Chapter 1. Physical Sound
1.3. Phase and Polarity
Polarity and phase are usually confused with each other, but they are not the same. The phase is the initial value of the wave when : it has to do with the time axis of the wave; polarity, on the other hand, has to do with the displacement axis of the sine wave and represents the sign of the sine wave (see Equation 1).
Figure 2 Phase and Polarity
The pressure sine function has the value Φ at t=0; because it is positive, it has positive polarity.
You will hear people talking (correctly) about switching the polarity of an audio signal; it means changing the sign of the signal. Some people (incorrectly) say “Switching the phase of the signal”; they mean switching the polarity, but their mistake is almost logical: for sine waves, this is equivalent to changing the phase by 180 degrees since two sine waves out of phase by 180° (π) will cancel out since sin(x)+sin(x+π)=sin(x)-sin(x)=0 . Note that two signals can be in phase but of opposite polarity and of course perfectly cancel each other out; that is in fact the only way to cancel two signals.
In audio terminology, “checking for phase coherence” or “checking the phase” means making sure that all waveforms stemming from recorded instruments roughly have peaks (or nulls) at the same time so that sound does not cancel out. This is used after recording an instrument to make sure that the signals do not cancel each other; this typically could happen when recording drums with multiple microphones: if the recorded sounds are of opposite polarity, some of their frequencies could cancel each other out, making the sound thinner. This can be very difficult to pull off, especially with multiple sound sources; the best result is achieved by listening to the different sources, switching polarity of each source in turn, and selecting the result with the best sound.
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