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When I first started recording at home many moons ago, I thought it would be awesome to put several mics on my guitar amp to thicken up the sound and get a nice stereo image. But a funny thing happened when I listened to the mix—it sounded thinner, not thicker. After investigating, I surprised to discover that my guitar sounded better with just one mic, rather than three. What could have been the problem?
If you’ve recorded with multiple mics in the same room, chances are you’ve experienced phase cancellation in one way or another. Something doesn’t sound quite right, or maybe the mix sounds thin? That’s phase cancellation at work. Most articles about phase cancellation read like a science textbook, but I’m going to break down phase cancellation as simply as possible—and tell you how to avoid it.
Just like water, sound travels in waves with peaks and troughs. Imagine you’re at the beach watching the ocean. Two waves from the left and right are traveling in towards the shore, but they collide before they arrive and cancel each other out. This is how I like to think about phase cancellation.
When you place a microphone in front of an instrument, the peaks and troughs of the sound waves cause the diaphragm in the mic to vibrate in different directions. And if you place a second microphone in front of the instrument at a distance or angle that’s “out of phase”, the peaks and troughs of both waves will literally cancel each other out. That’s why my guitar tracks sounded thin—the signals were out of phase, fighting amongst themselves and cancelling out frequencies.
The more mics you use simultaneously, the more issues you’re going to have with phase. If you’re recording with eight mics in a single room, some phase cancellation might be inevitable. But there are some simple steps you can take that will help you avoid the most common phase issues.
The 3:1 rule of mic placement is your best bet for fixing phase cancellation issues before they start. If you’re using two mics on an instrument, the second mic should be exactly three times further away from the sound source as the first mic. Let’s say you’re miking a guitar amp. You put the first mic right up against the grill, about an inch away from the speaker. The second mic should be placed three inches away from the speaker to avoid phase cancellation. Pretty easy, right?
The 3:1 rule isn’t always going to work. If you’re recording an acoustic guitar and you want to mic the neck for high frequencies and the sound hole for low frequencies, it may not be possible to space the mics 3:1. But there’s good news.
First, the high and low frequencies you’re capturing are going to be different enough that you’ll avoid some phase cancellation right off the bat. But if you listen back and the track still sounds thin, you can move the mics an inch or two and record a few seconds of audio to see if the sound improves. I’ve found that a small adjustment can make a big difference. That’s not very scientific, but it works.
If you’re recording with a lot of mics (a drum kit for example) mic placement and the 3:1 rule might not be enough to eliminate phase cancellation. Luckily, a lot of microphone preamps and DAWs allow you to “invert” or switch the phase. There are also some inexpensive “phase alignment” plug-ins available that can help. If you’re working with multiple tracks, you may need to experiment to find out which tracks are causing the issue.
If you’ve tried all of these methods and are still having issues, another simple fix is to delay or nudge one or more of the tracks by a one or two milliseconds. It won’t affect the timing of your tracks, and it can make a huge difference. Another thing to be aware of is your effects settings. Certain reverbs and delays can introduce phase problems that were previously not there. If a vocal or guitar track sounds thin or isn’t sitting right in the mix, try listening to it dry. If the problem disappears, adjust your settings or just try another effect.
Phase cancellation may not be the most exciting thing you’ve ever read about, but applying these simple steps can make a huge difference in the quality of your recordings. You’ll notice your mix sounds bigger, fuller and more locked in when you avoid phase cancellation. If you have any questions about phase, please let us know in the comments and we’ll try our best to help you solve them.