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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 but with inverted phase (also known as 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.

Modern active noise control is generally achieved through the use of analog circuits or Digital Signal Processing. Adaptive algorithms are designed to analyze the waveform of the background aural or nonaural noise, then based on the specific algorithm generates 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 will effectively reduce 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. 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 (e.g. 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.

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, 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. The wavelength in air of sinusoidal noise at approximately 500 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. This is because an engine's cyclic nature makes FFT analysis and the noise canceling easier to apply.

Different Types of Active Noise Control
Global Free Space Cancellation: The total annihilation of a sound field in three dimensions. May be utilized in submarines and household appliances.

Pros Cons
 * Reduces vibrations that cause material wear and increased fuel consumption
 * Can essentially eliminate sound from a single source
 * Cancellation source must be placed within very close proximity of acoustic disturbance
 * System must be able to generate a sound wave with a nearly identical amplitude to the source

Cavity and Duct Cancellation: Generally used in confined spaces where reﬂections from the walls create modal responses, which are generally present whenever the wavelength of the acoustic wave approaches or decreases below the dimensions of the cavity. This system is often used in air conditioning ducts and aircraft cabins.

Pros Cons
 * Can reduce sound in a larger area
 * Generally requires a more intricate system with a higher quantity of feedback microphones and speakers than other systems
 * Multiple speakers tend to interfere with each other
 * Usually not as effective as other methods due to system interference

Zone of Silence Cancellation: Provides a localized cancellation of sound ﬁeld intensity in a very small region of the overall sound ﬁeld. This is the method used in Noise Cancelling Headphones.

Pros Cons
 * A simple system that generally requires one or two microphones at the location requiring ANC
 * Limited to single user applications

Active vs. Passive Noise Control
Noise control is an active or passive means of reducing sound emissions, often incentivised by personal comfort, environmental considerations or legal compliance. Active Noise Control specifically refers to sound reduction by means of a powered system utilizing analog or digital circuits, microphones, and speakers. Passive Noise control refers to sound reduction by an unpowered system using noise isolating materials such as insulation, sound-absorbing tiles, or a muffler.

Advantages of active noise control
 * More effective at low frequencies.
 * Less bulky.
 * Able to block noise selectively.

Advantages of passive noise control
 * More effective at middle to high frequencies
 * Does not require a power source
 * Generally cheaper to implement

History of Active Noise Control
The first patent for a noise control system was granted to inventor Paul Lueg in 1934, describing 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. 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, , and Canadian patent 631,136. In 1986, Dick Rutan and Jeana Yeager used prototype headsets built by Bose in their around-the-world flight.