Polarity is not phase
People say "the phase is reversed" when they mean one cable is wired backwards. It is a common shorthand, and it hides a distinction that matters when you align microphones and loudspeakers.
Polarity
Polarity is the sign of a signal. Inverting it turns every positive voltage into a negative one and vice versa, at every frequency and every instant. A cable with the hot and cold conductors swapped, a loudspeaker wired with plus and minus reversed, or the invert switch on a console channel all invert polarity.
Properties of polarity inversion:
- It is equivalent to a 180° shift at every frequency simultaneously.
- It does not change the level or the time of the signal.
- It is a yes-or-no state.
Phase
Phase describes where a periodic signal is in its cycle. A phase shift is a displacement relative to a reference, measured in degrees (360° is one full cycle). Phase shifts come from:
- Time delay. A delay of t gives a phase shift of 360° · f · t, which grows with frequency, see the phase and time tool.
- Filters. Most filters change phase around their cutoff frequency, usually by a different amount at different frequencies.
A phase shift is a property of one frequency at a time. A signal with several frequency components, such as music, has a different phase shift for each.
Why the difference matters
| Polarity inversion | Delay or filter | |
|---|---|---|
| Effect at all frequencies | identical (180°) | varies with frequency |
| Cancellation when summed with the original | complete at all frequencies | only at certain frequencies |
| Can it be fixed by pressing one switch? | yes | no |
A delay of half a period puts a frequency into phase opposition, and also every odd multiple of it: 1 ms gives opposition at 500 Hz, 1.5 kHz, 2.5 kHz and so on, with full agreement in between (the comb filter, see Comb filtering). It never inverts all frequencies at once, so it cannot substitute for the invert switch.
Where each helps
- Polarity. A snare drum with microphones above and below: the bottom microphone sees the opposite pressure direction, so inverting its polarity restores the sum. Misaligned loudspeaker wiring: check with a pulse or a channel and polarity test.
- Delay. Two microphones at different distances: delay the nearer one to match the time of arrival, see delay and distance.
One more definition
A signal with a 180° phase shift at one chosen frequency is not the same as polarity inversion. People sometimes say "the signals are out of phase" and mean they cancel. That happens at those frequencies where the shift is 180°, and nowhere else. The remedy is to find the cause, not to switch polarity.
Draft: written by an AI assistant, awaiting technical review.
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