2. Wave Interaction & Interference · Concept 3 of 10
Phase Cancellation
Sound going quiet or hollow because two copies of it arrive with opposite timing and wipe each other out.
Two equal copies 180 deg apart cancel to silence; flip one to realign and gain +6 dB.
What it is
Two copies of the same sound arriving with opposite timing, so they wipe each other out and the sound goes quiet, hollow or thin.
Key facts
Phase is measured in degrees: 0 to 360 deg per full wave cycle.
180 deg out of phase = full cancellation; 0 deg in phase = +6 dB (double voltage).
Speed of sound in air ~343 m/s at 20 deg C (rises ~0.6 m/s per +1 deg C).
Wavelength = speed / frequency, so lambda = 343 / f in metres.
100 Hz wavelength = 3.43 m; half-wavelength that cancels (180 deg) = 1.72 m path difference.
1 kHz wavelength = 34.3 cm; 10 kHz = 3.43 cm (tiny, very easy to cancel).
Cancel when path difference = odd multiple of half a wavelength (1/2, 3/2, 5/2 lambda); reinforce at whole wavelengths.
Polarity flip = invert the waveform (push becomes pull): the 'phase' button or an XLR pin-2/pin-3 swap.
Comb filter from a fixed delay: first notch at f = 1 / (2 x delay seconds); 1 ms delay notches 500 Hz, 1.5 kHz, 2.5 kHz.
3:1 mic rule: keep a second mic at least 3x the source-to-mic distance away to cut phase bleed.
How it works
A sound source sends out a pressure wave (air pushed out, then pulled in).
Two copies reach one point via different paths or different wiring.
If one copy's compression lines up with the other's rarefaction, the pressures subtract.
At exactly 180 deg apart and equal level, they cancel to silence.
Real rigs are rarely perfect, so you hear weak, hollow, thin sound instead of total silence.
Flip polarity on one signal to realign them, and the lost body returns.
Real examples
Two mics on one snare (top and bottom): the bottom mic is inverted, so summing them thins the drum until you flip polarity on one.
A speaker wired backwards (+ and - swapped): bass disappears in the middle where it overlaps the correctly wired box.
DI plus mic on one bass amp: a slight distance gap creates a comb-filtered, hollow tone.
Stereo signal collapsed to mono: out-of-phase content (some reverbs, fake stereo) vanishes.
Subs and mains overlapping at the crossover: wrong polarity carves a hole in the low end.
How it helps in live sound
When bass or vocals suddenly sound thin, hit the polarity (phase) button FIRST and keep whichever is louder/fuller.
Snare top and bottom mics: invert one (usually the bottom) every time.
Kick in + kick out, or DI + amp mic: check polarity and nudge mic position/delay for max punch.
Apply the 3:1 rule: second mic at least 3x the distance the first mic sits from the source.
Align subs to mains with a measurement tool (Smaart/REW) or by ear at the crossover; flip sub polarity if the low end is weak.
Suspect a backwards speaker cable if one box sounds bass-light next to its pair, and re-terminate the +/- conductors.
Everyday analogy
Like stacking a photo on its own perfect negative so the picture vanishes, or two people pushing a swing in exact opposite directions so it never moves.
Watch out
Myth: 'phase' and 'polarity' are the same. Correction: the button labelled 'phase' actually flips POLARITY (an instant 180 deg invert at all frequencies); true phase shift comes from time delay and is frequency-dependent.
Fun fact
Active noise-cancelling headphones work ON PURPOSE by phase cancellation: a mic samples outside noise and the driver plays it back inverted (180 deg), erasing it before it reaches your ear.
Key takeaways
Same sound + opposite timing = it cancels; quiet, hollow, thin is the tell.
180 deg out = silence; 0 deg in = +6 dB louder.
Common causes: two mics on one source, a backwards-wired speaker, DI+mic combos.
Fix it fast: flip polarity on one signal and keep the fuller sound.
Low frequencies have long wavelengths, so bass shows cancellation worst and most audibly.