The 3:1 rule

[murf214@]

Couple questions about the rule. I understand that two mics have to be separated 3 times the distance they are from the sound source. My first question is can the ratio be greater? For example, 4:1 or 5:1. My second question, will there be phase problems if the two mics are different distances from the sound source? Example of this would be one mic up close and the second is a room mic.
thanks for any help.
Sean

 

[SouthSIDE Glen]

It's absolutely amazing just how pervasive the misunderstanding of the 3:1 rule its. Here's how it is supposed to read:

Any given microphone should be at least three times farther from the next closest miked source as it is from its own source.

All the 3:1 rule is meant to be is a guideline for reducing the amount or level of significant bleed into a microphone from an adjoining source that has it's own microphone, with the intention of keeping possible phase delay issues caused by such bleed to a minimum. Note that the 3 represents the distance to the next mic's source, not to the next mic.

If you really want to simplify the 3:1 rule, it can be stated this way:

Position your mikes to keep the bleed from the neighboring sources to an insignificant minimum so that the bleed does not cause phase issues when everything is mixed together.

That's it. That's it's full meaning. the 3:1 distance is just a general guideline (and a very good one, BTW) for minimum recommended distance based upon some very basic math rules, but is by no means meant to be a magic ratio that must specifically be adhered to. There is no magic to the 3:1 ratio itself. It could be 3.3:1 or it could 4:1 or it could be 100:1, or just about anything in-between. It can, in fact even be less than 3:1 if the bleed from the adjoining miked source is not actually creating significant phase interference with the signal in the adjoining mic.

With this understanding you'll see that the rule does not apply whatsoever to two mics miking the same source; i.e. that it has nothing to do with close mic vs. room mic.

G.

 

[KineticSound]

I understand that two mics have to be separated 3 times the distance they are from the sound source. My first question is can the ratio be greater? For example, 4:1 or 5:1. My second question, will there be phase problems if the two mics are different distances from the sound source? Example of this would be one mic up close and the second is a room mic.

The general rule is that you should use at least a 3:1 ratio to minimize phase issues. A greater ratio is fine, too.

In a room mic'ing setup, which is common for a variety of applications, observing the same rules should eliminate most issues. If you are having problems with phasing when using this technique, just flip the phase of the room mic.

 

[Farview]

The short answer is: The 3 to 1 rule is not about phase, it is about isolation. Being 3 times farther away from the source than the close mic, the far mic will be 9db quieter. (as long as you don't do something to adjust the levels) Since the far mic is that much quieter, the phase problems between the mics aren't an issue because they are so quiet that you can't hear them.

The long answeris here: http://www.independentrecording.net/irn/columns/jwal/index.php?id=50

 

[murf214@]

Thanks for the help guys.

 

[mixsit]

..In a room mic'ing setup, which is common for a variety of applications, observing the same rules should eliminate most issues. If you are having problems with phasing when using this technique, just flip the phase of the room mic.

When I read this I presume it means 'close and far' mics on the same source -in which case we're right back again. It does not apply there.
Typically the main contribution for a 'room mic not having time/phase problems is the multitude of random reflections in the far field averaging out with the primary line of site delay effects.
Again, what would be the point in applying the attenuation of 3:1 -and then wanting to bring the far mic back up in the mix undoing the attenuation you got by moving the mic back in the first place?