3:1 Rule of Thumb Illustrated

[bouldersoundguy]

The 3:1 rule of thumb is a guide for isolating multiple sources and multiple microphones by exploiting the inverse square relationship between distance and signal strength. The goal is to get each mic to pick up its intended sound source without picking up other nearby sources. This helps minimize phase problems (two mics, one source) and bleed problems (two sources, one mic).

Note that the 3:1 rule of thumb assumes similar sound levels from both sources. If the levels are substantially different you may need to increase the ratio to achieve sufficient separation.

Distance B should always be at least three times distance A:

The 3:1 rule of thumb doesn't apply to this situation:

 

[Farview]

+1. So many people are under the impression that the rule applies to stereo pairs. It doesn't.

 

[Dogbreath]

so this isn't about menage a trois?

 

[ashcat_lt]

The rule does apply any time you're mixing two or more delayed copies of the same sound. Yes, the thing with micing the same amp with two mics and yes the thing with the stereo pairs. Hell, it even applies to parallel processing!

The dips and bumps in the frequency response caused by phase interference will be too small to be noticeable if one is sufficiently louder than the other. That's really all the "rule" is trying to say. Distance is one way to get enough difference in level, but only if you don't then turn the distant mic up.

Now, in stereo micing we actually want those phase interactions to give us positional information.

Honestly, bsg, I'm having trouble understanding how you can be so correct in everything else that you're saying and then continue to argue my point. Leads me to believe that I am not expressing myself clearly enough.

Look at that "doesn't apply" picture. If both mics are the same, have the same amount of preamp gain, and faders set at the same levels, AND distance B is at least 3 x distance A, THEN phase interference will be barely noticeable because the direct sound in A will be 9db louder than in B. If you do anything at all to decrease that 9db difference - whether you move a mic or a fader - you will start to hear the comb filter. All I'm saying, but it is quite definitely a case where the 3:1 thing applies.

If the distant mic is meant for a room mic, then you can really look at it as more like one of the top two pics. The direct sound is source A and the reverb is source B. It may well be one of those cases you mentioned where one source is much louder than the other, but you specifically said that the "rule" applies in this case.

When you're using the distant mic to bolster the sound of the close mic, that's where you might be tempted to violate the rule by way of the faders. At 9db down, the distant mic wont be adding much noticeable. So turn it up. The distant mic will start to make a noticeable difference, and part of that will be comb filtering. Your options at this point are to either live with the comb filter (sometimes it's just what you need!), nudge or delay one of the sources, or use something like the IBC to smear the phase relationships enough that it stops sucking so badly.

 

[bouldersoundguy]

This is about the distance rule of thumb used when setting up to record multiple sources with multiple mics and get sufficient separation to have the freedom to mix as desired, without the bleed dictating what can and can't be done.

Certainly distance ratios matter for multiple mics on one source, but that's a fundamentally different thing than multiple mics/multiple sources. The "right" placement may or may not fit the 3:1 rule.

In the case of direct and ambient mics on one source maybe you can look at it as 3:1, but how exactly do you measure the ambient distance? In that case really you have to rely on your ears, experimentation and experience.

 

[toddyjoe]

I think both of you have have hit the nail on the head regarding the problem with most Internet discussions about the 3:1 rule -- most fail to emphasize the part about multiple sources versus a single source. As pointed out, the picture above stating "The 3:1 rule of thumb doesn't apply to this situation" can be misleading to the uninformed because the physics of the rule DO apply to that illustrated situation and to stereo pairs and other multi-mic set-ups on a single source. In those single-source situations, however, you sometimes WANT that phase interaction between different mics and the audible peaking and notching as it provides a sense of position, like in stereo pairs.

As noted above, the phrase "3:1 Rule" is intended to refer to "multiple source" and bleed situations where you want to avoid mics on one instrument or source messing with the phase and the sound of mics on another instrument or source. Unfortunately, this point sometimes gets lost when one sees a professional or "expert" mention the 3:1 rule in a web forum . All of a sudden you see folks regurgitating and blindly citing the "3:1 Rule" in every situation involving a microphone without understanding the intended application.

