A Crash Course In Phase Cancellation

One of the greatest threats to any recording session is phase cancellation. It’s a sneaky result that you end up with when you’re not careful with microphone placement, and can even turn up AFTER you’ve tracked everything if you’re not careful when editing your tracks.

If you’re not familiar with phase cancellation, it’s a relatively simple problem:

Any instrument that’s being picked up by more than one microphone is going to have varying distances to reach each mic. For example, it’ll take a sound twice as long to reach a microphone that’s 4 feet away from the source, as it would take for it to reach a microphone 2 feet away.

While everything is almost always going to be slightly out of phase with each other, there’s an acceptable limit that can be reached where the two mics are further out of sync than they are in sync. What I mean is that the waveform captured by one microphone is shifted closer to 180 degrees out of phase from the other, resulting in phase cancellation.

So what does this sound like to the end user?

In solo: not a thing. When a track is soloed, phase cancellation cannot happen. There is ONE audio source with no interference from another. Your soloed sound will sound natural and accurate.

But when you’re summing everything together and listening as a whole, phase cancellation sounds like weak, thin audio. Your snare drum can almost disappear from your mix if the snare mic isn’t in phase with the overheads. Guitars can be washed out if you mic the cab with two microphones if they are too far out of phase.

Luckily, there are a few easy ways to fix phase cancellation before it gets out of hand:

Option 1: The Mic

If you’re content with your soloed sounds but you’re having problems with phase cancellation, starting at the source is the way to go. Many microphones have a polarity switch, which effectively reverses the polarity knocking your phase 180 degrees back into sync with the other microphone.

Keep in mind – you’ll only need to flip the switch on one of the mics or you’ll end up in the same scenario. In a situation like the snare/overhead scenario we just talked about, the snare mic would be my first choice since it wouldn’t affect my overheads and their phase with each other or the rest of the kit. It’s always a good idea to recheck your phase each time a polarity switch is flipped.

If flipping the phase doesn’t work out for you, your mics might be at an awkward point where they’re out of phase, but not enough for the polarity switch to make much of a difference. In these situations, you’ll want to consider your mic placement, moving it if necessary.

Option 2: The Placement

Choosing mic placement can be a long and tedious process to really nail the tone you’re after, so I can completely understand hesitation about moving the mics. There are actually major records that have been released where two guitar mics were left intentionally out of phase because it helped the engineer achieve the sound they were after.

If your out-of-phase sound works for you, don’t change a thing. But if you’re losing something to the phase you want to get back and your polarity switch isn’t doing anything for you, consider trying some of these techniques instead.

  •      On drums, try an XY overhead pattern instead of a spaced pair.
  •      On acoustic guitar, make sure both mics are an equal distance from the guitar.
  •      On a close-mic’d speaker cabinet, try to match the distance from the grille cloth to the capsule of the microphones (not just the front of the mic).

Still not getting the sound you want? Then it’s time for Option 3.

Option 3: Fix It In The Box

Your DAW is a great tool for cleaning up the audio, and as a tracking engineer this is sometimes your only option when you need to clean up your phase cancellation issues. If you’re always recording the two mics at the same time, it can actually be a pretty easy fix.

What you’ll need to do at this point is zoom VERY far into your track to the point where the waveforms become apparent. What you’ll see on the two tracks once they’re side by side is that they match almost identically with one being offset from the other. At this level of granularity, you can actually see the phase issues based on the alignment.

Using your nudge setting, you can select the track you want to move (usually the same as the one you tried changing polarity on in Option 1) and nudge it forward or backward until the waveforms align and reinforce each other.

Just like after Option 1, you’ll want to make sure that changing the phase of the track didn’t affect its relationship with any other tracks. Getting something in phase with one thing can easily knock it out of phase with several others, which is why the in-the-box approach should be saved for a last resort. There may not be time to re-record if you find the issues are too large AFTER the session has ended.

On To The Mix

Once tracking is done, the engineer who tracked should be double-checking their work before sending it off to the mixer. If you’re mixing your own session, you get to carry on through the process either way. While some drum edits are left for the mixer, phase issues are rarely one of them.

Instead, the mixer should be evaluating things like the isolation between drums and their transients to make sure they cut through the mix. All of their decisions depend on having good, clean, in-phase source material to work with. The only time they should have to worry about phase is when they’re using spatial wideners, which can cause their own cancellation issues if they’re not mono compatible.

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