Phase cancellation is a potential issue whenever you are recording the same instrument with two or more microphones or even a microphone and DI method. Let’s talk about what phase cancellation is, what causes it, what it SOUNDS like it, and most importantly how to fix it if you have it in your mix.
Phase Cancellation
First, let’s identify what phase cancellation even is.
What is Phase Cancellation
As I covered in my overview of phase issues, phase cancellation is the result of audio reaching two recording sources, like two microphones, at different times to where the crests of one waveform are coinciding with the troughs of the other.
Let’s say we’re recording acoustic guitar with two microphones which are a few feet apart. If the two microphones aren’t placed the same distance from the guitar, the sound waves will reach the microphones at different times.
Even the difference of a few milliseconds can result in the two wave forms being completely out of sync with one another.
As this image demonstrates, the amplitude’s crests of one track can coincide with the troughs of the other track:
When the trough of one track coincides with the crest of the other, this causes the two waves to effectively cancel one another out, resulting in silence.
Sometimes there are practical reasons why we want phase cancellation.
Incidentally this is the basis of how noise cancelling earphones work. If you were curious what phase cancellation sounds like, that’s a perfect example.
Earphones and headphones with this setting feature tiny microphones which pick up outside noise. The device then inverts the polarity of that signal and plays that alongside whatever you’re listening to on the earphones.
If you’ve ever turned this feature on in a noisy environment you can hear firsthand the powerful effectiveness of phase cancellation.
In our mix, however, we obviously want to avoid noise cancellation as it makes our tracks mostly if not fully disappear.
Even slightly out of phase audio sounds weaker than it’s meant to, so let’s talk how to fix this problem.
How to Fix Phase Cancellation
If you have complete phase cancellation, the fix is to simply invert the phase of one of the two tracks.
Some digital audio workstations (DAWs) which you mix in allow you to do this on the track itself, others have specialized plugins.
My DAW, Ableton Live, allows you to invert the phase of a track through a stock “Utility” plugin:
Simply tick the L and R buttons underneath input to flip the phase 180 degrees, having this effect:
The crests are now in the trough positions which, in the case of complete phase cancellation with another track, will perfectly put your two tracks in phase with one another.
More often than not you don’t have complete but partial phase cancellation.
In this case you need to manually drag whichever track is slightly behind by a few milliseconds forward/earlier as I explained in my overview on how to fix phase issues.
You can also use a plugin like Waves’ InPhase to expedite the process a little:
Phase Cancellation Reviewed
Phase cancellation results from two recordings capturing the same source by a difference of a few milliseconds and specifically is defined by the crests of one track coinciding with the troughs of the other.
Be mindful of phase cancellation or issues in general any time you record one source with two or more microphones or even a DI alongside a microphone as this can result in slight timing discrepancies and with them phase issues.
This is a common problem when recording a drum kit with multiple microphones. For instance, the overhead mics typically capture the snare slightly after the snare microphone which is right up on the snare itself. When the two tracks are mixed together, the snare will sound weak when phase issues are present.
Simply inverting the phase of one track will correct the most extreme form of phase issues that is phase cancellation, but more often than not the phase isn’t completely out of sync and is more effectively fixed by manually adjusting the timing of one of the affected tracks by a few milliseconds.