I've been in a few conversations lately where we wind up talking about fixing phasing issues with drum mics. There's two ways to go about it. You can either go in the live room and move the mics, or you can "flip the phase" which really means hit the polarity button on the console or DAW. I'm going to be very clear which one I mean and you'll see why in a minute.
First of all, everyone says "flip phase" because it rolls off the tongue better than "reverse the polarity". No worries. We're all big kids here and we know what the deal is. But I'll be careful to say exactly what I mean though so there's no confusion about what we're talking about.
Let's start with what the difference is between the two techniques. Moving the mics subtly changes the arrival times of a sound to each mic. Two mics on the same signal will make some frequencies combine additively and others combine subtractively. What you wind up with is a comb filter effect and you just search around until you find positions that produce a comb that emphasizes frequencies that are pleasing. Inverting the polarity of one of the mics is an electrical process that doesn't change the arrival times at all.
Because you're dealing with two mics on a single source, you're dealing with time shift, that's phase. Pressing the polarity button is like going after the problem with a pipe wrench when what you really need is a jeweler's screwdriver. Yet we see people going about getting drum sounds all the time poking the polarity button and thinking they're getting to the root of the issue.
It's a complex one though. The simple answer for the longest time has been to just move the mics. But there's another tool available to people working in a DAW and that's micro delay. That means that if you really like where each mic is sitting individually, you can mess with how they interact with each other by applying tiny amounts of delay. Each foot is roughly equal to a millisecond so we're talking about getting into decimal places here.
I'm not going to sit here and write out the whole process for you because I'm hoping you're starting to see what I'm talking about. This doesn't just apply to the Brethren of the Knob and Fader who inhabit studios either. It's becoming common practice to take direct lines not just from the bass but also from guitars on stage and use micro delay to precisely line up that signal with the mics. In the age of the digital console and line array it's possible to get things razor sharp.