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In physics, phase cancellation is the effect of summing two waves that are out of phase with each other. Since the waves are not in phase, the crests and troughs will not match up, and the end result could be a wave that has less overall amplitude than both of the original waves.

Many times, in sound reinforcement systems and recording studios, phase cancellation will occur. If a single acoustical source is picked up by two different microphones, the difference in distance from the source and the microphone will cause the phase to be different. When the two signals are combined, some phase cancellation occurs, and the quality and tonality of the sound is affected. An example of such an alteration in sound fidelity would be a stereo recording played back on a monaural output, which may suffer from phase cancellation artifacts.

When two or more waves (vibrations) in a medium pass through a specific point at a specific time, they will all simultaneously try to move the medium there in the directions of their current vibration displacements (phases) and to distances from the medium's steady state position equal to their various displacements at that instant (magnitudes), adding their energies together — if they are vibrating such that the force on the medium at that instant from one or more vibrations is in opposite directions (opposing phases) from the other or others, the medium will be moved less from its resting state (phase cancellation will occur to some extent); if the opposed vibrations at that instant are exactly equal and opposite, the medium will not move at all (complete phase cancellation will occur) and it would be as if the various vibrations did not exist then and there.

One method of correcting phase cancellation is to invert one signal (inversion is not the same as phase shifting). In a loudspeaker setup, when either the left or right speaker is out of phase, the negative & positive polarities are reversed on one side. To correct this, simply flop the negative and positive (the red & black wires on the backside) on one speaker only.

• Destructive interference
• Phase (waves)#Phase difference
• Noise cancellation
• Directional microphone