Any time two or more mics are used on a single source, phase becomes an issue. Phasing problems occur when a sound reaches different mics at different times. The more mics you have open, the greater the chance for there to be phasing issues. Excluding stereo miking techniques for the moment, the basic rule is mics should be placed three times further from each other than from the source. If you are miking separate instruments, try to isolate the instruments from each other as much as possible. If you are using multiple mics on one source – a speaker cabinet, for instance, take the time to get the mics ‘phase aligned’. This can be done first visually, making sure the capsules are lined up relative to the speaker, and, secondly, with a pair of headphones on. Often times a slight variation in the placement of one of the mics, relative to the other, will produce an audible improvement – or degradation – in the combined sound. This is especially noticeable in the bass frequencies. Two mics in phase will get louder and fatter when combined. Two out-of-phase mics will get hollow sounding and thin when combined.
Also of importance is the electrical phase of two or more mics. If a cable is miswired somewhere in the signal path, it will put one of the mics out of phase. A great tool to have around the studio is a cable tester which shows not only continuity, but polarity. Take a few hours and make sure all your cables are properly wired. This will save time in trouble-shooting down the road during your sessions.