Following their migratory patterns through the pro audio landscape, engineers can encounter a wide variety of terrain, and can’t always assume that things are as they seem. Polarity is one variable that can be problematic. While evaluating various products over the years (including once very recently), I’ve seen entire consoles that were polarity inverted in-to-out, or a pair of review mics that were out of polarity with one another. We aren’t even consistent in how we refer to the concept. When an audio engineer says that two signals are “out of phase,” they quite typically mean “out of polarity.” The mislabeled Phase button on audio consoles or workstation channel strips (often labeled with the symbol Ø) is in actuality a polarity inversion control.
Back to basics: Phase discrepancies between two or more signals imply an offset based upon time. A pair of signals being 180 degrees out of phase is not the same thing as being out of polarity. A signal out of polarity is inverted as compared to a reference (easily achieved by inverting one of a pair of balanced signals by, say, connecting them with two XLR cables, one wired correctly and the other wired pin 3 hot on one end and pin 2 hot on the other end). With a 180-degree phase shift, two otherwise identical signals would be exactly a half cycle out of time with one another at a specific frequency; because the phenomenon is time-based, the phase relationship would vary with frequency — a phase specification requires frequency information to be useful. Given two identical signals except that one is polarity inverted, the signals would cancel out completely if added together. Given two otherwise identical signals that have a phase discrepancy, the signals will sometimes be subtractive, one to the other, and sometimes additive. With a sufficient phase shift, some frequencies will completely cancel while others will double in amplitude, with variations in between — the very definition of a comb filter.
OK, you probably knew all that, but it’s one of the fundamentals that is worth reiterating.
A related concept is Absolute Polarity. When a kick drum thumps, it is pushing energy towards our microphone. If everything is correct along the way, we get that same push out of our monitor loudspeakers. Some folks are quite sensitive to absolute polarity, others less so. Good engineering practice demands that absolute polarity be maintained through each stage of an audio signal chain. That push of air from our example kick should produce a positive electrical signal in the microphone, the mic cable should pass that positive signal on to the console and so on, through the entire chain (unless modified for effect).
You can’t always trust manufacturers to get it right. I’m aware of one monitor line that was manufactured out of absolute polarity, then corrected for later production runs. New installs, new gear and unfamiliar signal chains should be tested as a matter of course. There are some absolutes.