#1819130 - 10/03/07 10:53 AM
Phase issues
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Farview
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In the article "Beating phase issues in your DAW", the writers referenced the '3to1 rule' without making it clear that it only works for multiple sources, not multiple mics on the same source. In fact, the example they gave could be read as using two mics on the same guitar cabinet. (which doesn't work)
The 3 to 1 rule only addresses phase problems because it minimizes bleed. (and really only works when both mics have equal gain in the mix) 3 to 1 is also a minimum distance, not an exact point. The farther away the second mic, the less phase problems you will get. (with equal gain)
There is a lot of confusion on the subject and a lot of people running around thinking that 3 to 1 is some magic ratio that eliminates phase issues, it doesn't.
Your magazine normally does a wonderful job of explaining these sorts of things, I was suprised when this got through. Keep up the good work.
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#1819267 - 10/03/07 03:02 PM
Re: Phase issues
[Re: Farview]
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audiorulez
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The 3:1 rule most definitely applies to multiple mics on the same source as well. Whoever taught you it didn't needs to go back to audio 101.
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#1819285 - 10/03/07 04:15 PM
Re: Phase issues
[Re: audiorulez]
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Farview
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Even if you do use it for multiple mics on the same source, it won't do any good unless both mics have the same gain applied to them. Meaning, the mic that is farther away is lower in volume than the close mic.
The rule is about isolation, not some magic position where phase lines up.
Think about it, the smaller the delay, the higher the frequency that gets cancelled. The longer the delay, the lower the frequency. There is no possible way that two mics, one 1 inch away and one 3 inches away will have the same phase alignment as one mic 1 foot away with the second mic 3 feet away. So the rule couldn't be about aligning phase.
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#1819353 - 10/03/07 07:54 PM
Re: Phase issues
[Re: Farview]
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Southside
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Farview is correct.
I'm sorry to say that the article states the 3:1 rule incorrectly and uses an incorrect example to describe it.
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.
When double-micing a single source, however - like the git cab example described in the article - neither principle of the 3:1 rule applies. First the inverse square law, while still in effect, only applies if one mixes the two mics at unity gain; i.e. if the engineer actually desires the more distant mic to be a small percentage of the volume of the close mic. As soon as the engineer raises that fader to mix the two tracks, however, he is basically throwing the inverse square law out of the window because he is intentionally raising the volume of the distant microphone.
Second, assuming both mics are directional and pointed more or less towards the cab, and idea of using the 3:1 rule to take advantage of polar pattern rejection also goes out the window.
Third, as far as phase issues go, there are ALWAYS going to be phase discrepencies between two mics of differing distances micing the same source. The only question is at which frequencies those phase issues appear.
Placing one mic 3 inches away from the cab (as in the example) and another 9 inches away only truely affects those particular frequencies whose wavelengths are multiples of 3 inches (and, to a lesser degree, multiples of 1 inch). Move the rear mic one inch further away and now the frequencies that are those with wavelengs that are multiples of 7 inches. A big difference. And netither position will do anything as far as reducing overall phase effects across the spectrum the way that the 3:1 rule does when applied to multiple sources as intended.
Fourth, there is an added element when talking about a speaker cabinet. Which driver are we considering the source? Let's say you have a 4x10 cabinet, for example, and you place the close mic on the upper right speaker because that's the one that sounds the best, and you have the mic right on the grill because that's what gives you the sound you need.
With that setup, any distant mic more than a few inches away from the cabinet is not going to be micing the upper right speaker, but rather the combined sound of all four speaker elements, each of which will be a different distance from the distant microphone (unless the distant mic happens to be alligned with the direct center of the cabinet). This composite sound will have it's own complex phase properites because of the combination of the four physically seperated "sources", and will also have a much more complicated phase relationship with the sounds that the close mic picks up from the single element.
For all these reasons, the 3:1 rule is invalid when talking about multi-miking a single source and is designed and intended only to apply to the relationship between two mics meant for two different sources.
HTH,
G.
