Active control

Active Noise Control (ANC) or Active Noise Reduction (ANR) works by analysing noise and producing an inverse or antiphase noise to cancel it out to create quiet zones. The technology has been used in luxury cars for some time and is popular because it can cut vehicle weight by reducing the requirement for heavy sound-limiting materials. It is also used in some turboprop aircraft. Active control has been extensively used for many years in noise-cancelling earmuffs, military headsets and flight helmets, where the low-frequency active noise reduction neatly complements the inherent higher frequency passive noise reduction of these devices. It is now widely used in high-end domestic headphones.

The University of Southampton is one of the few universities with research expertise in active noise control. Our project research team combines the expertise of the consultants from ISVR Consulting with the research of the ISVR. We are now collaborating with the high-speed train manufacturer, CRRC Sifang, on computational and field research into reducing noise for passengers on their latest ultra-high-speed trains .

Active noise control over a large space has previously been limited by the expense of microphones, microprocessors and other computing equipment, but the cost of these components is reducing, and the methods developed by Southampton are becoming more attractive. However, active noise control works best for low frequency noise – such as tonal noises at low speeds – rather than the high-frequency noises on a high-speed train, where just one carriage would require a multitude of microphones and computer equipment for the technology to be effective.

Another form of active noise control, sometimes known as active noise cancellation, is used in some communications systems and in smart phones.  A primary microphone close to a caller's mouth picks up speech but with some background noise.  A secondary microphone further from the mouth picks up background noise with little speech.  By correlating the signals from the two microphones it is possible to subtract a lot of the unwanted background noise from the wanted speech.

In the past a distinction was often made between Active Noise Reduction (ANR) and Adaptive Noise Cancellation (ANC) but this distinction is now often lost. Adaptive noise cancellation uses a reference signal which is correlated with unwanted noise interference to filter out the inteference from a wanted microphone signal. For example, a tacho signal or accelerometer signal from an engine can be used as a reference to adaptively filter out the engine noise from a wanted speech signal from a microphone.

 Some close-talking boom microphones on headsets are also described as ‘noise-cancelling’.  These are usually pressure-gradient microphones which naturally discriminate between distant and nearby sound sources and do not use signal processing.

See also:
Applications of Active Noise Control .  Malcolm Smith, Simon Roberts and Gergely Orosz of ISVR Consulting.  Video of a Powerpoint presentation given by Malcolm Smith at the Institute of Acoustics workshop on Low-Noise Design, September 2019 (approx 19 minutes from Vimeo).