The Stereo 3:1 Rule

  • Thread starter Thread starter mixsit
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Yeah, you see the 3:1 rule described wrong at a number of sites. To be fair to Tweak's Guide in its entirety though, they usually get things right.

Until recently it was usually described wrong at HR too. Like this 3:1 rule thread. Total confusion despite the authoritative tone of parts. But hey, no big deal. To me it doesn't matter if the tracking engineer knows correct scientific terms, just the sound quality they get.
 
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There is also the stereo 3-1 rule, which has it that if the Left Mic is 1 foot from the source, then the right mic must also be 1 foot from the source but three feet from the 1st Mic.
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I guess we can just throw coincident pairs out the window, then. Not to mention actual stereo mics with two capsules in a single case ;).

G.
 
alright then, once and for all, somebody please explain it clearly, and then lets have everone agree on it. I had a notion of what I thought it meant, and now I'm not so sure anymore.
 
andyhix said:
alright then, once and for all, somebody please explain it clearly, and then lets have everone agree on it. I had a notion of what I thought it meant, and now I'm not so sure anymore.
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First of all, I have no idea what drugs may have been slipped into my food or which northsider may have hacked my username (;)) back when that XLR thread occured. I have no memory of that thread, but I can tell you that I was quite wrong in my definition there. Yikes! My aoologies to any who particiipated in that thread a couple of years ago, I have no idea who stuck the needle in my arm on that one. :confused:

The 3:1 rule only applies to mics on seperate sound sources and does not apply to stereo miking techniques.

Here's the real deal:

The 3:1 rule states that the distance between individual microphones should generally be at least three times the distance from each mic to it's individual source. This applies solely to microphones on seperate sources and is meant to minimize phase issues that can be caused by bleed from nearby sources. It is not meant at all to address phase issues that can be caused by multiple mics on a single source.

The principles behind it are two-fold: First and foremost is the application of the inverse square law relation between distance and volume; i.e. that any time you double the distance from the sound source, the perceived volume level drops to 1/4th the volume at the original distance. The 3:1 rule is designed to try and ensure that any leakage of sound into a microphone from another source is not going to be of high enough amplitude to cause major phase cancellations.

Second, is that the rule helps take advantage of the directional polar patterns of cardioid or figure-8 microphones. The greater the peripheral distance between sources, the more off-angle the source of the leakage and the more it will be rejected by a directional microphone. Again with the same purpose; to reduce the amplitde of leakage so that it's phase effect on the primary source is minimalized.

HTH,

G.
 
Hahaha, if you guys knew me for real, you'd know I was about as far from either a Mafioso (last time I "Czeched", the Bohemian Mafia didn't exactly having people shaking in their boots :) ) or a right wing politician (in a perfect world, Cheney would have stepped in front of the shotgun. At least the other guy would have had better aim).:D

I challenge you guys build up 5K+ posts over a few years and a) make zero mistakes, and b) remember evey thread and post from back then. At least I - unlike the majority of Internet users - recognized my past mistake, owned up to it, and corrected it.

G.
 
mixsit said:
There is also the stereo 3-1 rule, which has it that if the Left Mic is 1 foot from the source, then the right mic must also be 1 foot from the source but three feet from the 1st Mic.
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Nevermind the science, 1+1=2 not 3 :)
 
I was always under the impression that it applied to stereo miking techniques, and I think I may have (I hope not) gave advice to a couple people about the 3:1 rule applying to stereo miking. :o

Figured it was true though cause pretty much every single website out there explains it that way.
 
danny.guitar said:
I was always under the impression that it applied to stereo miking techniques, and I think I may have (I hope not) gave advice to a couple people about the 3:1 rule applying to stereo miking. :o

Figured it was true though cause pretty much every single website out there explains it that way.
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Lot's of wikiality out there, for sure. While not perfect, it's best to go to sources that actually go through an editing process, though that's no guarantee either as we'll see in a second.

