Learn the difference between these two critical concepts that are often confused with each other
As you are likely aware, the world of audio production is littered with jargon. We like our 'verbs wet, our tracks warm, and our bits unjittered. We are meticulous about our A/D/A, question the need for 192k, and fret about busting 0dBFS. Yet while we're able to get a handle on all this arcane knowledge, the concepts of phase and polarity are regularly confused. It's not just the average user that has problems—a lot of gear mistakenly labels the polarity switch as a phase switch.
Let's take a look at a basic sine wave:
Note how it starts at zero, rises to +1, returns to zero, drops to -1, and finally returns again to zero, representing one complete cycle. (In this illustration, there are three complete cycles.)
If we flip the waveform around the zero point, everything is reversed—where the waveform originally went positive, it now goes negative - like a mirror image of the original waveform. In the illustration below, the darker waveform represents the original signal, and the brighter waveform represents inverted polarity. This is how polarity switches (and mislabeled phase switches) on mixers and effects units operate— they simply reverse the electrical signal. It's important to note that polarity reversal is neither time nor frequency dependent.
Phase is a result of offsetting a signal in time. This can be the product of acoustical, electrical, or mechanical operations. For example, if you were to use two mics to record a single sound source, with one mic at the source and the other mic some distance away, the sound would reach the farther microphone a moment later in time. The result would look something like this:
The darker waveform looks like an echo of the first waveform, although much too close to the original wave to be perceived as such. Instead, the combination of the first and second waveforms will act like a comb filter and alter the timbre of the sound. (Phase shifter effects work by applying an LFO to the delay time, resulting in an active, pleasing filter effect.)
What's the difference?
With the right amount of delay, it is possible that the delayed waveform can be 180-degrees out of phase with the original waveform—virtually identical to a polarity reversal. But this would only be true for a fixed frequency.
In the illustration above, the darker waveform is delayed by one-half cycle of the initial frequency—180 degrees out of phase. But as the frequency increases, the initial and delayed waveforms coincide until, in this example, they're almost completely in phase.
Even though phase and polarity are often confused, it should be clear now that they're really very different.