Haven't checked the article yet but will.
I appreciate the way they explain the difference between 180 degrees out of phase and reversed polarity:
What's confusing about the '180 degrees out of phase' term is that it is sometimes used to refer to a situation where the second channel has not been delayed, but has had its waveform flipped upside down, so that the peaks become troughs and vice versa — a process more unambiguously referred to as polarity reversal. This scenario also results in silence at the combined output, hence the common confusion in terminology, but it's very important to realise that the total phase cancellation here is brought about by inverting one of the waveforms, not by delaying it. In this example, it might seem like we're splitting hairs, but in practice the distinction between time delays and polarity becomes much more important.
(italics mine)
Yeah, this is a pet peeve of mine. I will always say
polarity when I mean signal inversion. That button on a mixing board or preamp is not a
phase switch its a
polarity switch! Label it correctly manufacturers, you know better!
And except in special cases, total cancellation only happens when summing a polarity inverted copy with the original. A delay will produce total cancellation only at frequency multiples which correspond to the delay time. That's a comb-filter. The minimal delay required to produce a 180 phase shift that nulls a high frequency sinewave is short. The delay required to do the same with a low frequency sine is far far longer. And the minimal delay required to produce a 180 degree shift for any frequency in-between requires a different delay time.