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Thread: 3:1 Rule of Thumb Illustrated

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    3:1 Rule of Thumb Illustrated

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    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:


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    +1. So many people are under the impression that the rule applies to stereo pairs. It doesn't.
    Jay Walsh
    Farview Recording. I am also the forum spokesmodel for Terasyne Amplification

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    so this isn't about menage a trois?



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    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.
    Last edited by ashcat_lt; 08-13-2013 at 15:19.

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    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.
    Last edited by bouldersoundguy; 08-14-2013 at 10:08.

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    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.

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    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.
    Jay Walsh
    Farview Recording. I am also the forum spokesmodel for Terasyne Amplification

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    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.

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    Quote Originally Posted by ashcat_lt View Post
    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.

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    Quote Originally Posted by ashcat_lt View Post
    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.

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