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

sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...

by the addition of a second sound specifically designed to cancel the first. The concept was first developed in the late 1930s; later developmental work that began in the 1950s eventually resulted in

commercial airline An airline is a company that provides air transport services for traveling passengers and freight. Airlines use aircraft to supply these services and may form partnerships or alliances with other airlines for codeshare agreements, in which ...

headsets Headset may refer to: * Headset (audio), audio headphone(s), particularly with an attached microphone * Head Set (band), an American alternative rock band * Headset (bicycle part), a bicycle part that connects the fork to the frame * Head-mounte ...

with the technology becoming available in the late 1980s. The technology is also used in road vehicles,

mobile telephone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whil ...

s, earbuds, and headphones

Explanation

Sound is a pressure wave, which consists of alternating periods of

compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression * Gas compression *Compression ratio, of a ...

and

rarefaction Rarefaction is the reduction of an item's density, the opposite of compression. Like compression, which can travel in waves (sound waves, for instance), rarefaction waves also exist in nature. A common rarefaction wave is the area of low relativ ...

. A noise-cancellation speaker emits a sound wave with the same

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...

but with inverted phase (also known as antiphase) relative to the original sound. The waves combine to form a new wave, in a process called

interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extr ...

, and effectively cancel each other out – an effect which is called

destructive interference In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...

. Modern active

noise control Noise control or noise mitigation is a set of strategies to reduce noise pollution or to reduce the impact of that noise, whether outdoors or indoors. Overview The main areas of noise mitigation or abatement are: transportation noise control, ...

is generally achieved through the use of analog circuits or

digital signal processing Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are ...

. Adaptive algorithms are designed to analyze the waveform of the background

aural Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is audito ...

or nonaural noise, then based on the specific algorithm generate a signal that will either phase shift or invert the polarity of the original signal. This inverted signal (in antiphase) is then amplified and a transducer creates a sound wave directly proportional to the amplitude of the original waveform, creating destructive interference. This effectively reduces the volume of the perceivable noise. A noise-cancellation speaker may be co-located with the sound source to be attenuated. In this case it must have the same audio power level as the source of the unwanted sound in order to cancel the noise. 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, reducing noise in some spots while doubling noise in others. In small enclosed spaces (e.g. the passenger compartment of a car) global noise reduction can be achieved via multiple speakers and feedback

microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...

s, and measurement of the modal responses of the enclosure.

Applications

Applications can be "1-dimensional" or 3-dimensional, depending on the type of zone to protect. Periodic 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 headphones Noise-cancelling headphones reduce unwanted ambient sounds using active noise control. This is distinct from passive headphones which, if they reduce ambient sounds at all, use techniques such as soundproofing. Noise cancellation makes it possible ...

, active

muffler A muffler (North American and Australian English) or silencer (British English) is a device for reducing the noise emitted by the exhaust of an internal combustion engine—especially a noise-deadening device forming part of the exhaust sys ...

s, anti-

snoring Snoring is the vibration of respiratory structures and the resulting sound due to obstructed air movement during breathing while sleeping. The sound may be soft or loud and unpleasant. Snoring during sleep may be a sign, or first alarm, of ob ...

devices, vocal or center channel extraction for

karaoke machine Karaoke (; ; , clipped compound of Japanese ''kara'' "empty" and ''ōkesutora'' "orchestra") is a type of interactive entertainment usually offered in clubs and bars, where people sing along to recorded music using a microphone. The music is ...

s, 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 more expensive. Noise reduction is more easily achieved with a single listener remaining stationary but if there are multiple listeners or if the single listener turns their head or 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. The wavelength in air of sinusoidal noise at approximately 800 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-dimensional space 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. This is because an engine's cyclic nature makes analysis and the noise cancellation easier to apply. Modern mobile phones use a multi-microphone design to cancel out ambient noise from the speech signal. Sound is captured from the microphone(s) furthest from the mouth

oise signal(s) Oise ( ; ; pcd, Oése) is a departments of France, department in the north of France. It is named after the river Oise (river), Oise. Inhabitants of the department are called ''Oisiens'' () or ''Isariens'', after the Latin name for the river, I ...

and from one closest to the mouth esired signal The signals are processed to cancel the noise from the desired signal, producing improved voice sound quality. In some cases, noise can be controlled by employing active vibration control. This approach is appropriate when vibration of a structure produces unwanted noise by coupling the vibration into the surrounding air or water.

Active vs. passive noise control

''Noise control'' is an active or passive means of reducing sound emissions, often for personal comfort, environmental considerations or legal compliance. ''Active'' noise control is sound reduction using a power source. ''Passive'' noise control is sound reduction by noise-isolating materials such as insulation, sound-absorbing tiles, or a

rather than a power source. Active noise cancelling is best suited for low frequencies. For higher frequencies, the spacing requirements for free space and zone of silence techniques become prohibitive. In acoustic cavity and duct based systems, the number of nodes grows rapidly with increasing frequency, which quickly makes active noise control techniques unmanageable. Passive treatments become more effective at higher frequencies and often provide an adequate solution without the need for active control.

History

The first patent for a noise control system——was granted to inventor Paul Lueg in 1936. The patent described how to cancel sinusoidal tones in ducts by phase-advancing the wave and cancelling arbitrary sounds in the region around a loudspeaker by inverting the polarity. In the 1950s

Lawrence J. Fogel Dr. Lawrence Jerome Fogel (March 2, 1928 – February 18, 2007) was a pioneer in evolutionary computation and human factors analysis. He is known as the inventor of active noise cancellation and the father of evolutionary programming. His scient ...

patented systems to cancel the noise in helicopter and airplane cockpits. In 1957 Willard Meeker developed a working model of active noise control applied to a circumaural earmuff. This headset had an active attenuation bandwidth of approximately 50–500 Hz, with a maximum attenuation of approximately 20 dB. By the late 1980s the first commercially available active noise reduction headsets became available. They could be powered by NiCad batteries or directly from the aircraft power system.

References

External links

BYU physicists quiet fans in computers, office equipment

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