Editing Active noise control (section)

== 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 waveform.

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-canceling headphones]], active [[muffler]]s, anti-[[snoring]] devices, vocal or center channel extraction for [[karaoke machine]]s, 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-dimensional 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&nbsp;Hz is double the distance of the average person's left ear to the right ear;<ref>{{cite book |url=https://books.google.com/books?id=ZgdkVhHUjEMC&pg=PA26 |page=26 |title=Understanding and crafting the mix: the art of recording |last=Moylan |first=William |publisher=Focal Press |year=2006 |isbn=0-240-80755-3}}</ref> 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.{{efn|The average head is about {{convert|21.5|cm|in|abbr=on}} from ear to ear. Assuming the speed of sound is 343 meters per second (1125 feet per second), the full wavelength of a tone of 1600&nbsp;Hz reaches from ear to ear. A tone of half that frequency, 800&nbsp;Hz, has a wavelength twice as long. A single such tone coming from the side will appear at the two ears 180 degrees out of phase—one ear compared to the other. An active noise control tone coming from a different angle will not be able to attenuate the original tone in both ears at once.}} High-frequency sounds above 1000&nbsp;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 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 (the noise signal(s)) and from the one closest to the mouth (the 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 the vibration of a structure produces unwanted noise by coupling the vibration into the surrounding air or water.