Hopefully this thread pops up in searches regarding the "3:1 Rule" and corrects at least a few people's misunderstanding. Good post.

 

[Farview]

Even though the 3-1 distance will always have the same effect in every situation, it's not useful positioning a stereo pair of mics. It's also pretty pointless in double micing a single source, since the mix volume dictates the outcome.

In an isolation scenario, with two mics on two sources, the sources have to be relatively the same volume and the same relative mix level for it to work.

The 3-1 rule is about isolation, that it prevents comb filtering is just a side effect and not the point.

 

[ashcat_lt]

This is such a poorly understood topic I definitely think it's good to have the discussion. Honestly, ive been down this road on a couple of different forums...

It's been years since I held it in my hands, but I seem to recall the Yamaha Sound Reinforcement Handbook talking about 3:1 in context of stage monitor positioning as well, pretty much the opposite of the cases cited above, but it still applies and for the same reasons.

It is something to consider any time you are combining two delayed versions of the same signal. It's always the relative volume that matters.

But it is a rule of thumb. It is meant to inform our decisions, not constrain them.

 

[bouldersoundguy]

Look at that "doesn't apply" picture. If both mics are the same, have the same amount of preamp gain, and faders set at the same levels

But if you're capturing one source with two mics at different distances you're not necessarily going to leave the gains the same. If you are going to use both mics you're going to make them both loud enough to hear, and that will obliterate the level difference caused by the distance difference. There is no bleed from unintended sources, only phase interactions between the signals from the two mics. That's why 3:1 doesn't apply to a single source.

With multiple sources the bleed is from sources you don't want in the mic. That's what the 3:1 distance rule is intended to address.

 

[miroslav]

This is such a poorly understood topic ...

.....I seem to recall the Yamaha Sound Reinforcement Handbook talking about 3:1 in context of stage monitor positioning as well, pretty much the opposite of the cases cited above, but it still applies and for the same reasons.

There are a few different applications where 3:1 is used...which is why the "rule" tends to get blurry.

 

[ashcat_lt]

But if you're capturing one source with two mics at different distances you're not necessarily going to leave the gains the same. If you are going to use both mics you're going to make them both loud enough to hear, and that will obliterate the level difference caused by the distance difference. There is no bleed from unintended sources, only phase interactions between the signals from the two mics. That's why 3:1 doesn't apply to a single source.

With multiple sources the bleed is from sources you don't want in the mic. That's what the 3:1 distance rule is intended to address.

Yes, you're violating the rule, and should be slapped!

Or rather, you should be aware. Rather than wondering why the hell it doesn't sound as awesome as you would think from listening to the two individual mics in solo and imagining what it will sound like when you mix them, you will at least not be surprised. What you do with that information is your own business, though I offered suggestions of "solutions" above.

That kinda brings us back to the OP in the other thread where this debate originated, and is now sort of running in parallel. All I've been trying to say is that if you're looking to mix the direct signal in both mics in a ratio smaller than 3:1, you're going to have audible phase issues. This is a point you have conceded a number of times. Where's the argument?

 

[bouldersoundguy]

That kinda brings us back to the OP in the other thread where this debate originated, and is now sort of running in parallel. All I've been trying to say is that if you're looking to mix the direct signal in both mics in a ratio smaller than 3:1, you're going to have audible phase issues. This is a point you have conceded a number of times. Where's the argument?

The amount of phase interaction between two mics on one source doesn't depend on the distances, it depends on your mix levels. And there is no bleed to prevent. That's why 3:1 doesn't apply to one source.

 

[Seafroggys]

Interesting read. I did a lot of 2:1 recording a month ago by accident, and I got no weird phasing effects. Kinda shocked me because the 3:1 rule has been pummled into my head since when I took Audio 101 in college.