Edited by Southside (10/03/07 08:16 PM)
Edit Reason: Typo patrol
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#1819440 - 10/04/07 05:52 AM
Re: Phase issues
[Re: Southside]
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miroslav
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As soon as the engineer raises that fader to mix the two tracks, however, he is basically throwing the inverse square law out of the window because he is intentionally raising the volume of the distant microphone.
Yeah.
I always thought the whole point of using a distant mic was to have some differences.
What I mean is…
When you take a distant mic and intentionally adjust the level to match the close mic…and then also go and align it in the DAW to match the close mic…
…you lose the whole effect of having a distant mic. 
I find that the best "rule" for using multiple mics on same or different sources is to go with what your ears are telling you.
There is no "math" that will work in every situation...
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#1819459 - 10/04/07 06:37 AM
Re: Phase issues
[Re: miroslav]
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audiorulez
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I agree with Miro, the whole point of distant mics is the time/fq difference. Time and level matching are moot, just turn the close mic up more if you're going to do that.
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#1819482 - 10/04/07 07:33 AM
Re: Phase issues
[Re: audiorulez]
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Farview
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I agree with Miro, the whole point of distant mics is the time/fq difference. Time and level matching are moot, just turn the close mic up more if you're going to do that.
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#1819494 - 10/04/07 08:11 AM
Re: Phase issues
[Re: Farview]
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miroslav
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Right.
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#1819509 - 10/04/07 08:41 AM
Re: Phase issues
[Re: miroslav]
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Southside
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I always thought the whole point of using a distant mic was to have some differences. What I mean is… When you take a distant mic and intentionally adjust the level to match the close mic…and then also go and align it in the DAW to match the close mic… …you lose the whole effect of having a distant mic. I find that the best "rule" for using multiple mics on same or different sources is to go with what your ears are telling you. There is no "math" that will work in every situation...
You don't necessarily lose the "whole effect" of the distant mic. It will still have a different sound than the close mic; you'll still be picking up more of the whole cabinet, and usually, but not necessarily, a signifigant change in the bass timbre. Also, if the mic is far enough away, you'll be picking of signifigantly more of the effects of the room. Simply mixing in at higher volume and time aligning that distant mic won't take away those effects; in fact they will make them more pronounced in the overall mix.
Additionally, though you probably alreay know this, time aligning the track will not eliminate many phase issues, it will only serve to align the initial attack of the dominant component of the wave; i.e. the overall sound will be time aligned. But because we are not sending a simple single frequency sine wave through the cabinet, but instead a wave consisting of an almost infinite number of fundamentals and harmonic frequencies, each with their own wavelengh and frequency, most of the phase discrepencies between the close mic and the far mic will still exist, though perhaps rotated in phase somewhat.
You are right, the ultimate judge has to be one's ears as afar as mic placement goes. Sometimes a particular mic placement may sound good soloed, but once time aligned and mixed with the original, the phase issues turn sour. other times the opposite may happen.
As you say, there is no math or rule in the form of distance that says that the two mics should be such and such of a distance from each other, or more, or less. It's all dependant upon the actual composition of the sound coming out of the cab, and which phase discrepencies sound good and which ones sound bad for that sound and that room.
G.
Edited by Southside (10/04/07 08:45 AM)
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#1819515 - 10/04/07 08:57 AM
Re: Phase issues
[Re: Southside]
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miroslav
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You don't necessarily lose the "whole effect" of the distant mic. It will still have a different sound than the close mic...
Yeah...I know...I was just exaggerating a bit.
See...while everyone has their SOP...
....I always felt that if I was going to use 2-3 mics, and have a close one, a mid and a distant mic...for me, it is mostly the differences in their distances from the source that would be the "usable" aspect. And I would then blend those mics in the mix for a total sound, using the time differences to get some room/delay flavor going.
If they are all the same level and time-aligned...then differences are minimized...and to me, would sound rather odd, having a mic 10' away, but time-aligned with the mic that is 10" away.
