3:1 rule

[mstaylor]

I have seen this mentioned before but I have never fully understood what it is and how to apply it. Say I am micing a choir, three rows deep. We place mics accross the front, how does this apply to placement?

 

[bishopthomas]

The 3:1 rule states that the microphones should be three times the distance from each other than they are to the source. So if you have three mics four feet away from the choir then each microphone should be 12' from each other. This isn't always possible, though, as you could imagine.

 

[museav]

3 to 1 Rule - WikiRecording
Audio-Technica - Microphones, headphones, wireless microphone systems, noise-cancelling headphones & more : Two Common Problems
3:1 rule
Mic Primer - Multiple Microphone Use | Heil Sound
Sennheiser USA - Houses of Worship, audio sound systems, voice, microphone, properties

The issue is that when using multiple microphones any sound can potentially be picked up my multiple microphones with each at a slightly different distance from the source. If you combine those mics in the mixer that difference in distance from source to mic translates to a difference in relative phase between microphones, which in turn affects the frequency response of the resulting summed signal. The idea of the 3:1 rule is to try to create a combination of relative phase and relative level difference such that this effect is minimized.

This is actually more a guideline than a rule and you always need to verify what works in any given situation, but is is a basic concept that's good to try to follow for choir or area miking.

 

[mstaylor]

Thanks guys. I have heard an explanation at some point but wasn't sure what the ratio was for.