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?