6 min read
Phase cancellation: why two good microphones can sound worse than one
Sum a boom and a lav and the dialogue can get thinner, not fuller. Here is why, and how to catch it before the mix.
Two microphones capture the same voice at slightly different distances. Sound travels about 34 cm per millisecond, so a small distance difference becomes a time offset, and a time offset becomes cancellation at the frequencies whose half wavelength matches that delay.
What you actually hear
Total cancellation is rare. What you get is comb filtering: a series of notches spread across the spectrum that makes the voice sound hollow, phasey or thin. It is most obvious when you sum boom and lav to mono, and it can vanish when you solo either track, which is why it gets missed.
The 3 to 1 rule
When using two microphones on one source, place the second at least three times further from the first than the first is from the source. The level difference between the two arrivals is then large enough that cancellation stays inaudible.
Fixing it after the fact
- Nudge one track in time until the transients line up; a few samples is often all it takes.
- Flip polarity on one track and listen: if it suddenly sounds fuller, the two were fighting.
- Choose one microphone for the line rather than blending both, then crossfade between them.
- Use a high pass on the lav so the two tracks overlap less in the low mids.
Polarity is not phase
Inverting polarity flips the whole waveform instantly. Phase differences come from time offsets and vary with frequency. The polarity switch fixes reversed wiring and some close mic pairs; it cannot repair a genuine time misalignment, which needs a delay adjustment.
Always check in mono
Broadcast, laptop speakers and phone playback often fold your stereo mix to mono. A mix that only reveals its cancellation in mono is a mix that will collapse for a large part of the audience. Fold down and listen before you export, and inspect the summed file if anything sounds suspicious.