3-to-1

[polyestr]

All,

I am trying to understand the 3-to-1 rule as it applies to capturing a mono-compatible stereo image of an acoustic guitar.

Application of the rule is obvious in situations where a mono image of each of two different sound sources in close proximity are being captured - like a singer with an acoustic guitar (1 vocal mic, 1 mic in front of guitar), in which the rule is a guideline which helps you approximate the necessary separation to make sure each signal bleeds into the other signal's mic at least -9dB relative to its level in its own mic, so that the resultant comb filtering in a mono mix is inaudible.

But I don't understand why it is recommended when dual-micing an acoustic guitar (or dual-micing any single sound source, rather than two sources) -- since both mics are capturing the sound hole's energy. In my case I have a mic pointed at the 12th fret, and another one over my right shoulder (I'm right handed) pointing just below the sound hole. Since the mics share a lot of common energy, I don't see how the 3-to-1 rule helps, or how the majority of comb filtering can even be avoided, with the guitar moving while it's being played and all. I'm not saying it doesn't help, but perhaps someone has a simple explanation that will help me understand.

As a side question, in terms of the stereo mix of the dual guitar mic situation, is there any rule about panning? I always let my ears guide me, but if there's any scientific motivation for panning the channels a certain way it would be nice to know.

Thanks!

[farview]

The short answer is: In the situation you describe, the 3 to 1 rule doesn't apply. It's not a stereo micing technique.

[JoshT]

Well, I'm sure someone will link to it, might as well do it sooner rather than later.

http://recforums.prosoundweb.com/index. ... 790/0/0/0/

Here is a really long debate about the meaning of the 3-1 rule, I'm sure you will either find your answer or just get more confused.

[joel hamilton]

Well, I'm sure someone will link to it, might as well do it sooner rather than later.

http://recforums.prosoundweb.com/index. ... 790/0/0/0/

Here is a really long debate about the meaning of the 3-1 rule, I'm sure you will either find your answer or just get more confused.

IMHO, things like this are best left to self proclaimed internet messageboard guru's, not people who actually like to record music.

[kayagum]

I think you're overthinking the whole thing. Even the 3:1 ratio has comb filtering- but a lot less severe than if it's less.

If you're truly obsessed with mono compatible stereo, just mic it XY or MS and be done with it. But it's not necessary.

[A.David.MacKinnon]

The real world answer is to move the mics around til it sounds good and check it in mono to make sure nothing crazy is happening.
Your ears are better tools for the job than your eyes and a tape messure.

[farview]

I'd just like to note that the 3 to 1 rule only works when both mics are at the same gain. If you turn up the far mic by 9db (to make it as loud as the near mic), the benefit goes away.

For stereo acoustic guitar, I use XY and point it where it sounds good.

[PT]

I've been really confused about this. Specifically when micing a guitar amp speaker with a close mic and a mic further back.

If you have a close mic on a speaker, it's like 1" away. So if you do the 3-to-1 rule, the second mic should be, what, 4" away? This makes no sense. And a common practice seems to be "I put a 57 right on the grill cloth and a condenser 3 FEET back." I never hear of anyone "measuring" 1 inch then multiplying that by 3 and then adding to that 1" to place another mic 4" back.

So I've never understood...if the mic is literally right up the ass of the speaker, where, scientifically speaking, would be a good place to put the second mic to reduce comb filtering?

I know, I know. I do use my ears and have gotten decent results. But I'd just like to understand the physics a little better of using two mics at different distances on a mono source.

[ashcat_lt]

PT - You've left out the whole "at least..." part of the 3:1 rule. 3' is at least 3 times as far away as 1", so you'll be fine. Of course, that's assuming you don't turn up the distant mic to where the direct signal from the amp is more than 9db below the direct signal in the close mic. In fact, I don't think it makes much difference how far apart the mics are as long as one of them is so low in the mix that the notches aren't noticeable.

[PT]

PT - You've left out the whole "at least..." part of the 3:1 rule. 3' is at least 3 times as far away as 1", so you'll be fine. Of course, that's assuming you don't turn up the distant mic to where the direct signal from the amp is more than 9db below the direct signal in the close mic. In fact, I don't think it makes much difference how far apart the mics are as long as one of them is so low in the mix that the notches aren't noticeable.

Ahh "at least"! That's starting to make a little more sense.

And you've brought up another point I noticed from the prosoundweb "discussion" that the distant mic can't really be turned up too much?

I'm obsessed with both the close and distant mics needing to sound 100% awesome independently of each other. So I could have the choice to use one or the other at mix time. But I've found that with a well placed close mic, the spot in which the distant mike blends best with the close mic, isn't always the best spot for the distant mic on its own. So I end up only turning the distant mic up a little in the mix.

Is this a common experience? Or do I just suck at micing amps?

I guess I've always wanted that 3-to-1 rule to be a magic formula to getting the perfect sound from both the close and distant mics. But maybe that's not the case?

[farview]

I guess I've always wanted that 3-to-1 rule to be a magic formula to getting the perfect sound from both the close and distant mics. But maybe that's not the case?

That's just it, the 3 to 1 rule is about isolation, not phase.

As long as the distant mic is at the same gain as the close mic, the distant mic will be 9db quieter than the close mic. Because it is that much quieter, the comb filtering isn't an issue.

You can accomplish the same thing at any distance by turn one mic down at least 9db quieter than the other one.

[JoshT]

I thought I posted this, I guess it disappeared. Anyways, as I understand it, the 3-1 rule is about 2 mics on 2 different sources of sound, not 2 mics on one source of sound.

Nevertheless, as Joel alluded to, who cares about the specifics as long as it sounds ok.

[willowhaus]

Just to add a little nit-pickery...

...since both mics are capturing the sound hole's energy. In my case I have a mic pointed at the 12th fret, and another one over my right shoulder (I'm right handed) pointing just below the sound hole.

You're not really capturing just the sound hole's energy. The sound of an acoustic instrument comes from not only the holes, but also from the resonant top, back, & sides. That's why simply pointing a mic at the hole doesn't really capture the sound - because the "sound" is a combination of the above.

To me (I could be wrong, of course) it sounds like your setup would probably be more than 3:1 anyway, assuming that your 12th fret mic is fairly close - distance from mic 1 to 12th fret area, vs. distance of mic 2 from below the soundhole.

At any rate, regardless of 3:1 - I agree that what matters is whether it sounds good or not.

[polyestr]

Yes, unfortunately I'm the only one involved in this particular project. I'd love to have the luxury of stepping outside my body and moving mics around while continuing to sit there and play.

Anyway, this is kind of what I was guessing about 3-to-1 .. its a shame the Internet is as full of information as it is of misinformation. Lucky to have boards like this full of such learned folk to settle the score..

I have been able to obtain beautiful results with the mic positioning described in my original message. Even still, in a situation where there may be important rules, it's nice to know I'm not breaking any. So the fact that 3-to-1 doesn't apply here, I guess, is pleasing.

I know drum overhead mics need to be placed equidistant from a snare head to avoid phasing problems. Does this also apply to the positioning of my mics relative to the acoustic guitar's sound hole?

I guess I'm worried about subtle phasing and comb filtering that may not be immediately apparent during tracking.

[farview]

I guess I'm worried about subtle phasing and comb filtering that may not be immediately apparent during tracking.

If you are working in a DAW, you can always slide the tracks to get rid of phase problems that you might accidently get.

If you are using two mics to get a stereo effect, it might be a moot point since the mics will be panned away from each other.

If you are looking for mono compatibility, you might try XY or MS.

If you are just trying to get mono with two mics, just make sure the distant one is quieter than the close one. (or vise-versa)