It's just not as natural sounding doing that.
So yeah...I don't bother equalizing their levels or time aligning them, and I use the full flavor of their differences.
Or...I just use one mic and place it where I get the right tone of instrument and room.
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#1819523 - 10/04/07 09:10 AM
Re: Phase issues
[Re: miroslav]
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Southside
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Gotcha. Agreed on all points :).
G.
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#1819693 - 10/04/07 12:17 PM
Re: Phase issues
[Re: Southside]
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audiorulez
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The 3:1 rule doesn't apply if you're close micing the same source with 2 mics obviously, since you're not observing the 3:1 rule. You will encounter issues with phase and time alignment, it is unavoidable, since 2 mics cannot physically occupy the same space.
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#1819806 - 10/04/07 02:51 PM
Re: Phase issues
[Re: audiorulez]
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Farview
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It doesn't apply when you have a close and a distant mic either.
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#1819855 - 10/04/07 04:28 PM
Re: Phase issues
[Re: audiorulez]
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Southside
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There are no time alignment issues with two close mics that are the same distance from the source.
G.
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#1820078 - 10/05/07 08:15 AM
Re: Phase issues
[Re: Southside]
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audiorulez
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There are no time alignment issues with two close mics that are the same distance from the source.
G.
Since it is impossible for 2 mics to occupy the same physical space, there will ALWAYS be time and phase alignment issues with 2 close mics on the same source.
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#1820086 - 10/05/07 08:31 AM
Re: Phase issues
[Re: audiorulez]
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Southside
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They don't have to share the same physical space to be equidistant from the source.
Two mics placed on the voice coil gap, one on the left side of the dustcap, one on the right side of the dustcap, will be equidistant from the source and will have no time alignment issues with each other.
For a bit farther away, any two mics placed anywhere on an arc who's center is the sound source will be equidistant from the source and will not have any time alignment issues with each other.
G.
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#1820312 - 10/05/07 06:48 PM
Re: Phase issues
[Re: Farview]
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mixsit
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Hi all. I'm not around here much, only as I tend to make HomeRec' my default home away from home.. and sort of the one who stirred this up over there. 
Maybe '3:1' is for whatever reason one of those things that falls into that crack where often enough it's 'used', or taught and talked about without actually understanding mechanics of if.
It seems that way anyway.
Here's few bits to add to the effort- The same things really, boiled down, from different angles;
Let's take 'phase' out of 3:1. Teach 3:1 is attenuation -only.
A Coherent pair is time aligned. So is an equi-distance spaced pair -except for signals from the sides. (Here we see the only valid explanation I can think of for all the times 3:1 is shown as on a 'single instrument'. I.E, the neck and body are two sounds from one guitar, but for some reason we are presumed not to want one mic to hear the other side? Phooey. Equi-distant works too. 
We don't necessarily always 'record things in phase'. Often we mix 'out of time' signals that sound good -in spite (or as often because of) some frequencies being in and out. Think multi-mic'd drum kits before track sliding. What does ORTF bring to the table that 'XY does not?
Time (delta T) is frequency on your phase shifter. Relative volume is your depth knob.
3:1 fun teasers;
If my 1st mic is at zero inches, then how far back am I supposed to put the 2nd one?
If my snare is at 2" does that mean everything at 6" and beyond is in phase, or is it in and back out every 6"?
Craig has class. His response 'over there makes we wish in retrospect maybe I'd toned down the title a bit.
We move on.
Cheers guys.
Wayne Smith
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#1820361 - 10/05/07 11:06 PM
Re: Phase issues
[Re: Southside]
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audiorulez
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They don't have to share the same physical space to be equidistant from the source.
Absolutely 100% wrong. As previously stated, it is impossible for two mics to occupy the same physical space. thus there will always be a tonal difference resulting in phase and time misalignments.
Two mics placed on the voice coil gap, one on the left side of the dustcap, one on the right side of the dustcap, will be equidistant from the source and will have no time alignment issues with each other.
