Here’s a great little video explaining how to use phase reverse in Logic.
Phase cancellation can happen if you have two mics recording a single source, and because of the differences in time it takes for the sound to get to the mics, the sound can be out of phase and some parts of the two waveforms can actually cancel each other out. If you reverse the phase on one of the signals, then you could eliminate this problem.
This could also happen if you are layering kicks for example in your productions.
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