Michael Jones
New member
I've seen several references to a 3:1 mic/distance ratio on this board. Someone, pretend I'm an idiot, and explain what this is about. I'm assuming it has to do with eleminating phase problems when stereo micing?
harley96
New member
The 3-1 rule is for every foot away the mic is away from the the sound source any other mics should be atleast 3 feet away from eachother.You have two mics and mic A is 2 ft. away from the sound source so mic B should be atleast 6 ft. away from the first mic.This helps to prevent multiple microphone interference,hence phase cancellation. Another way to describe it is to prevent phase cancellation when signals are mixed during mixdown. If you have two mics intended for 2 different sound sources both mics should be atleast three times the distance away from eachother as they are from there intended sound source. 
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Michael Jones
New member
OK, thanks Harley. So this rule applies to multiple mics AND multiple sound sources? Because, in an X-Y or ORTF stereo micing application, this 3:1 rule would seem to not apply?
harley96
New member
I am assuming in a xy configuration that any incoming signal arrives at both mics at the same time and in phase with eachother so this technique prevents phase cancellation.When mixed down two mics in phase with eachother will add where as two mics out of phase with eachother will subtract from eachother.The worst case scenario if both mics are 180 degrees out of phase with eachother they will totally cancell eachother out.
Michael Jones
New member
Ah, yes, of course. Good point. Thanks again for your help.
PowerCouple
New member
DANGER!!!!!!
The 3:1 is for MONO recording. Nothing to do with STEREO
Phase problems when recording, say two singers, in a stereo configuration (semi-coincident, Blumlein, separate pair) will only be noticeable if you use them in MONO later, adding the different signals what may cause cancellations. 3:1 is for MONO recording of more than one source simultaneously and in the same room or phisycal space.
When using STEREO techniques that may be used later as MONO, you should use MONOCOMPATIBLE micking techniques (MS, XY) that make stereo effect by using difference of intensity and not difference of time (what produces cancellations when used in mono mixes)
The concept harley posted is correct. You should have three times the distance A (source1 to mic1 and source2 to mic2) between the two mics A and B so you will avoid cancellations when mixing and recording the signals together. Cancellations ocurr due to time difference between mics, what produces a "comb filter". The reason why 3:1 is that when you separate the mics three times the distance, you have in mic A a decreasing of incoming sound from source B of about 9.5dB and approx. 10dB of difference makes the comb filter (cancellations) inaudible.
Sorry, it's hard for me to translate it to English, but I hope you'll get it...
Peace.
PC
The 3:1 is for MONO recording. Nothing to do with STEREO
Phase problems when recording, say two singers, in a stereo configuration (semi-coincident, Blumlein, separate pair) will only be noticeable if you use them in MONO later, adding the different signals what may cause cancellations. 3:1 is for MONO recording of more than one source simultaneously and in the same room or phisycal space.
When using STEREO techniques that may be used later as MONO, you should use MONOCOMPATIBLE micking techniques (MS, XY) that make stereo effect by using difference of intensity and not difference of time (what produces cancellations when used in mono mixes)
The concept harley posted is correct. You should have three times the distance A (source1 to mic1 and source2 to mic2) between the two mics A and B so you will avoid cancellations when mixing and recording the signals together. Cancellations ocurr due to time difference between mics, what produces a "comb filter". The reason why 3:1 is that when you separate the mics three times the distance, you have in mic A a decreasing of incoming sound from source B of about 9.5dB and approx. 10dB of difference makes the comb filter (cancellations) inaudible.
Sorry, it's hard for me to translate it to English, but I hope you'll get it...
Peace.
PC