Active Noise Control

This is an electronic method of reducing or removing unwanted sound by the production of a pressure wave of equal amplitude but opposite sign to the unwanted sound. When the electronically produced inverse wave is added to original unwanted sound the result is sound cancellation.

This method of noise control is becoming increasingly popular for a variety of uses. It is sometimes considered a miracle "cure-all" for noise problems which, at the present time, is not the case. For example noise cancellation in 3D spaces, such as living areas, is very difficult to achieve. However it can be more successful locally, eg for a passenger sitting in an aircraft or car. There are many institutions and companies around the world working on the technology to increase the circumstances where ANC can be used effectively.

In its general form as applied to the vehicles, the technique involves continuous monitoring of the selected parameter(s) whose effect is to be controlled and producing, by means of an electronic method, a cancelling signal. In the case of Active Noise cancellation, for example, the engine noise is the monitored parameter, selected because of its order related nature, and the cancellation is provided by a loud-speaker in the passenger cabin at a carefully chosen location.

Active noise cancellation systems measure that pressure wave with a microphone located near your ear, reverse the signal, and then feed the resulting "anti-noise” into the headset. The outside noise and the anti-noise signals cancel each other out, leaving silence.

Active-noise control systems need to have a phase accuracy of roughly a quarter wavelength: after that, the anti-noise starts adding to the background sound, rather than cancelling it out. The size of this bubble of silence is roughly 0.35 metres for a 250-hertz tone (roughly middle C on the piano), which has a wavelength of about 1.4 meters. So if somebody is standing next to you and the thing is turned on, they will perceive your quiet as a series of unpleasant shrieks and squeals.

This sensitivity to location means that active noise control systems do not work well above 150Hz in large spaces where the ear location is not well defined. One way of controlling this space is to use headsets where the position of the speaker and microphone relative to the ear is well defined and the volume is small.

Commercial headsets with active noise cancellation became available for airplane pilots in the early 1990s and hit the music scene by the end of the decade.

When Jeana Yeager and Dick Rutan made the first around-the-world flight in an aircraft without refuelling back in 1986, they each wore a pair of Bose anti-noise headsets. Rutan and Yeager said they could not have withstood the psychological stress of the trip without the peace-and-quiet that the headsets provided.

The hiss you hear in active noise cancelling headphones is the ‘self-noise’ or ‘noise floor’ of the electronics. Take just about any audio system, turn it up very loud with no signal being played, and put your ear close to a speaker in a very quiet room and you will hear some hiss.

Reference

S J Elliott, P A Nelson, "Active noise control", IEEE Signal Processing Mag

S. J. Elliott, "Active Control Of Structure-Borne Noise", Journal of Sound and Vibration, Volume 177, Issue 5, 10 November 1994, Pages 651-673

S.J. Elliott, "Hardware for Active Control, Signal Processing for Active Control", Academic Press, London, 2001, Pages 439-465

See also: Acoustics, Noise Control.

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Subjects: Noise & Vibration