On most mixing consoles above a certain price point, there’s a button on each channel with the “∅” symbol. It may be (mistakenly) labeled “phase.” Mythology runs rampant on the meaning and use of that switch, and most of that confusion is steeped in stories from someone who doesn’t understand it telling others who understand it even less.
This misunderstanding leads to the statement I’ve heard for years and still do to this day: “Inverting the polarity is the same thing as the signal being 180 degrees out of phase.”
The terms “phase” and “polarity” refer to two different things and cannot be used interchangeably. Phase implies a shift in time relative to some reference while polarity is simply a reversal of the positive and negative terminals of an audio connection. That could mean swapping pin 2 and pin 3 of an XLR connector, the tip and ring connection of a TRS connector, or the plus and minus leads of a loudspeaker terminal.
As you can see in Figure 1, nothing has changed with time. The graph shows our friendly sine wave test signal in proper polarity, and the graph on the right shows the sine wave signal out of polarity.
So let’s clarify. It’s more properly termed a polarity switch because all that the button does is reverse the wiring between pins 2 and 3 at the XLR input on that particular channel. It does not affect the phase. I know that many of the engineering folks at the manufacturers still call it a phase button, but they should know better.
When one looks at a graph of, for example, a sine wave (Figure 2), with the signal delayed in time by 180 degrees, it looks like the polarity has been reversed. And that’s probably where the misunderstanding begins. If we don’t see that the signal has been delayed in time, then we could be misled to believe that polarity and phase are the same.
Under what conditions would a sound operator choose to push that polarity switch? Rarely. But here’s one that may help to know. Let’s say that you’ve decided to double-mike the snare drum, with one microphone on the top head to capture the main “body” of the sound and a second microphone on the bottom head to get the “snap” of the snares.
Think about it and realize that the acoustic energy reaching the top mic when the drum head is hit (down first) is roughly the opposite condition for the acoustic energy received at the mic on the bottom head. If the polarity of the mic on the bottom of the snare is not inverted, the sound will seem thin because the low frequencies are cancelling; just press the polarity switch on the channel with the bottom snare mic and the snare will sound “fat” again.
I’ve also run across situations where the switch comes in handy when something was miswired. In one case it was a mic cable that had been terminated incorrectly, and it was a situation where changing the cable at that moment wasn’t possible. In multiple other cases, the polarity reversal was in a new unit that was shipped from the manufacturer. (It happens.)
Just remember that the “phase” switch on your console input channel does not “shift the phase by 180 degrees” because that would involve a shift in time. It’s nothing more than a polarity reversal. And from this day forward, start calling it a polarity switch rather than a phase switch!