3:1 Clarification

[corban]

Can somebody who really knows their stuff please clarify the 3:1 rule for everyone here, including myself? I've been coming across a lot of advice on the site about using it for one source, and I believe it's intended for two source recording. I don't know the truth enough to presume to set it straight. A nice link to a solid article would work just fine, thanks.

 

[Lionel_Hutz]

It's the same source because the same soundwaves coming in at different times, for instance a mic further away causes phase cancellation. Say for example you have miked your rock crash cymbal with a mic. The mic is say a half a foot from it. Then you miked your medium crash cymbal(which is usually right next door to your rock crash) Since your mic is a half a foot from the rock crash you'd want the medium crash mic 3 times that distance away from the rock crash mic. So you do the math in figuring out the distance!

 

[corban]

That is hardly the response I was looking for. Here is an article that does some work in clarifying things.

http://www.prosoundweb.com/install/spotlight/bartlett/phase/phase.shtml

Having read this article now, though, I'm thinking that this rule could easily apply to recording one source as well as two sources. I think the misinformation I heard earlier was what the three refers to and what the one refers to.

Damn, I'm sick of this thread already.

 

[jake-owa]

That's how I've aways heard it also Lionel.
It stands to reason that the rule is BS. The fact is that each frequency has waveforms of a different length and placing the mic at a distance based on the distance of the first mic is not going to be very likely to place the two mics in phase for all frequecnies.

Using two mics always causes some amount of phase cancellation. Use your ears and not some "rule". Take some headphones and monitor both mics while moving them around along the sound radiation path. Find the best place for the two mics. Some amount of cancellation can be benificial but you HAVE GOT to listen.

There is no formula that is better than good listening skills.

 

[corban]

There is no formula that is better than good listening skills.

I agree, although it's nice to have a guideline to start from and listen from there. Let's call it the 3:1 guideline rather than rule.

 

[FALKEN]

tracking one time we put a 421 over the drummers shoulder micing the snare, so It was exactly 2' away. we kept getting phasing with the overheads till we measured them exactly 6' away, and everything was MUCH better.

 

[bennychico11]

It stands to reason that the rule is BS. The fact is that each frequency has waveforms of a different length and placing the mic at a distance based on the distance of the first mic is not going to be very likely to place the two mics in phase for all frequecnies.

true that each freq has different lengths, but the rule IS mathematically proven. it's all about reducing the comb filtering to as low as you can get it....1dB or less. that way it's inaudbile to the human ear. it's explained more in depth on that page he linked to.

and yes, the ear is the best way to learn where you are having phasing issues. but it's okay to cheat every once and awhile and do something like record the two mics for a few seconds and then visually check the wave forms on a computer

 

[Lionel_Hutz]

Three To One is a popular saying so I figure it was invented by somebody who really knows what they are doing. (If anybody knows the brainchild of that term please let me know.) But I agree with Jake, trust your ears first, but I also agree with Corban, three to one is a good starting point and maybe not so much of a rule, but a solid guideline. Thanks Falken for proving the three to one rule to have worked for you.

 

[Harvey Gerst]

That's how I've aways heard it also Lionel.
It stands to reason that the rule is BS. The fact is that each frequency has waveforms of a different length and placing the mic at a distance based on the distance of the first mic is not going to be very likely to place the two mics in phase for all frequecnies.

It ain't BS. The idea here is that placing the 2nd mic 3 times further away will diminish the output of the 1st source enough to where phase cancellations will be way down, while you get a clean recording of a 2nd source. It's not about wavelengths; it's about getting some separation between two mics while trying to record two different things.

Levels drop off with the square of the distance. If you're miking two different acoustic guitars and the mic for the first acoustic guitar is 18" from the guitar body, then the mic for the 2nd acoustic guitar should be at least 6' from the first guitar to prevent the sound from the first guitar from interfering.

 

[mshilarious]

First off the rule is for two mics (or more) and two sources (or more). It isn't really applicable for two mics on a single source. If you do the near/far mic thing on a single source, and increase the level of the far mic to match the near, yes, you can have phase cancellation. Nothing will prevent that IF you match levels, regardless of distance, 3:1, less, or more. If you leave the far mic down -9dB or more, you won't be able to discern the phase problem; it might even sound musical.

That's how I've aways heard it also Lionel.
It stands to reason that the rule is BS. The fact is that each frequency has waveforms of a different length and placing the mic at a distance based on the distance of the first mic is not going to be very likely to place the two mics in phase for all frequecnies.

It isn't the time delay getting you "back in phase" that helps you, it's the drop in amplitude. The mic that is 3 times as far will be -9dB with respect to the near mic (assuming the same mic and same level). Thus, any phase cancellation that occurs will have a mild effect, as I said above, perhaps even musical.

Using two mics always causes some amount of phase cancellation. Use your ears and not some "rule".

Yes! And you will find your ears are happiest if there is a difference in level between the two mics for a single source.



Edit: Arrgh! First beaten by the Bear, now by Harvey. I guess I know when I've been licked. Back to AA ball for me

 

[freak1c]

Just noticed this thread after giving a long explanation of the 3:1 rule for spaced pairs in the proffesional vocals don't have mono image thread.
c'mon now, this is simple stuff.