If they are on opposite sides of the dustcap then they are not aiming at the same source then, are they.
For a bit farther away, any two mics placed anywhere on an arc who's center is the sound source will be equidistant from the source and will not have any time alignment issues with each other.
Once again, they are not aiming at exactly the same place, because, once again, two mics cannot occupy the same physical space. Basic physics.
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#1820466 - 10/06/07 07:44 AM
Re: Phase issues
[Re: audiorulez]
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Southside
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Wow. There are so many different and extremely basic things wrong with that post on so many levels that I'm not even sure where to begin.
Who else cares to start?
G.
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#1820482 - 10/06/07 08:08 AM
Re: Phase issues
[Re: Southside]
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audiorulez
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How about starting with how 2 microphones can occupy the same physical space. I can't wait to hear that one.
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#1820499 - 10/06/07 08:57 AM
Re: Phase issues
[Re: audiorulez]
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Southside
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They can't.
Now here's a question for you.
Why would two microphones have to occupy the same physical space to be the same distance from a source?
G.
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#1820539 - 10/06/07 10:26 AM
Re: Phase issues
[Re: audiorulez]
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mixsit
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How about starting with how 2 microphones can occupy the same physical space. I can't wait to hear that one.
A Coherent XY pair works at being coherent (at least for our purposes). And if you pan them in don't they do a pretty good mono? And to what ever degree they don't, at that point you'd use one mic any way. The same would apply to a space pair, ORTF, etc. except for sound from the sides.
What is it we are try to solve here?
fix- I just realized I've been useing the term 'coherent pair instead of 'coincident pair.
Edited by mixsit (10/07/07 09:55 AM)
Edit Reason: Yogi n' Boo Boo
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#1820642 - 10/06/07 05:18 PM
Re: Phase issues
[Re: mixsit]
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miroslav
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What is it we are try to solve here?
At this point...how to best split a hair.
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#1820820 - 10/07/07 06:47 AM
Re: Phase issues
[Re: miroslav]
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audiorulez
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Quite simply, if they are not in the same exact physical space, the sound each mic captures will not be identical, and, when combining them, this difference created by time and phase alignment issues. It may not be a problem, it may actually be a desired effect. However the bottom line is unless both mics are in the same physical position, which is impossible, there will be differences in what each mic "hears" with respect to the other, because of the difference in time and phase alignments to the source.
Miro may call it splitting hairs, I call it reality.
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#1820846 - 10/07/07 08:03 AM
Re: Phase issues
[Re: audiorulez]
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miroslav
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I don't think anyone is arguing that two mics can actually occupy the same spot...so it's a rather moot point to keep stressing that they can't. Everyone agrees with you on that.
What is being said (and the way I am interpreting it) is that sound waves do NOT travel as multiple, individual "rays" from a single source, with each mic getting a different "ray" that arrives at a different time and at a different phase.
Rather, they travel out from a point source as one continuous (frequency varying) wave that travels from the point source in a perfect arc.
That means (and what some are pointing out) that if you place two mics along that arc, they are equally distant from that single source. They will NOT be out of time alignment or phase alignment.
The sound wave reaches each mic at the same time, and the positive/negative portion of the sound wave is hitting each mic at the same point…on that equidistant arc.
But...I feel that even when taking two mics, and pointing one at the left side of a cone and the other at the right side, while keeping each of them equidistant from each of their points...is not going to introduce anything negligible as far as phase issues.
Even though you are pointing each mic at its point source…the two point sources are instantly combined as signal passes from the speaker…and so each of the two mics get relatively the same complex blend of sounds from those two point sources.
There will not be any time issues since they are equidistant from their sources, which are firing at the exact same time, even though they may be different source points on each side of the cone…
…hence my comment about “splitting hairs”.
Sound transmission (and its capture) is a very complex process…and trying to reduce it to a mathematical formula that can make or break a tracking session is IMO a bit extreme…maybe even a bit silly at times. I know there are guys that will break out a tape measure when setting up mics, and then they use a calculator to check their position…and if that works for their SOP, great.
