All audio waves have a positive and a negative side. When a microphone’s capsule is pushed away from the front of the mic by a sound pressure wave, this typically generates a positive voltage to the mixing desk. When the capsule is pulled forward by the soundwave, it produces a corresponding negative voltage. It’s these fluctuations from positive to negative, the opposing phases of a soundwave, that represent the original soundwave.
Phasing, the alignment of these peaks and troughs, is not an issue with one microphone, but what if you have two microphones on the same instrument? Phase misalignment and cancellation become a real headache.
When recording a snare drum, most engineers will use a mic facing the skin of the drum with another underneath to capture the rattling snares. This poses a problem: when the snare skin is struck, the downward pressure will pull the capsule in the top mic and create a negative voltage, but the mic underneath will have its capsule pushed down and create a positive voltage.
In this case, the two mics are said to be 180 degrees out of phase: combine them in a mixing desk and the positive signal from the top will largely cancel out the negative version of the same signal coming from the bottom. The snare will sound weedy, weak and ‘wrong’.
To counteract this, an engineer must ‘invert the phase’ of one of the microphones, reversing its polarity, and thus add to the other mic’s signal where previously it was cancelling it out. Clever!