These two sources get it right:
Understanding Audio by Daniel Thompson said:
When individually miking proximate sources, such as several horns in a horn section, the distance between individual microphones should generally be at least three times the distance from each mic to its respective sound source. Known as the 3-to-1 rule, this principle is based on the fact that the increased distance between each source to the more distant mics will mean reduced leakage of the non-principal sources into each mic. Each source will have its greatest level at it's principal mic, and signifigantly reduced level at each of the other mics,...It is important to note that the 3-to-1 rule does not apply to stereo miking techniques.
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Modern Recording Techniques by David Miles Huber said:
Whenever individual instruments are being miked close (or semi-close), it's generally wise to follow the 3-to-1 distance rule. This principle thates that in order to reduce leakage and maintain pgse integrity, for every unit of distance between a mic and it's source, a nearby mic (or mics) should be seperated by at least three times that distance (Fig. 4.3) It should be noted that some err on the side of caution and avoid leakage even further by following a 5-to-1 distance rule.
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Bobby Owsinski, however, gives ambiguous advice at best, and not-quite-right advice at worst, that can be quite misleading:
Recording Engineer's Handbook by Bobby Owsinski said:
Simply put, the 3-to-1 principle states that in order to maintain phase integrity between microphones, for every unit of distance between a mic and its source, the distance between any other mics should be at least three times that distance. For example, if a pair of microphones is placed over the soundboard of a piano at a distance of one foot, the separation between the two mics should be at least three feet.
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The problem with the Owsinski quote is that the idea behind 3:1 is not to maintain phase integrity between mics, but rather to decrease phase interference that can be caused by leakage.

And the example of miking the piano soundboard is a poor one that can go many ways. First, he mentions a "pair of mics". Obviously if they are a coincident or near coincident stereo pair, they will most decidedly NOT follow the 3:1 rule, nor should they. Second, if they are not ment as a stereo pair, but rather as a pair of different mikes grabbing the same source in order to mix mic flavors, the 3:1 rule is not necessarily in play.

G.
 
Most of the articles I've read (I hate reading Wiki) were from searching for stuff like "Acoustic guitar stereo miking". I think I've read a few different ones, and all of them were apparently wrong.
 
After running into this 'Stereo 3:1' thing which was a new twist on this for me, I pulled these three. There are tons of references saying spaced pairs on a single 'source' may benefit from 3:1, they just don't tend to call them a 'stereo mic' technique. Ok, fine.

These first two get into the why the case could be made for 3:1 to apply to a pair on a single source -If sound is coming from left and right it could be viewed as having cross feed, as in the case for micing two sources. My feeling is whether it is important enough to want to attenuate the cross feed will depend- It's relative contribution, the width and depth of the source and the mic positions, the goal of the track..

In the first one Bruce Bartlett makes a good point andcase; The cross feed from a 'wide source (relative to mic position) is an average of the sounds coming from the different angles and widths of the instrument. Thus the effect of the individual direct straight-line differences are blended with the others, and diluted. Sounds exactly like what happens to the direct line combing of a second 'depth mic' as it is moved far enough back to be a room mic..

The 3:1 rule cannot be applied to miking a choir with a few mics. Why? Most of the singers are somewhere between the mics, and those singers will be picked up with some phase interference. However, since each singer is in a different position relative to the mics, each singer is heard with a different coloration. The effect averages out over all the singers and so is not very audible.
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http://www.prosoundweb.com/install/spotlight/choir/choir.shtml

The graphic to the left demonstrates an example of the 3:1 Rule in use. By placing the mics spaced above the hammers, a bright percussive sound is captured without phasing issues. This setup yields a great stereo mix, with bass notes on the left mic, and treble notes on the right.
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http://www.sonicus.net/stereo.html

I've never close miced the hammers on a piano in a studio (PA' yes), but drums, acoustic guitars etc, I have had no problem skipping 3:1. (paying attention instead to equal distance, or going with the color')

This third one fails completely. :rolleyes: The cardinal/common/obvious rule in all cases, :) Again, here is '3:1 trying to fix 'depth' time differences. :rolleyes:

