Introduction

Phase refers to the relationships between soundwaves when the meet in time and space. Phase is essential to an understanding of ...

How reflected sound behaves in an acoustical space, such as a room.
The correct setting up of a monitoring system.
The correct setting up of microphones in a multi-mic setup.
Mixing audio signal in a mixing desk.
Phasing effects.
Flanging effects.

In phase

Two identical sound waves of the same frequency are said to be In-Phase when their compression and rarefaction cycles coincide exactly in time and space. If two waves of the same frequency sum together in-phase the resulting wave will be of the same frequency but have the combined amplitude of both waves.

Out of phase

Two sound waves of the same frequency are said to be Out-of Phase when their compression and rarefaction cycles diverge. In this case they will cancel each other out.

These situations represent two extremes and between them it is possible to have situations where the two soundwaves are partially in and out of phase.

Phase differences can be the result of time delays between two identical soundwaves. Two signals radiating from two sources (say a stereo speaker system) equidistant from the listener will arrive at his ear in-phase. If one speaker is placed further or nearer than the other then the signals will be out of phase to some degree dependent on the distance.

Phase effects between complex sounds

It is important to remember that most sounds contain harmonics at many different frequencies. The effect of combining identical and complex waves is to create a more complex pattern of summing and cancelling which can sound very dynamic if variations in the delay time between them are introduced, as in Phasing, Flanging, and Chorus effects.

Standing waves

Standing waves (also known as room modes) occur when reflected low frequency sound waves combine or diverge with direct sound waves. Points of high and low amplitude occur as they sum together or cancel each other out. This happens when one of the dimensions of a room (length, width, height) is a direct multiple of the wavelength of the sound. Control room design attempts to mitigate this unwanted effect by employing absorption devices and making walls non-parallel.