Suppose you’re recording a singer/guitarist. There’s a mic on the singer and a mic on the acoustic guitar. When you monitor the mix, something’s wrong: the singer’s voice sounds hollow or filtered. What you’re hearing is the effect of phase cancellation.
In general, if two mics pick up the same sound source at different distances, and the signals are mixed to the same channel, this might cause phase cancellations. These are peaks and dips in the frequency response caused by some frequencies combining out of phase. The result is a colored, filtered tone quality that sounds like mild flanging.
To prevent this problem, follow the 3-to-1 rule: The miking distance should be less than 1/3 the distance between mics. For example, if two mics are 12 inches apart, they should be less than four inches from their sound sources to prevent phase cancellations