Phase cancellation
Two copies of a sound arrive out of step and subtract from each other, making the mix turn thin, hollow, or weak.
See it
What it is
When two versions of the same sound arrive at different points in their wave cycles, one can push while the other pulls. Add them together and some frequencies get quieter or vanish. Two identical waves offset by exactly half a cycle can cancel completely; the small timing offsets in real recordings usually make a hollow series of cuts called comb filtering instead.
Look for it whenever one source reaches more than one mic, or reaches a mic and a direct input: drum kits, guitar cabinets, bass DI plus amp, and stereo recordings folded to mono. Compare the tracks in mono, watch a correlation meter, flip polarity, and nudge timing while listening for the low end and center image to return. The old 3 to 1 rule is the cheap defense at the source: keep the second mic at least three times as far from the source as the first one is.
Gotcha: polarity and phase are not the same fix. A polarity button turns the whole waveform upside down, which helps an almost 180 degree mismatch. A time delay creates a different phase offset at every frequency, so it needs alignment, not merely a flip. Do not force every multi-mic recording into sample-perfect alignment either; some spacing is the sound, and the mono check decides whether it is a problem.
Ask AI for it
Repair the phase cancellation between these two recordings of the same source in Pro Tools. Sum them to mono, use the Trim plugin's polarity button to test an inversion, measure the transient offset in samples, then use Time Adjuster to delay the earlier track until the important peaks reinforce instead of hollowing out. Check the result on a correlation meter and by ear, preserve any intentional stereo spacing, and report the polarity choice and final sample offset.