What is Phase?
What is phase exactly? Why is it so important?
At its core, phase is simply the difference in time vs. amplitude between two sources. The easiest way to visualize this is by picturing the simplest waveform, a sine wave.
Pretend you have two sine waves, each of which is going from +1 to -1 at the extreme top and bottom. If there was absolutely no phase difference between the two, and they were both playing at the same time, we would end up hearing a sine wave twice as loud.
Why? Because each wave would hit +1 and -1 at the same time. Thus, they are added together giving us the appearance of one sine wave ranging from +2 to -2.
But what if we they were not in phase? If the two sine waves were perfectly out of phase with one another, one sine wave would be at +1 while the other would be at -1. The end result would be a constant 0 or no sound at all.
As audio engineers, we deal with much more complex waveform than simple sine waves. As such, we rarely encounter true cancellation due to phase. Instead we often encounter partial cancellations that might knock down some frequencies, boost others, and mess with our stereo field.
After you have correctly identified which tracks have phase problems, you will have a couple different options. Here are a few tricks:
- If you are close miking a source with two or more mics, but something sounds off, slightly adjust the mic position. Usually you will need to move the capsule less than an inch! Having someone else move the mic while you listen always helps too.
- For stereo tracks (a L-R of a stage for example), make sure you follow the 3-1 rule (assuming you are still recording). This rule states that for every foot back you are from a source, you should spread your stereo mics out another three feet. Simple easy fix.
- If you recorded a source from two very different angles (top and bottom for example), you may have two recordings that are almost 180 degrees out of phase (the snare drum is a common case). In these situations, try flipping the polarity/phase of a track. If you suddenly hear a bunch of bass and mids come to life, you are golden.
- Sometimes the polarity flip trick does not work quite perfectly. It might fix one problem but still produce a slightly phasey sound in another way. To remedy these situations, a phase rotation is a better option. While a polarity switch lets you choose between 0 and 180 degrees, a phase rotation gives you a full 360 degrees of adjustment to perfectly dial in the phase relationship.
- Another option is to zoom in very tightly on your tracks, so you see individually recorded samples. If you constantly see that one track is going positive while the other is negative, just slightly nudge one the tracks to line up with the other. Look for transients as good reference points.
- For cases when you’re using parallel processing, and you hear phase problems when you solo the tracks, you will need a time delay. While almost every plugin should have proper delay compensation, not all do. You can either add a specialized plugin to delay the original source, or you can simply guess-and-check using the nudge method above.