Active noise control
Active noise control (ANC), also known as noise cancellation (NC), or active noise reduction (ANR), is a method for reducing unwanted sound 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 headsets with the technology becoming available in the late 1980s. The technology is also used in road vehicle and in mobile telephones. Explanation. Sound is a pressure wave, which consists of alternating periods of compression and rarefaction. A noise-cancellation  speaker emits a sound wave with the same amplitude 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, and effectively cancel each other out – an effect which is called destructive interference. 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 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 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, anti-snoring devices, vocal or center channel extraction for karaoke machines, 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 [noise signal(s)] and from one closest to the mouth [desired 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 muffler 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 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 1980's the first commercially available active noise reduction headsets became available. They could be powered by NiCad batteries or directly from the aircraft power system.