After writing What is Phase? last week, one of my readers emailed me, asking me to address the 3:1 rule of microphone placement. (Thanks Mike!)
What is the 3:1 Rule? According to the handy Sweetwater glossary,
…when using two microphones to record a source, normally you will get the best results by placing the second mic three times the distance from the first mic that the first mic is from the source. Confusing? An example: If the first mic is 1 foot from a source, the second mic should be placed 3 feet from the second mic. Using the 3:1 Rule will minimize phase problems created by the time delay between mics.
This rule originated when engineers were micing multiple sources in the same vicinity. The same principle applies. If you are recording two different sources of sound, their respective microphones should be at least three times further apart than they are close to their respective sources. Keep in mind that rules are meant to be broken; you may prefer the sound created by ignoring the 3:1 Rule – experiment and let your ears be your guide!
Before you scroll down to the comments section and start asking me how the 3:1 Rule came into existence, let me just say that I honestly don’t know. I’m not exactly sure why it’s 3:1 as opposed to 2:1 or 5:1. However, what I do know is that a lot of people who are much smarter than me developed this theory, and it seems to work well.
The basic idea here is that you want to get the 2nd microphone at least 3 times the distance away from the first. When you get the microphones this far apart (or farther), then you provide enough separation between the two mics to keep the combined recorded signal from having phase issues.
A great example of this is recording a choir. Most new engineers try to put as many microphones in front of the choir as possible.
It looks something like this:
I used to get all sorts of calls at Sweetwater from churches who weren’t happy with their choir sound. Nine times out of ten, they would have too many mics on the choir. Each of those mics was picking up so much similar information as all the other microphones, and the choir ended up sounded thin and small.
The solution? Less is more. The 3:1 Rule would look like this:
Perhaps 3 feet away from the choir is a bit close, but it really depends on the application. You could move the microphones back one foot, then have the microphones be 12 feet from each other. Even a pair of microphones would sound better than seven mics. However, with a source as wide as a choir, three microphones seems to be a good number.
As the Sweetwater glossary stated, the 3:1 Rule is just like any other “rule” you’ll run across in the recording world. Regardless of what everyone says you should do, do what sounds best.
[Photo Credit - mrgilles]
i’ve just met your blog, and i must say that is awesome!
“do what sounds best” should be the motto of every sound technician out there, regardless of what the “book” says!
Thanks Det. And welcome!
As a rule of thumb, 3:1 works quite nicely.
In your example with multiple mics in front of a choir all mics face more or less the same direction, and evidently there is some kind of stage involved. In other words, there _is_ more space available. No harm in using a 4:1 or higher ratio, but in the result you’ll have to take care to get a proper stereoscopic result of the different sections provided with a mic.
However, in a comparably small room, say, a broadcasting studio, you may have mics placed in different directions to record/broadcast an interview situation. There’s no room for a higher ratio, and if there’s more room the intimacy of the interview will be lost. But a smaller ratio like 2:1 may cause severe undesired comb filter effects.
another 3:1 rule I just learned was keeping the microphone a good distance from a source that has a lot of movement. While recording drums, the drummer and singer insisted a microphone on the splash. Whenever we listened to it we kept hearing a long whirling gong song on the tail end of the initial hit. (almost like Time Warp, but with sound) My fellow engineering buddy told me we were too close because the splash had a lot of movement available on the hardware it was sitting on. The measurement of the lowest point to the highest point the splash would move (hardest the drummer would hit it) was about 4 inches. Using the 3:1 rule meant that in order to get rid of that weird sound we would have to have the mic around 12 inches away. Having a splash mic was useless, since we had overheads going. It’s not a black and white rule (use your ears rule) and it depends on the how much movement the transient has when played or struck too.
For some reason, when I read the definition, it sounded like it said that if the first mic is 3ft from a speaker, than the second mic should be 6 feet away from the speaker (regardless of any distance away from the mic). Then I continued reading and it cleared up, especially from the diagram.
I’ll be sure to keep this in mind. Good future reference.