The switch: same sound, opposite polarity
Two oscillators play exactly the same note. Pressing Ø flips the second channel — every rise becomes a fall. The sum collapses to nothing.
The meter shows the level of the sum relative to one channel alone. In phase you get +6 dB — twice the sound pressure. Fully inverted you get silence. Note that a 180° phase shift gives exactly the same result here as the Ø switch, because only one frequency is involved.
Two mics on one source
In the real world there is no clean 180°. There is a difference in distance, which means a delay — and that delay cancels some frequencies while boosting others. This is the comb filter everyone calls “a phase problem”.
Sweep the delay and you hear a flanger — those are the notches moving along the frequency axis. Now press Ø: notches and peaks trade places. There is no absolute “correct” here, only the option that sounds fuller — usually the one where the low end survives.
The mono test
Stereo wideners run on exactly this principle: send one signal to the right and its inverse to the left. Sounds wide and impressive — until someone plays it back through a single speaker.
Watch the L+R meter: it already sits lower than the other two. The high shimmer layer lives only in the difference between the channels, and in mono it evaporates. This is why you check in mono before sending anything to radio, a Bluetooth speaker or a club PA.
Where this catches you in the studio
Snare top and bottom
The bottom head moves opposite to the top one. Without polarity inversion on the bottom mic the body of the snare cancels, and all you keep is the rattle.
Bass: DI and amp
The amp signal arrives later, after a metre and a half of air. Nudge the DI track in time until the bass is fattest — this is not an EQ problem.
The 3:1 rule
Two mics on one source? Keep them at least three times further from each other than from the source. Then the notches are too weak to matter.
Polarity ≠ phase
The Ø switch flips every frequency by the same amount. A phase shift comes from a delay and hits each frequency differently — step 1 versus step 2.
Every sound here is generated live in the browser. Worth using headphones or a pair of monitors — on a laptop speaker the cancellation in the low end is far less obvious.