Phase cancellation
Phase cancellation – where two versions of the same signal exist and are combined, and if the delay between the two signals means that the peak of one waveform combines with the trough of the other, then the resultant is nothing. The two identical waveforms have cancelled each other out. While total phase cancellation is rare and difficult to achieve, phasing problems can occur when two microphones spaced unequally apart are picking up the same instrument. The combined signal at the mixing desk then has the two waveforms interfering with each other and this creates a swirling sound or a high-pitched whine. Phase problems like this may be avoided if the second microphone is placed more than three times the distance from the source as the closer mic.
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