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I have read that when recording through multiple microphones, that each microphone should be far enough away from each other such that the distance between mics is at least three times the distance from each mic to its intended source, to avoid phase issues. This is known as the 3:1 rule.

I'm interested to know where this rule came from. Is there something special about the geometry of a 3:1 ratio, or is it just a rule of thumb? Why 3:1 and not some other ratio?

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    That doesn't make sense. How can two microphones be both be 3 times further from each other than they are from the something between them? The most they could ever be is double, if they are placed directly opposite each other. I don't think you are talking about mic'ing an instrument in stereo. The rule as described in the answer is about mic'ing two nearby instruments in mono, so that they don't get picked up on each others mics too much.
    – rjmunro
    Feb 11, 2011 at 0:13
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    Right, the 3:1 rule is not about stereo, but using multiple microphones in general. In stereo recording you generally will see each stereo pair as one source in this case.
    – Lennart Regebro
    Feb 17, 2011 at 9:36
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    I was sure that you guys were wrong, but I checked out the recording text where I learned about this rule to make sure and you're both quite right. The rule isn't for micing one instrument in stereo, but rather when mic'ing two or more separate sources. Thanks very much for pointing that out! I'll edit the question to make more sense :)
    – Warrior Bob
    Feb 17, 2011 at 15:46

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Short answer is that it's just a good rule a thumb.

Basically you run into phasing issues when the same sound reaches two different mics at different times at similar levels. The 3:1 rule is to ensure that the sound level in the more distant mic is enough lower than the sound level in the near mic that that effects of the phase difference will not be noticable.

These articles have good detailed explanations.

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