Easy now, didn't mean to raise anyone's ire. It's a simple fact of geometry that it is impossible to have two mics three times as far apart as they
both are from the
same source (as would be the case with a stereo pair recording a single source). Draw a diagram or get out the tape measure.. can't do it in this 3 dimentional space universe. I completely respect your experience and the reputation of GEFELL, but the information on their site is wrong, perhaps their english translation is to blame.
Now if the source is large like a chorus or orchestra, it is possible to have one
part of the large source close enough to one of the mics to achieve a 3:1 ratio between that one
part of the large source and the two mics. Other parts of the source will have lower ratios. That's not what we are really talking about, and confuses the issue by breaking up a single large source into multiple parts.
If you can show me a diagram or actual mesurements of a spaced pair setup you have used to record a single source with a spaced pair following the 3:1 rule you will convince me.. & we can dig up the bones of Pythagoras & Euclid and re-invent geometry.
No offense meant to you. The rule does of course exist, it just doesn't apply to a spaced stereo pair.. not because anyone says so, but because it cannot be done.
You are 100% incorrect The 3:1 rule does exist argue your point with GEFELL one of the most respected names in audio.
I have been a sound engineer for 20 years I know my mic techniques and not because I read them from a book because I have used them. This rule can be applyed to close micing and to distant micing BUT it is only a guidline I was only trying to say that this rule dose exist and it is used sometimes. The fact that someone says this rule does not exist does not make it so. Furthermore this rule does not always have to be used it depends on your situation.
Chris Church
Here is the url for the info I have posted. http://www.gefell-mics.com/stereo_mic_techniques.htm
Wide stereo
Set-up: Two directional or omnidirectional spread using the 3:1 rule
Primary use: STEREO RECORDING OF ENSEMBLES AND ORCHESTRA'S
Stereo image: EXCELLENT
Center image: OKBest for: Working with the mid to far field
Advantage: Better stereo imaging
Notes:
The wide stereo employs a 3:1 rule whereby the distance between mics is 3 times greater than the distance of the source. For large orchestra, an additional center mic is often used. This provides better stereo imaging but does not make for good mono compatibility.
Actually RESPECTFULLY, you have it backwards phase cancellation happens when two mics pick up the same source at the same exact time and amplitude and frequency. The polar patterns overlap making this happen. This normally happens at one frequency like for example if I had two Omni mics on a single singer and he or she was hot at 2.5k I would need to apply the three * to one ratio to avoid phase cancellation and the peek frequency or dominant frequency.
Your scenario of the rule needing to be applied with two different sources would be almost impossible to get any kind of phase problem. Hence the need to space mics with such open polar patterns (OMNI) in the first place. This is the reason why you never see a true stereo one point Omni mic; it would be a phase cancellation nightmare.
Aaah, I see, the much misunderstood 3:1 rule.
And now, sorry for the emphasis.
The rule of thumb is ONLY of any use when you use two mics to record two DIFFERENT sound sources and want the two sources to be SEPARATED. So if you want one channel to have the lead singer and one to have the backup singer it is applicable. If then the singers are each one meter away from his/her mic, then the mics should be at least three meters from each other. Then each mic will basically only have one singer, allowing you to mix and match without much problem with phase issues and bleed between the mics. As with any rule of thumb it certainly does not work in every circumstance. It is a useful starting point for mulitmicing things in the studio or on the stage.
It has ABSOLUTET NO RELEVANCE for the distance between the two mics that make up a stereo pair. Here we want both mics to capture every source to some degree. The arrival time and volume differences are queues for our brain to recreate the stereophonic landscape. How to setup the mics is ruled by what stereo image you get, and then both mics need to get at least part of the picture, exactly as your ears.
Once more, sorry for shouting -- this seems to be a very common misconception.
Gunnar
Sorry Chris, but Gunnar is correct. The 3:1 rule is for multiple mics on separate sources. it is a multi-miking rule and does not apply to a single stereo pair. You are correct in noting that comb filtering is caused by the interference pattern of a single source, picked up by two mics and then summed again (either electronically or in air). The 3:1 rule works by getting each source/mic pair far enough away from the next that any inevital bleed is or or less inconsequential for phase issues. The rule has nothing to do with polar patterns other than more discriminating patterns can, by attenuation, help to reduce bleed and limit the comb filtering that the 3:1 rule aims to achive using distance. The different path lengths between the source and the 3:1 separated mics proivides both attenuation and phase differences between the signals of all the mics that are picking up that souce, both intentionally (mic on that source) and unintentionally (bleed from other mics).
It is physically impossible to apply the 3:1 rule to stereo micing, follow me here:
For stereo miking where some part of the source (say, the center of the stage) is equidistant from each mic, you cannot have more than a 2:1 ratio of source to mic vs. mic to mic distance. The only geometry that would acheive a 2:1 ratio would be to put the mics in a line with the source on the midpoint of that line, centered between the mics. As soon as you move the source away from the line the ratio drops. An equilateral triangle of source and mics would have a 1:1 ratio.