But I think for most situations, you put up the mics…and use your ears.
Edited by miroslav (10/07/07 08:27 AM)
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#1820854 - 10/07/07 08:33 AM
Re: Phase issues
[Re: miroslav]
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audiorulez
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So, according to your theory, if I stand exactly 10ft away of my marshall at 90 degrees to the front of the cabinet, it'll sound exactly the same as if I stand 10 ft away at 10 degrees. Fat chance.
In addition, unless you have measured the exact distance to within ten thousanths of an inch from diaphram to diaphram, then they not going to, even if sitting side by side, receive the same signal at the same time, even if it was identical, and this will affect a summed difference, due to phase and time alignment, or misalignment actually.
However it's quite simple to prove in fact. Simply put two matched mics side by side as close to each other and as close to equal distance from the source as humanly possible on a single source. Record a full range sample of each on separate tracks. Now, invert one of the mics in the pair, and hear the difference.
Finally, if indeed two mics did capture exactly the same result, what would be the point? If indeed they were identical in every way, it would be no different than taking one mic and duplicating it in your software. The result of combining such tracks would simply be more apparant volume.
Again, while these differences may be minor, and even desired, the fact remains there will be anomalies due to issues with time and phase alignment, no matter how exact you place the two mics. This is basic audio physics. There's no need to (once again) make this a p*ssing contest. Sound in a particular place in time is unique, and any changes to the x y or z axis will create a change in the properties of said sound with relation to another place, no matter how small the variants. It's simple, basic mathematics, which we all know from our early audio education is what sound is, mathematics.
While the combined effects of these anomalies may very well be the desired result, to say that putting two mics equal distance from a source they will capture the same response from the source and not be affected by the difference in time or phase relations is incorrect. Only if you could put both mics, perfectly matched, in the same physical place could this possible occur, and still there would be the inconsistants of many other variables in the electronics, that, again may be slight, are still there, since no two components arem exactly identical.
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#1820859 - 10/07/07 08:56 AM
Re: Phase issues
[Re: miroslav]
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Southside
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What is being said (and the way I am interpreting it) is that sound waves do NOT travel as multiple, individual "rays" from a single source, with each mic getting a different "ray" that arrives at a different time and at a different phase.
Rather, they travel out from a point source as one continuous (frequency varying) wave that travels from the point source in a perfect arc.
That means (and what some are pointing out) that if you place two mics along that arc, they are equally distant from that single source. They will NOT be out of time alignment or phase alignment.
The sound wave reaches each mic at the same time, and the positive/negative portion of the sound wave is hitting each mic at the same point…on that equidistant arc.
There are only two things that can cause a time delay in the reception of a sound: a change in the distance that the sound has to travel, or a change in the medium through which the sound has to travel that actually slows the sound down. Mere angle or or direction has nothing to do with the phase or timing of the sound (one exception, the sound will be 180° out of phase if you put a mic behind a loudspeaker, but that's because of the mechanics ofthe loudspeaker, not because of the physics of sound.)But...I feel that even when taking two mics, and pointing one at the left side of a cone and the other at the right side, while keeping each of them equidistant from each of their points...is not going to introduce anything negligible as far as phase issues.
Even though you are pointing each mic at its point source…the two point sources are instantly combined as signal passes from the speaker…and so each of the two mics get relatively the same complex blend of sounds from those two point sources.
There will not be any time issues since they are equidistant from their sources, which are firing at the exact same time, even though they may be different source points on each side of the cone… I know there are guys that will break out a tape measure when setting up mics, and then they use a calculator to check their position…and if that works for their SOP, great.
But for the rest of us, you're right, that kind of esoterica is unnecessary. And for those with a very misguided knowledge of the subject, it is nothing but trouble.So, according to your theory, if I stand exactly 10ft away of my marshall at 90 degrees to the front of the cabinet, it'll sound exactly the same as if I stand 10 ft away at 10 degrees. Fat chance.
