Polarity vs Phase
Definition
Polarity and phase are two different properties that are often confused. Polarity describes the sign of a signal: inverting it turns positive values into negative ones at every instant and at every frequency. Phase describes a time relationship between signals at a given frequency, and it is shifted by filters, distance and processing delay. The Ø button on a console inverts polarity and does not shift phase.
In the field
When two microphones on the same source are in opposite polarity, their sum loses level, most noticeably in the low frequencies, where the two signals are nearly identical. Checking polarity is therefore standard practice whenever several microphones pick up one source. If inverting one microphone makes the sound worse, the problem is a time offset, which is corrected by moving the microphone or by adding delay.
Related lessons
Sources
- Shure, « How to Avoid Comb Filtering with Podium Mics » : annulations entre micros sur une même source · shure.com
- Phase (waves) (Wikipedia) : la phase comme relation temporelle à une fréquence donnée · en.wikipedia.org
- Ballou, Handbook for Sound Engineers : polarité et phase