The 3:1 rule applies more to using two differently spaced mics on a source, one close and one distant for example. The second mic should be three times farther from the first mic as the first mic is from the source.
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http://duc.digidesign.com/showflat.php?Cat=&Number=1206631&Main=1206453

Ding. :D
What'd ya think?
 
mixsit said:
In the first one Bruce Bartlett makes a good point andcase; The cross feed from a 'wide source (relative to mic position) is an average of the sounds coming from the different angles and widths of the instrument. Thus the effect of the individual direct straight-line differences are blended with the others, and diluted. Sounds exactly like what happens to the direct line combing of a second 'depth mic' as it is moved far enough back to be a room mic..
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What Bartlett is describing is what's referred to as the "near field" of a sound source, which is often defined as being a radius roughly equivalent to the width of the source or instrument eminating the sound. In his case that would mean miking icloser to the choir closer than the distance from the left-most singer to the right-most singer.

This also applies to acoustic instruments like guitar and paino. Mic closer than 7' to the business side of a 7' grand piano, and your inside that instrument's near field. Inside of about 3 feet or so and your inside the near field of an acoustic guitar. Inside of 10' and your inside the near field of a 10" loudspeaker.

He is right in that the 3:1 rule does not apply when stereo miking within the near field of an instrument or group of instruments. Simple self-experiment will show that the sound within the nearfield of an instrument can chane depending upon location within that nearfield. It is possible to multi-mic within the near field with non-stereo intentions to catch different timbres in a non-stereo mix. In such cases 3:1 can sometimes make a difference, but Bartlett is right that the interplay within the nearfield is so complex as to often make something simplistic alike 3:1 fairly unpredictable and lind of useless as a general rule.

When the 3:1 rule does start applying for sure, however, is when moves the mics in to close mic and seperate individual sources within that wade field of sound. For example, at some point, if you move close enough, the choir no longer is a choir, but rather individual singers. Give each one a seperate mic, and one should apply the 3:1 rule. Same thing with a horn section. Mic the horn section as a whole (as if the horn section were an individual instrument), in mono or stereo, and the 3:1 rule does not apply. Mic the individual instruments, however, and it does.

Close mic the piano strings (which you really should try, it's the most relaible and flexible method I have found, especially in a sub-par room) using a stereo pair, and there is no 3:1 rule. Close mic them using seperate microphones intended to catch seperate timbres of the piano intended to be used as seperate source tracks in a non-stereo way, and the 3:1 rule should be considered. Why? Because one is now using the sifferent sounds of the piano as basically different instruments, no different than close miking the individual singers in a choir with their own individual mikes is breaking the near field of the single choir into seperate instruments.

One could consider an entire band as a single instrument with a near field the side of the stage or as seperate sound sources with their own near fields within the overall near field, or with their own opportunity for close miking. A drum kit has it's own near field of some 4 or 5 feet which can be treated as a mono source, a near fueld which can be stereo miked, or individual sound sources which can me close miked.

What untimately needs to be considred is not just the source, but how it is to be used. That use needs to be combined with how big we are defining the "instrument" and how we want to use it in the mix:

1. Far field multi-miking is going to be treating the source as a mono source within a stereo space. No 3:1 there, assuming typical stereo miking techniques.

2. Near field multi-miking, if done as a stereo mix has no 3:1; it's stereo miking. If done as a non-stereo blending of two different overall sounds within the nearfield, then use your ears only, because 3:1 is not relaible.

3. If close miking individual mono sources within the overall group (individual drums in a kit, singers in a choir, parts of a piano, etc.) then 3:1 applies the most.

G.
 
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snow lizard said:
What's wrong with Northsiders??? :mad:
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:D. Nothing at all. Just a friendly jab a few folks I know who have a friendly rivalry against anybody who lives south of Soldier Field.

(that used to read south of Monroe St, but they seem to have mentally annexed all the new construction west of the Museum Campus as part of the North Side now. :rolleyes:)

G.
 
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