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Saturday, July 6, 2013

PHASE COHERENCE IN MULTIPLE MIC PLACEMENT!


Hello and welcome to this week's article!
Today we're going to talk about Phase Coherence, a problem that anyone who has recorderd an acoustic drumkit or a guitar cab with more than one microphone, has already had to deal with.
When miking a snare or a speaker with just one microphone, we don't have to worry about phase, because it's just listening to the source by itself, but when we use two or more microphones on the same source, it's important that the sound hits the capsule at the same time, because if it doesn't, the frequencies taken by the two microphones will cancel (partially or completely) each other out.
Phase can therefore be described as the time difference between two microphones in hearing the same waveform.

Let' make an example: let's record with two microphones a snare drum or a guitar cabinet: we'll end up with 2 mono waveforms.
Now we must zoom into the waveforms very close to see if the two waves are "in phase" one with the other.
To check if they're in phase, they must look as the image below:



If they're in phase, the two waves will help each other in reproducing the sound as precisely as possible, if they're partially or completely out of phase instead, they will cancel some frequency or even cancel completely each other (this is almost impossible, though).

The solutions are to move one of the microphones slighly closer or farther to the source in order to align the phase better, if we have just a sligh, partial frequency cancellation, or to flip the phase with our DAW, if the tracks are completely out of phase.
Usually we need to flip the phase when we record a source with two microphones on the opposite sides, especially when miking a snare drum with a microphone at the top and another one at the bottom of it: in this case it's very likely that we need to flip the phase of one of our two tracks.
To flip the phase, usually in almost every DAW is present a control, with this symbol:




By activating it, the phase of the track will invert, and the waves will sound much fuller, without any unwanted cancelling.
It's important to check the phase coherence of the whole drumset when miking an acoustic drumkit, by trying to flip each track, one at the time, and see if we notice an increase or decrease in tone, to get the best reproduction achievable.

This article must not be confuse with phase issues that may emerge in the mastering phase, and which are relative to the single, final stereo track. Click here for the dedicated article.

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