I tend to go back and forth on some of my mixing approaches. For example, I thought I had sworn off using drum overhead mics in exchange for under-miking cymbals, but lately I've been flying my good ol' overhead mic on the drums again. When I do use an overhead mic, I've become a fan of using an X-Y stereo mic to capture more of an overall kit sound than simply cymbals. Using a stereo mic is advantageous for this because it ensures phase coherency at the capsule. Phase coherency is one of those things I probably overlooked for too many years in my career, until the light bulb finally clicked and I could hear what the lack of it was causing. A phase-coherent overhead mic guarantees that the snare and toms captured by that mic won’t have phase problems resulting from the left and right capsules "hearing" those drums at different times. So, aside from the effects of the mic’s frequency response, I know that the drums coming through that mic will sound like the actual drums.
Phase issues can still come into play with this approach, however, when I start blending in close mics with the overhead. If my overhead mic is three feet above the kit and my snare mic is only a few inches off the snare, the snare will arrive at the overhead roughly 3 ms later than at the close mic. If the snare in these two mics combines in my mix at more-or-less the same levels, that time or "phase" offset will create phase cancellation—also known as “comb filtering”—in the sound of my snare. If I employ a little bit of math from my Smaart training, I can estimate that the time difference will create a null right around 160 Hz, which means it will take some of the meat out of our drummer's wonderfully fat snare. Additionally, the time offset can also smear the transient of the snare and dull its punch in the mix.
One of the tools I've been using to approach these types of phase issues is Waves' InPhase plugin. I keep InPhase instantiated at all times on a stereo utility channel. I would love to use it all over the console since there are so many things I like to check with it, but this seems to be the best way to optimize my DSP use on the Avid VENUE. To make use of my one instance, I have a series of snapshots programmed to patch different instruments into the plugin where I might want to check phase. For example, I have snapshots that patch my top snare mic or tom mics into one channel of the plugin and one side of my overhead mic into the other. I also have snapshots to patch my multiple guitar mics on the same amp so I can compare the phase between those mics. Basically, if I’m using multiple mics on an instrument, chances are good I have a snapshot programmed to patch those mics to InPhase.
So here’s a typical scenario involving my snare and overhead mic. I’ll start my process during soundcheck or rehearsal by recalling my snare snapshot and pulling the plugin up on the console. A quick click of InPhase's Capture switch gets the ball rolling with a visual inspection of the waveforms captured from the mics. InPhase’s window makes it very easy to see the difference in time between the two mics, but I try not to get too hung up on what I see because ultimately what we hear is the overriding factor. Next I'll click the "Mono Mix" button and solo my InPhase channel up in headphones or on my nearfields to hear what's happening when those two mics combine. If I like what I hear, I'll leave things alone, but in most cases I'll start adjusting the delay on my snare mic to line up its waveform with the overhead right in InPhase's window. I listen to the combined result as I do this, until the snare gets its body and punch back. Then I'll take the delay time from InPhase and dial it in on my snare mic before moving on to another set of mics.
I used to employ a more convoluted version of the same process with recorded audio in Pro Tools, trying to measure the distances between peaks in the waveforms, but it was always time-consuming and not something I could easily do until there was downtime. InPhase, on the other hand, lets me do this on the fly while the band is playing, and I get to hear the results in real time. I love this because it saves me time from dealing with the technical side of mixing and allows me to spend more of my time approaching things musically.
If you've never experimented with the difference getting things in phase in your mix can make, I highly suggest you give InPhase a try.