Active noise control

Modern active noise control is generally achieved through the use of analog circuits or digital signal processing.

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.

Several commercial applications have been successful: noise-canceling headphones, active mufflers, anti-snoring devices, vocal or center channel extraction for karaoke machines, and the control of noise in air conditioning ducts.

High-frequency waves are difficult to reduce in three dimensions due to their relatively short audio wavelength in air.

Noise control is an active or passive means of reducing sound emissions, often for personal comfort, environmental considerations, or legal compliance.

Passive noise control is sound reduction by noise-isolating materials such as insulation, sound-absorbing tiles, or a muffler rather than a power source.

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.

In 1957 Willard Meeker developed a working model of active noise control applied to a circumaural earmuff.