The letter one is applied to cancel unwanted (primary) noise by a superimposed (secondary) sound that is equal in amplitude but opposite in phase. Due to the three-dimensional sound propagation, zones of constructive and destructive interference exist at the same time, but at different locations.

Active Control of Tonal Noise
The success of active noise control is demonstrated using a dummy head that is placed on a conventional bed. Two microphones that were integrated into the mattress are used as error sensors. The anti-noise is generated by two loudspeakers that are mounted above the dummy head. After activation of the control system, a reduction in noise of 20 dB was measured.