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Yes...So When you have two mics which are an identical distance from the source, sound will arrive at the mics at exactly the same time. They will need to have the same polarity in order to be in phase. If you move one mic further away you will lose that relationship but you will have positive or negative reinforcement as the initial peaks and troughs of the first mic line up with the decaying peaks and troughs of the second. Or not. When these waveforms line up, even if displaced time wise they are in phase.
In the case of the snare bottom mic or top and bottom toms you would almost always have one mic flipped in polarity against the other because they are both equal distance to the source and pointing in opposing directions.
So looking at it from a practical point of view, lets consider kick snare and an overhead for simplicity. You mic the kick and the snare according to positions that you've chosen as sounding subjectively 'best'. You cant move those, they're close mics. So then we put up an overhead and look at it's relationship with the kick. The easiest way to do this is to ask the drummer to play some kicks and print them to the DAW. Zoom in and have a look. You'll see the kick in the close mic and then the kick in the overhead a few ms later. How do the waveforms relate to each other? Is the kick in the overhead aligned with the decaying close mic? Do you need to move the overhead up or down to change this relationship? How does it sound? The situation you want to avoid is one where you flip polarity on one mic and the sound doesn't really change. Provided they're similar in level, this would mean that they're half in, half out.
So now we look at the snare. We might find that the snare needs to be reversed in poloarity to reinforce with the overhead because that's the position for the overhead which worked best for the kick. We might find that with that kick position the snare is half in half out and that we would prefer to favour the snare and move the overhead due to the fact that the overheads have more snare information than kick low end anyway, which would mean that we have more to lose with a dodgy phase relationship. But whatever, there's no reason why you might not have a flipped polarity on snare top, and a positive polarity on kick inside, etc etc. It can be any way around. The thing to avoid is recording all the drums half in half out, because then you cant just flip polarity and correct it.
J
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