MicsAfter 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:

Choir 1

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:

Choir 2

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]

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  • Rob

    Are 2 different speakers of a 4 speaker cabinet considered the same sound source?

    • You’d have to listen to ’em to find out.

    • Billy

      It depends on whether the speakers are in or out of phase. If they’re 180° out of phase, they’ll completely cancel. If you’re putting two capsules the same distance from two speakers that are in phase, you’re golden.

      Yes, I realize this is ancient.

  • Rodney

    How about using two mics on one source. Do I place the source between both mics?

  • john white

    How about this one?

    I’m installing Kickports into a double bass drum kit, micing two bass drums at the Kickports. The drums are 16″ deep. So if the mics are going to be at the kickport openings, would I cut the holes in the heads for the kickport placement so that the mics would be 48 inches apart? This would be treating the beater as the sound source. 3 x 16″ = 48″

    -or- just simply 3 foot apart from each other? This would be treating the kickport opening as the sound source, I’m guessing.

    I can rotate the drum heads on the shells to adjust the distance between them, but they have Ludwig logos on them. Something tells me I’ll be paying the school of hard knocks for the education, the price of new heads once I figure out the distance.

    Thanks

    • If you’re talking about two different kick drums entirely, then the 3:1 rule doesn’t apply. The 3:1 rule has to do with using two microphones on the SAME source.

      • George Simpson

        No, wrong, it only applies to different sources…
        but besides, you’d find that a either mic would be picking up the other drum: think about it, it must logically be more important.

        –>think about the volume difference. If you can avoid spill between them, then its OK. Sounds like one way to do it is put the holes furthest apart but angling so the mics don’t see the other drum. I’ve never heard it used much in drums. Sometimes you might space the OHs further to create better mono compatability, but its done by ear. You could do this by another means than distance in fact.
        3:1 is only to reduce the level of spill in microphones, basically. when the mic on another source could pick up the first source, there’s potetial for phasing. But not if the signal is quiet enough.
        Makes absolutely no sense on a single source as you can see hopefully, because either you’d get a really quiet useless 2nd mic, or for the reason that there’s no second source in the mic, so you could just process and line up the mics anyway.

    • Mike

      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.

  • Why are you putting ice cream cones in front of choirs, Joe? : )

    #BestGraphicEver

  • Edith Ballistics

    Phase is dependant upon frequency, and even with simple math you can see that when using multiple mics it is impossible to completely prevent phase cancellation (i.e. making some frequencies reduced or ‘null’). Without belabouring the math, the 3:1 rule evolved in part because an odd number is mathematically ‘friendlier’ than an even number (eg. 2:1) as there is less chance for the frequencies and their overtones to line up and cause phase cancellation. When mic-ing some acoustic instruments, you can get a great sound using a large diaphragm condenser 1 foot away and a small diaphragm condenser 2 feet directly behind it — preserving the 3:1 ratio.

    • George Simpson

      Is that true though? 3:1 rule is nothing to do with lining up frequencies, that mistake is always spread around. Its about volume. It should give a 10dbspl cut in volume (perfect scenario) between spill in microphones, which is just a healthy number. Phase only become a big problem when the two signals are of similar volume. It also might ensure that the signals have different frequency content too, or even ambience, so they don’t cancel. Note, it says AT LEAST 3:1, so why would signals line up at 5.299384:1 ??? doesn’t make sense.

      It won’t do anything for a single source, where you just have one mic more distant…unless you leave the distant mic inaudible at -10 compared to the other. The ways to get those in phase is: line up the mics after in the DAW, delay one to the point where its ambience no longer affecting phase, or hope that the signals are different enough because its further away chosing a different type of mic. 3:1 works only with different sound sources. This could be a single instrument, like miccing an acoustic guitar at the fretboard, and soundhole. Often its used in the case of ensembles of players/singers.

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  • Clare Springett

    Hello,
    I just wanted to know what you thought might be the best mic to use for Miking up a choir? I know that there are lots of great mics out there that would work well, was just wanting your opinion.
    Cheers
    Clare Springett

    • Edith Ballistics

      For choral miking I tend to lean toward super-cardioid (sometime called ‘shotgun’) mics like the RØDE NTG-2 or AKG-568, especially if the mics are going to be some distance away from the group (to prevent ‘bleed’ from other sound sources)

    • If you have the means, DPA makes the best choir overhead mics.

      If you are on a budget, don’t laugh, the Heil PR 30 is unreal on choirs. I’ve been doing HOW with choirs for 15 years. These are the only reasonably priced mics that get it done, and it’s really strange… Heil isn’t a staple brand for this sort of work, and it’s a dynamic mic that needs 48v? super strange, but i kid you not, it has a ton of headroom, and can take a lot of gain before feedback. With a GEQ on your choir bus and monitors, you’ll have no problem ringing them out.

      Use 3 on a choir of 60+. they have a wide pickup. Anything under that can be handled by 2…

      Sound: I’d describe them as warm and present. They have a hefty low end (being originally built for drums) so your basses can sound really heavy if you want that. They’re also not too crispy sounding like most stage choir mics or an SM81, but if you want that sheen, it’s there for the taking.

      Noisefloor: They’re not super hissy, and if you’re micing a live choir, don’t even worry about it. If you’re in the studio, it can be easy to over-compress into a hissy mix, so make sure you’re using a high bit rate/sample rate. I would recommend being careful about how much tube saturation you add before the record head or in the preamp… I’d add that “to-taste” during mix.

      Overall: seriously, the heil mic is a heck of a mic on choirs for the price. no joke.

      • I hear nothing but good things about Heil mics. My drummer uses a bunch to record drums.

  • 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.

    • CamBam

      Me too haha

  • Joe

    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.

  • not_a_monkey

    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.

  • det

    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!