Phase describes where a waveform is at any given time in its cycle.
The starting point of a wave is 0 degrees, the peak of a wave is 90 degrees, the next neutral pressure point is 180 degrees, the peak low pressure zone is 270 degrees, and the pressure rises to zero again at 360 degrees.
If two or more waveforms are involved in producing a sound, their relative amplitudes can often be different at any one point in time. If the waves are completely in phase they will combine to make a new waveform with the same frequency but will be double in amplitude. This is known as constructive interference.
If the same two waves are combined while being completely out of phase by 180 degrees, they will cancel each other out resulting in no amplitude. This is known as destructive interference.
When two sound waves with the same frequency but different starting points combine, the resulting wave is said to have phase shift. This new wave will still have the same frequency as the original wave but will have increased or decreased amplitude depending on the degree of phase difference.
Phase interferences are a common occurrence in the recording environment. For example, waves can often be heard to be in phase near a boundary when an incident (primary) wave combines with the reflective wave. This means that when a microphone is positioned close to the boundary, there will be roughly a doubling rise in amplitude in comparison to a microphone that is positioned away from a wall. Note that their will not be a doubling if the frequencies are absorbed or diffracted by the surface.
The most common type of interference within a studio environment is phase shift. An example can be seen when two microphones pick up the same sound source at different distances. Because it takes longer for the sound to arrive at the more distant microphone there is effectively a phase difference between the two waves.
It is imperative that microphones are checked before a recording for phasing issues. If microphones are facing in the opposite direction one microphone must be phased reversed to prevent cancellation of the sound.
Reflected waves can interfere with incident waves, producing patterns of constructive and destructive interference in a room. This can lead to resonances called standing waves. Reflection of waves in strings and air columns are essential to the production of resonant standing waves. To help eliminate comb filtering or other phasing issues, the 3 to 1 rule should be adhered to at all times.