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On many sound cards and mixers there is a button "Phase invert". What is its purpose? When should we use it?

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To avoid the effect of Phase Cancellation, as pictured below:

This happens when you record with multiple microphones and their phases are cancelling each other.

You should use it when visually you notice that one signal is cancelling the other, inverting one of the channels should solve the issue.

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    "When visually..." i think it's actually best to use your ears. Starting with one mic at unity, and one with the fader all the way down, start raising the level of the muted mic, and if you hear the bass fall away, you have cancellation. Much quicker and MUCH more reliable. At least it has been for me. (+1 btw, great answer.) – user42882 Feb 10 '18 at 16:33
  • @AytAyt: That's fine if you're in a position to mute/unmute one of the mics at will. – Lightness Races with Monica Feb 10 '18 at 16:35
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    @LightnessRacesinOrbit you're missing that you only need to check once, during sound check. In the decade and a half i've been doing this, i've learned that it's always possible to get a quick soundcheck in. You seem to mistakenly think you need to check phase constantly or something. Once at the beginning is fine. Alternatively you can do it afterwards, in your daw, then use that knowledge to nudge the waveforms into place. – user42882 Feb 10 '18 at 16:42
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    How do I check for cancellation? @AytAyt +1 – Mazura Feb 10 '18 at 17:27
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    Indeed it is better to use your ears, though I'm afraid depending how much the phase is dislocated you can't hear immediatly some subtle differences. Usually after visualising users tend to understand better the sound and sometimes they can start to hear better the effect of phase cancellation. Particulaly, I don't trust entirely on my ears, they might not be trained enough to hear some small details. I'll leave this paper here about "Visual influences on auditory spatial learning": ncbi.nlm.nih.gov/pmc/articles/PMC2674475 – Hugo .L Feb 11 '18 at 13:29
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When you are combining several microphones capturing the same sound sources that happen to have different phase conventions.

I've recently made a comparison between Røde and Oktava mics placed right beside each other and matching their gain pretty well on the mixer. When you added the signals without phase reversal, most of the bass was gone.

Now the point here was for comparison purposes, so the end product contained either one or the other, but if I had wanted to create a musically useful product by combining two microphones, I'd have been pretty annoyed (my mixer doesn't have phase inversion switches, but my DAW has). Also when switching in full sound, a phase mismatch would cause loud clicks.

In contrast, there is the trick of tying two microphones to each other, sing straight into one, and combine them phase inversed. That make most sound from a larger distance cancel and helps against acoustic feedback.

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    Wouldn't that trick have a different effect per frequency based on the wavelength of the sound vs the distance between the two microphones? – Random832 Feb 10 '18 at 19:47

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