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
