As is evident from the discussion of the phase of a waveform, phase is, strictly speaking, always relative. The term absolute phase therefore actually refers to the phase relative to some standard. To the extent that this standard is accepted by all parties, one can speak of an absolute phase in a particular field of application.

High fidelity

In the realm of high fidelity reproduction of music, absolute phase refers the phase of the reproduced sound waves relative to the original sound waves, or to the relative phase of the various channels of stereo or multi-channel reproduction. In most cases, it is actually a question of the polarity of the channels, i.e., phase shifts of 180°. Some audiophiles claim (see, for example, [1]), that reversing the polarities of all the channels simultaneously makes a perceptible difference in the sound quality, even though the relative phases of all the channels are preserved.

Power electronics

When dealing with power electronics, the phase of the voltage and current at various points in the system relative to one another are important. If the points of interest are widely separated in space, it is difficult to measure the relative phase. To solve this problem, the phase of the signals relative to absolute time (UTC) is measured using instruments relying on GPS. Comparison of two absolute phases in this sense allows the relative phase of distant signals to be computed. See [2].

Signal processing

In signal processing a pulse or finite wavetrain can be considered as a signal of a single frequency modulated by an envelope, or as a superposition of an infinite number of infinitesimal waves of different frequencies. In the first case, one may speak of the phase of the wave with respect to the envelope as the absolute phase. In the second picture, it is a question of the relative phase of the component frequencies. For examples of physical effects due to the phase of signals with the same power spectrum, see [3] and [4].