A "phasing" effect can occur when two similar instruments play the same note at nearly the exact same pitch (including vibrato).
If the pitches are exactly the same, at a given point where a listener (or microphone) might be, the effect can range from the note being twice as loud (due to reinforcement) to disappearing entirely (due to phase cancellation)!
More typically, the pitches aren't exactly the same, but are very close, as can easily occur when both instruments are synthesizers.
In this case, as the pitches subtly "dance" around each other, the harmonics produced by their closeness, sometimes heard as "pulsing", shift in interesting ways.
This shift is often called a "phase shift", or just "phasing".
For example, consider my MIDI sequence for Pachelbel's "Jauchzet dem Herrn" which can exhibit this phasing when played back on some synthesizers.
Note that the fugue near the end of the piece calls for unison of the two choruses (that is, four-voice SATB). That means each voice has two identical instruments playing identical notes, since the rest of the piece is for eight voices.
This sequence represents that straightforwardly. The result, on my synth, is some "phasing" effects as identical instruments make nearly identical sounds for identical notes, one on the left and one on the right. It's an interesting effect, though I've sometimes used the global randomizing-note feature (set to 2 or 3 out of the range of 0 through 7) on my synth to help bring out the voices more distinctly when practicing to this and other sequences.
Copyright (C) 2000 James Craig Burley
Last modified on 2000-06-14.