Absolute Phase - Sound Reproduction

Sound Reproduction

In the reproduction of sound by headphones or loudspeakers, absolute phase refers the phase of the reproduced signal relative to the original signal, retaining the original polarity. A positive pressure on the microphone is reproduced as a positive pressure by the loudspeaker or headphones driver. For instance, the plosive "p" sound from a vocalist sends an initial positive air pressure wave toward the microphone which responds with an initial inward movement of the microphone diaphragm, away from the vocalist. To maintain absolute phase, a loudspeaker reproducing the sound would send an initial positive pressure outward from the loudspeaker, toward the listener.

In audio, a change in polarity refers to an equal phase shift of 180° at all frequencies, usually produced on one channel by reversing the connections of two wires. Some audiophiles claim that reversing the polarities of all the channels simultaneously makes a subtle perceptible difference in the reproduced sound, even though the relative phases of all the channels are preserved. The ear is sensitive to the periodicity of a waveform at low frequencies; tests have shown that absolute phase can sometimes be heard by test subjects listening under monaural conditions (a single loudspeaker, or headphones sending the same signal to both ears.) Audio engineer Douglas Self concludes "there is a prima facie case for the audibility of absolute phase", especially for high impulse sounds such as percussion. The concept of absolute phase is rendered irrelevant for any instrument with strings (such as a guitar or piano), or for two or more instruments played together. Complex sounds such as these have an undetectable phase relationship. In practice, the absolute phase of an audio system can be assumed to be inaudible.