 

[mixsit]

..That kinda brings us back to the OP in the other thread where this debate originated, and is now sort of running in parallel. All I've been trying to say is that if you're looking to mix the direct signal in both mics in a ratio smaller than 3:1, you're going to have audible phase issues. This is a point you have conceded a number of times. Where's the argument?

Let me try a different tack on this.

There are a few different applications where 3:1 is used...which is why the "rule" tends to get blurry.

So much if the time if it's 'blurry it's from what's being left out in these discussions, either in not being specific enough or being incomplete.
Of course if you move a mic away -far enough to attenuate so you don't hear the mix of the delayed signals, well ok you have 'applied this.
One good reason it doesn't IMO belong and ought not to be implied to fit close + far' is that it totally allows clouding of the intent and the reasoning of it.

Ie; If you are double micing close + far, what is the reasoning?
The second / far mic's position for it's tone and/or ambience'.


You want to hear / use this other mic's contribution?, you're going to blend them?

Then you either like the sound of the 2nd mics placement and the phase interaction with #1, and/or mess with the time alignment -or maybe end up just blending a little in.

Now.. this 'distance / ratio /attenuation thing..?

It's not just irrelevant, being injected there.. What's happened is it has set people up for it being misunderstood.

 

[Bobbsy]

Nice post, bouldersoundguy.

One area where this definitely applies is with the typical "mics across the front of the stage" layout very common in live theatre sound. A smaller number of mics farther apart rather than lots of mics can actually sound better because of the destructive interference if they get too close together.

It's worth saying though that, just as you say, this is a rule of thumb. For example, you can cheat on this in the theatre if you're actively mixing line by line (quite common) and avoid having adjacent mics up at the same time. Note that this doesn't void the rule of thumb--if anything, it proves it--but gives you a way of working around it.

 

[Farview]

Again, the 3-1 rule is about isolation. in a stereo Mic setup or a near-far Mic setup, you are not trying to isolate anything. That is why it doesn't apply.

 

[bouldersoundguy]

Interesting read. I did a lot of 2:1 recording a month ago by accident, and I got no weird phasing effects. Kinda shocked me because the 3:1 rule has been pummled into my head since when I took Audio 101 in college.

Directional mics can add some flexibility of placement.

 

[Rexinator]

Hmmmm. This discussion about the 3:1 rule differs from my understanding. I thought the guideline applies to the difference between two mics being used on one source, not the distance of each mic from the sound source. For example, when micing a choir, if two mics are placed ~ 3 feet in front of the vocalists, they should be ~ 9 feet apart. Is this outdated? If so, there is a lot of misinformation on the internet because a quick google search pulls up numerous references consistent with what I described. And they state that the main objective is to minimize comb filtering. So it's not simply a matter of placing the second mic farther from the source such that the recording level is diminished and thereby reduces phase problems. This explanation doesn't actually make sense to me, so I'm not defending it.

 

[PDP]

Sorry if this doesnt belong in this thread, but I was pretty surprized when I saw this picture of Van Halen recording at Sunset Sound. Is it a common thing to record a band playing that loud without any isolation?

It sure looks like David lee Roth is planted for the picture, but the rest looks real.

http://www.sunsetsound.com/

 

[miroslav]

Is it a common thing to record a band playing that loud without any isolation?

Well...from the picture, you can't really tell if they are playing at all.......I mean, it could just be a publicity shot.

That said, it's not uncommon to record a band as if they are playing live. How much isolation is used between the players/instruments, is relative to the production goal....though I would say in the majority of multi-player tracking sessions, the norm is to have "some" isolation between them, but it might be nothing more than a basic gobo, just for minor separation (not really isolation)....with maybe just the lead vocals getting isolation....though there are times when artists like to just sing "live" in the same room as the players and let it all bleed.
Mic bleed is not necessarily a bad thing at all....especially with talented musicians that can deliver the same quality take after take, so even if splicing/comping/editing occurs....it all blends very well even with the bleed.