Active noise control
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'''Active noise control (ANC)''' (also known as '''noise cancellation''', '''active noise reduction (ANR)''' or '''antinoise''') is a method for reducing unwanted [[sound]].
== Explanation ==
Sound is a [[pressure wave]], which consists of a [[compression]] phase and a [[rarefaction]] phase. 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]]. Depending on the circumstances and the method used, the resulting soundwave may be so faint as to be inaudible to human ears.
A noise-cancellation speaker may be co-located with the sound source to be [[Attenuation|attenuated]]. In this case it must have the same audio power level as the source of the unwanted sound. Alternatively, the transducer emitting the cancellation signal may be located at the location where sound attenuation is wanted (e.g. the user's ear). This requires a much lower power level for cancellation but is effective only for a single user. Noise cancellation at other locations is more difficult as the three dimensional wavefronts of the unwanted sound and the cancellation signal could match and create alternating zones of constructive and destructive interference. In small enclosed spaces (eg the passenger compartment of a car) such global cancellation can be achieved via multiple speakers and feedback microphones, and measurement of the modal responses of the enclosure.
Modern active noise control is achieved through the use of a computer, which analyzes the waveform of the background [[Noise (environmental)|aural]] or [[Noise (physics)|nonaural]] noise, then generates a [[polarisation]] reversed waveform to cancel it out by [[interference]]. This waveform has identical or directly [[Proportionality (mathematics)|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 {{US patent|2043416}}, 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 helicopter and airplane cockpits including those patented by [[Lawrence J. Fogel]] in the 1950s and 1960s such as {{US patent|2866848}}, {{US patent|2920138}}, {{US patent|2966549}} and Canadian patent 631,136. In 1986, [[Dick Rutan]] and [[Jeana Yeager]] used prototype headsets built by [[Bose Corporation|Bose]] in their around-the-world flight.<ref>[http://www.acoustics.org/press/147th/active-noise.htm Special Lay-Language Paper for the 75th Anniversary Meeting of the Acoustical Society of America, May 2004]</ref><ref>[http://scholar.lib.vt.edu/theses/available/etd-04222002-143554/unrestricted/02rudissertation.pdf The effects of noise on speech intelligibility and complex cognitive performance]</ref>
== 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 one or two microphones and speakers to be effective. Several commercial applications have been successful: [[noise-cancelling headphone]]s, active mufflers, and the control of noise in air conditioning ducts. The term "1-dimension" refers to a simple pistonic relationship between the noise and the active speaker (mechanical noise reduction) or between the active speaker and the listener (headphones).
Protection of a 3-dimension zone requires many microphones and speakers, making it less cost-effective. Each of the speakers tends to interfere with nearby speakers, reducing the system's overall performance. Noise reduction is more easily achieved with a single listener remaining stationary in a three-dimensional space but if there are multiple listeners or if the single listener moves throughout the space then the noise reduction challenge is made much more difficult. High frequency waves are difficult to reduce in three dimensions due to their relatively short audio wavelength in air. Sinusoidal noise at approximately 1000 Hz is double the distance of the average person's left ear to the right ear; such a noise coming directly from the front will be easily reduced by an active system but coming from the side will tend to cancel at one ear while being reinforced at the other, making the noise louder, not softer. High frequency sounds above 1000 Hz tend to cancel and reinforce unpredictably from many directions. In sum, the most effective noise reduction in three dimensions involves low frequency sounds. Commercial applications of 3-D noise reduction include the protection of aircraft cabins and car interiors, but in these situations, protection is mainly 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 plug]]s. Bigger noise cancellation systems are used for ship engines or [[tunnel]]s. An engine's cyclic nature makes [[FFT]] analysis and the noise canceling easier to apply.
The application of active noise reduction produced by [[engine]]s has various benefits:
*The operation of the engines is more convenient for personnel.
*Noise reduction eliminates [[vibration]]s that cause material wearout and increased [[fuel]] consumption.
*Quieting of submarines.
== See also ==
*[[Active vibration control]]
*[[Noise control]]
*[[Noise-cancelling headphone]]
*[[Coherence (physics)]]
== External links ==
{{wikibookspar|Active noise control|Acoustics/Active Control}}
*[http://www.sensear.com/ Sensear]
*[http://byunews.byu.edu/archive04-Mar-activenoisecontrol.aspx BYU physicists quiet fans in computers, office equipment]
*[http://slashdot.org/article.pl?sid=04/05/27/1641254 Slashdot discussion of noise suppression]
* [http://doctord.dyndns.org:8000/Pubs/POTENT.htm Anti-Noise, Quieting the Environment with Active Noise Cancellation Technology], ''[[IEEE]] Potentials'', April [[1992]]
*[http://www.signalsystemscorp.com/anc_status.htm ANC commercialization status]
*[http://chrisruckman.com/ancfaq.htm ANC FAQ]
*[http://www.headwize.com/projects/noise_prj.htm Noise-Canceling Headphones construction]
*[http://www.analog.com/library/analogDialogue/archives/34-02/noise/ Principles and Experiments of an ANC for a computer box]
==References==
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[[Category:audio engineering]]
[[Category:Noise reduction]]
[[de:Antischall]]
[[fr:Contrôle actif du bruit]]
[[nl:Antigeluid]]
[[ja:消音スピーカー]]
[[zh:靜音機]]