Created on 2013-02-01 00:14:00
The acoustic vibrations that are sound and the electrical vibrations that represent sound as audio signals are waves.
At a given moment in time, two waves of the same frequency can be at the same point in their wave cycle (in phase) or at some different point in the cycle (out of phase). Waves are referenced to circular motion, and a complete cycle of a wave is 360 degrees of angle. 0 degrees, 360 degrees and any integer multiple of 360 all represent the same point on a circle, and all describe signals that are in phase.
Differences in phase may be observed between any two signals. In audio testing, phase is typically measured between channels in multichannel devices, and between the input and output of a device under test.