Understanding phase can help you diagnose problems when miking or mixing. This post is a clear explanation and helpful overview of the topic from Mark Garrison's book Encyclopedia of Home Recording.
The word phase is used to refer to points along the progression of the cycle of a sound wave (see Fig. 73). Points along this progression are designated in degrees, just as if the sound wave was a circle. The point where the sound wave is furthest into positive amplitude is 90 ̊, where it reaches the center line again is 180 ̊, the point furthest into negative amplitude is 270 ̊, and so on.
When two identical signals are not perfectly aligned, they are said to be “out- of-phase”. When out-of-phase signals are combined, they will begin to cancel each other out in areas where one is in positive amplitude and the other is in negative amplitude and boost each other when they are both in positive or both in negative amplitude (see Fig. 73), resulting in a washy modulated sound. When two identical signals are 180 ̊ out-of-phase (a vertical mirror image) they will cancel each other out completely, resulting in silence.
Most problems with out-of-phase signals come from delay effects or from poor stereo mic placement. When phase problems result from delay effects, the solution is usually as simple as adjusting the delay time until the phasing stops. Phase issues from mic placement can be avoided by using coincident pairs (mics placed with the capsules together) or by following the 3:1 rule (mics should be three times as far from each other as they are from the instrument). Because phase problems only show up when the two signals are being played through the same speaker, it is very important to always check mixes in mono.
When two mics are used on the same instrument from different sides, for example miking the top and bottom of a snare drum, the resulting signals will usually be 180 ̊ out-of-phase from each other. This can be fixed by reversing the phase on one of the signals. Many devices have phase reversal circuits, which are usually labelled with the “Ø” symbol. If a phase reversal circuit is not available, a mic cable can be made to reverse the phase by swapping its hot and cold leads at one end. This is usually a last resort, as it makes the cable unusable for any other application and creates the possibility of accidental phase reversal by inadvertently using the modified cable.