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The Significance of Absolute Phase

What is the significance of having absolute phase? Most of us are familiar with hearing about phasing between two sources (like two speakers in a PA system). If they are in phase it usually means all is well, and if they’re out of phase with each other there is usually some problem. But this does not address the concern over the polarity of the original sound source as compared to the amplified signal. For example: If a drummer hits his bass drum and creates an initial pressure wave, which way should your speakers move first to accurately reproduce that? These questions arose years ago when engineers were first figuring out how to make recordings.

The initial pressure wave from that bass drum will cause a microphone diaphragm to be pushed into the microphone (or at least away from the sound source). Most (not all) microphones generate a positive voltage when that happens. If “absolute phase” (see WFTD absolute phase) is maintained the resulting movement of the speaker will reproduce that initial pressure wave with the correct polarity in the listening position (the speaker will move out).

Modern studies have concluded that the human ear can be very sensitive to phase anomalies. For example: It is now understood that at very low frequencies the ear only responds to positive pressure on the ear drum. Consequently the motion of a speaker reproducing an initial pressure wave can have great significance. In practice, however, the purity of phase in modern multitrack recording is smeared. Placing multiple microphones around a drum set, for example, will generate so many signals slightly out of phase with one another that it can become impossible to distinguish whether absolute phase is maintained. Furthermore, ported loudspeakers contribute to this smearing at very low frequencies due to phase anomalies created by sound emanating from the port.

If you have never experimented with absolute phase you should do so. It’s as easy as reversing the polarity of the wires on your speakers to try it. There is often a noticeable difference in some sounds, especially sounds with significant transients (see WFTD archive transient) in them. There is so little care taken with this in most recording that you have no way of knowing if any CD you play on your stereo is playing back in absolute phase.

Editor’s note: Special thanks to Eric and Earthworks Microphones for his contribution to the facts of this tech tip.

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