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Phase Cancellation And Comb Filtering

[This article is part of The Home Recording Show Podcast, coming soon!]

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Whenever we use two or more microphones on a single source we need to be aware of phase cancellation and comb filtering. Due to the time delay of the sound waves hitting the microphone elements at different times, partial phase cancellation will occur.

When the signals from the microphones are combined you may find that the result is a thin, boxy, or hollow sound. This is called comb filtering. Due to the complex waveforms being combined, some frequencies will be in-phase and some will be out of phase.

Here’s an example.

I have a guitar combo amp miked with a 57 style dynamic mic, positioned right up against the grillcloth.

(clip-1)

I have an AT 2020 condenser microphone about 9 inches back from the grill and 1 inch to the right of the 57.

(Clip 2)

I also have a second AT 2020 at the rear of the amp.

(Clip 3)

All these clips sound ok on their own, but the point of this is to combine the tonal qualities of these different positions to make one guitar sound.

Lets play them all together.

(clip 4)

Not too pretty, that thin, boxy, hollow sound is really not what I’m looking for.

So what do we do? We play around with the phase invert buttons in our DAW.

Here is the sound with the 57 phase inverted.

(clip 5)

With the 2020 in the front inverted.

(clip 6)

and with the 2020 in the back inverted.

(clip 7)

That last clip is pretty much the sound I’m after much thicker and aggressive, some EQ and I’ll be happy.

For emphasis on the sound of comb filtering here is a clip of the 3 mics combined while I move one of the mics further away and then closer to the amp.

(clip 8 )

You can minimize the effects of phase cancellation by getting the microphone capsules as physically close together as possible.

Also using different kinds of mics will color the sound differently.

One last thing, keep in mind that you can get this kind of comb filtering when using just one mic on a guitar amp if reflections off the floor or nearby wall get into the mic. Moving the amp, tilting it, and putting down carpets under the amp will all help eliminate that problem.

Let’s discuss.