 

[jake-owa]

It ain't BS. The idea here is that placing the 2nd mic 3 times further away will diminish the output of the 1st source enough to where phase cancellations will be way down, while you get a clean recording of a 2nd source. It's not about wavelengths; it's about getting some separation between two mics while trying to record two different things.

Levels drop off with the square of the distance. If you're miking two different acoustic guitars and the mic for the first acoustic guitar is 18" from the guitar body, then the mic for the 2nd acoustic guitar should be at least 6' from the first guitar to prevent the sound from the first guitar from interfering.

Well I was talking about three to one on a single source situation.
That makes a lot more sense the way you explained it on a dual source.

 

[freak1c]

damn people, there's tons of info on this, simple stuff. Also the 3:1 thing isn't gospel or anything, I've never seen anybody break out a tape measure on a session, phase issues might be there or not using almost any technique (not xy), wether they're a problem or not is up to you, the mics aren't cemented in place, move em around. But it does mean 2 mics on 1 source= stereo, get it? lots of mic techniques using up to 5 mics on 1 source. this is 101 stuff people.

 

[jake-owa]

Got any resources to refer us to freak, besides forum chatter?

Anything scientific?

Are you saying Harvey is wrong?

What makes you the expert on the subject if you don't mind my asking?

I think we all understand that 2 mics on a source is a stereo image assuming both mics are panned somewhat away from each other.

 

[XLR]

Thank you Harvey

It ain't BS. The idea here is that placing the 2nd mic 3 times further away will diminish the output of the 1st source enough to where phase cancellations will be way down, while you get a clean recording of a 2nd source. It's not about wavelengths; it's about getting some separation between two mics while trying to record two different things...

Thanks Harvey for weighing in on this. The topic's been going around in circles for weeks in another thread also. I think the Crown website has a good description.

Tim

 

[bigwillz24]

So I don't misinform anyone

The following is copied directly from "Modern Recording Techniques, Fifth Edition" by David Miles Huber & Robert E. Runstein...


Chapter 4 Microphones Design and Application Page 115-116

"Whenever individual instruments are being miked close (or semi-close), it's generally wise to follow the 3:1 distance rule. This principle states that in order to maintain phase integrity ... for every unit of distance between a mic and its source, a nearby mic (or mics) should be seperated by at least three times that distance." (Figure 4.33)

"Although the close miking of a sound source offers several advantages, a mic should be placed only as close to the source as necessary, not as close as possible. Miking too close can color the recorded tone quality of a source."


Make your own interpretation of what that means...

 

[freak1c]

It ain't BS. The idea here is that placing the 2nd mic 3 times further away will diminish the output of the 1st source enough to where phase cancellations will be way down, while you get a clean recording of a 2nd source. It's not about wavelengths; it's about getting some separation between two mics while trying to record two different things.

Levels drop off with the square of the distance. If you're miking two different acoustic guitars and the mic for the first acoustic guitar is 18" from the guitar body, then the mic for the 2nd acoustic guitar should be at least 6' from the first guitar to prevent the sound from the first guitar from interfering.

What do you meen by "Levels drop off with the square of distance?

 

[Harvey Gerst]

What do you meen by "Levels drop off with the square of distance?

Do a search on "Inverse Square Law".

 

[bennychico11]

What do you meen by "Levels drop off with the square of distance?

he talks about it in that link corban gave us.

and really, if you guys think about it....the 3:1 rule IS for both a single or double source. when you're mic'ing a vocalist and his guitar at the same time, both mics will pick up both sounds of the instruments. so even though your intent is to only mic one of the double source....you're actually getting two, single instruments, in one each mic. so in other words...the whole room acts as ONE source that you're micing. i guess you could say this could be useful too when mic'ing a vocalist up close and another far away to capture the room sound. you can see it as one source (the vocalist) or two sources (the vocalist and the room WITH the vocalist).
maybe i'm thinking too much into this.

also the link above mentions that 3:1 is a minimum guideline. 4:1 is even more ideal

 

[NYMorningstar]

The following is copied directly from "Modern Recording Techniques, Fifth Edition" by David Miles Huber & Robert E. Runstein...


Chapter 4 Microphones Design and Application Page 115-116

"Whenever individual instruments are being miked close (or semi-close), it's generally wise to follow the 3:1 distance rule. This principle states that in order to maintain phase integrity ... for every unit of distance between a mic and its source, a nearby mic (or mics) should be seperated by at least three times that distance." (Figure 4.33)

"Although the close miking of a sound source offers several advantages, a mic should be placed only as close to the source as necessary, not as close as possible. Miking too close can color the recorded tone quality of a source."


Make your own interpretation of what that means...

What a great book and one I would recommend for everyone to read.

There really isn't more than one interpretation to what he is saying. The picture he refers to (Figure 4.33) has two mics, two sources, simple. <b>The rule is not for dual miking one source</b>. This "is not" about a close and distant mic on one source
The rule is simply to prevent leakage from a nearby mic when you are <b>close miking two or more sources</b>.

The subject Dave is talking on is 'Close Microphone Placement'. He explains that a sound 6' from a mic is generally insignificant in level when compared to a sound that originates 3" originating from the pickup.
In other words, if we don't hear it, there is no phase issue. Follow this rule and you will maintain phase integrity.

A couple other suggestions he gives to prevent leakage is to use directional mics or use an acoustic barrier.