Phase, part 1http://www.music.columbia.edu/cmc/courses/g6630/Phase.html
Phase is a time relationship between two sounds, waveforms, or signals in a circuit. When the time relationship is coincident, the two signals or sounds are "in phase." And their amplitudes are additive. When the time relationship is not coincident, they are "out of phase" and their amplitudes are subtractive. This has ramifications both acoustically and electronically.
Example - Close miking a piano with two microphones. When you strike a key, the sound from the string will reach the closer mic sooner then the mic that is farther away, creating a slight timing or phase difference in the microphone signals. If those two signals are played on a stereo system the effect should be a stereo "image" of some sort. Our ears capture the same kind of timing and phase differences and the brain uses these differences for sound localization.
Now if the two signals from the two piano microphones are summed to mono, the phase difference between them could result in phase cancellation. In that case, certain frequencies, depending on the music content and the microphone distances, are reduced in volume as a result of the subtractive amplitude of the out of phase signals. (Conversely, if frequencies are in phase, the volume will increase.) The result could be a change in timbre of the piano, a decrease in volume of certain notes, or even a kind of distorted, dirty sound.
Whenever you are miking in stereo, especially close miking, it's a good idea to listen, then sum the two channels to mono and listen again. Then reverse the phase of one channel and listen again. This should tell you what frequencies are being canceled when you sum to mono. If you don't like what you hear, reconsider your microphone placement - phase is working against you.
ProTools has a Reverse Phase effect in the audio suite.
Sound reflections in a room can create similar cancellation effects, which recording engineers must consider in the room setup for a recording session.