Down with noise [active noise control]

Annoying noise in the passenger cabins of propeller aircraft, the rumble in air-conditioning systems, and the sounds disrupting headset communication are being reduced these days by active noise control, thanks to advances in digital signal processing. The technique relies on the principle of destructive interference between two sound fields, one field is generated by the original or primary sound source, the other by a secondary sound source setup to interfere with, and cancel, that unwanted primary sound. The primary source may be an engine and the secondary source, a loudspeaker with an electronically controlled output. Destructive interference is at its most efficient when the two sound fields can be accurately aligned in space over an acoustic wavelength. It works best on low frequency sounds, whose acoustic wavelengths are large compared to the zone in which the noise is cancelled.