When determining where to place mics on stage or in the studio, be aware that microphones placed close together, even though aimed at different sound sources, may not always pick up just the intended sound source. A mic can easily pick up "leakage" from adjacent sound sources, depending on its polar pattern, frequency response, sensitivity, and how it's angled relative to the unintended sound source. This "leakage" can degrade a mix.
To see how, let's say we have two singers side by side, each with their own microphones.
The sound from singer No. 1 will arrive at the mic directly in front of her first, and then arrive at singer No. 2's mic slightly later. When these two signals are mixed together, comb filtering will result, producing an uneven and often unnatural-sounding frequency response. The degree of comb filtering will depend on how much "leakage" sound level comes from mic 2.
The range of frequencies affected by comb filtering will also depend on the off-axis frequency response of mic 2 (which frequencies are picked up better off-axis) compared to the on-axis frequency response of mic 1.
If these comb-filtering nulls are broad, they will be quite audible. And because they are acoustically generated, they can't be subsequently boosted by an electronic equalizer. So, it's best to listen for them during rehearsal or a sound check, and make changes in mic placement should they occur.