One has to do with the design of the loudspeaker driver itself; it is designed with a bi-lateral symmetry; it faces a certain direction. Not all sound sources are like that, or if they are, their asymmetry and directionality is not readily apparent. From the gong of a bell (circularly symmetrical) to the strike of thunder (radially symmetrical) to the striking of a cowbell {complexly symmetrical) there are many, many sources for whom your statement about miking a cab is inaccurate. The difference is not the physics of sound, but rather the mechanical design of the sound source.
Another has to do with the dispersion characteristics of the various frequencies; higher frequencies tend to be perceived as more "directional" (an oversimplification, but it will serve here) whereas bass frequencies tend to be perceived as more "non-directional". For this reason, there will be a change in the AMPLITUDES of the various frequencies as one moves off-axis of the face of the driver causing a perceived difference in the quality of the sound, but no changes whatsoever in the timing or the phase of the sound.
A third reason for the difference is that you're hearing the room and not just the cabinet; i.e. you're hearing differences caused by reflections of the primary sound, which themselves will have different phase and timing relationships. But that ha nothing to do with any phase or timing changes in the direct radiated sound from the source.In addition, unless you have measured the exact distance to within ten thousanths of an inch from diaphram to diaphram, then they not going to, even if sitting side by side, receive the same signal at the same time, even if it was identical, and this will affect a summed difference, due to phase and time alignment, or misalignment actually. However it's quite simple to prove in fact. Simply put two matched mics side by side as close to each other and as close to equal distance from the source as humanly possible on a single source. Record a full range sample of each on separate tracks. Now, invert one of the mics in the pair, and hear the difference.
In a real room with reflections and bass modalities and such, and with mics that are not perfectly matched (which even matched mics are not perfect), and with an inperfect sound source, and the results will not null, you're right.
All that proves is that there are many variables which can affect the outcome. It has NOTHING to do with the basic physics of primary sound disperison itself, which state that position alone has zero effect on phase or timing as long as the distance is the same.
This is basic audio physics. There's no need to (once again) make this a p*ssing contest. Sound in a particular place in time is unique, and any changes to the x y or z axis will create a change in the properties of said sound with relation to another place, no matter how small the variants. It's simple, basic mathematics, which we all know from our early audio education is what sound is, mathematics.
Simply put: There is no term for angle of reception in the equations for calculating distance, time and phase in sound disperison.
End of thread.
G.
Edited by Southside (10/07/07 09:56 AM)
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SouthSIDE Multimedia Productions
Independent Recording Network
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#1820862 - 10/07/07 09:01 AM
Re: Phase issues
[Re: audiorulez]
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miroslav
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So, according to your theory, if I stand exactly 10ft away of my marshall at 90 degrees to the front of the cabinet, it'll sound exactly the same as if I stand 10 ft away at 10 degrees. Fat chance.
There are differences in amplitude due to the cabinet/speaker's design, but the time it takes for the sound wave to reach each mic is identical.
And...while the amplitude may be different, the phase at each mic will be identical.
Also...two ears, are not the same as a microphone.
What you hear with your two ears is MUCH more complex than what a mic hears...
...not to mention that the reflections within the room will be different for each mic if their angles are different, as each mic will be a different distance from the outside walls.
Like I said....it's too complex to just focus on the mic and the point source…hence why split hairs over just the “spot” each mic occupies.
But…I will like to see some of the other folks chime in so this doesn’t end up being only a two person debate at this point.
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miroslav - miroslavmusic.com"Just because it happened to you, it doesn't mean it's important."
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#1820863 - 10/07/07 09:07 AM
Re: Phase issues
[Re: Southside]
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miroslav
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...the sound source in this case is not a point source, it has dimension.
I know what you mean...
...it's a figure of speech.
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#1820879 - 10/07/07 09:53 AM
Re: Phase issues
[Re: miroslav]
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Southside
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Loc: Chicago, IL USA
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I know you get it, miro ;). That was more for audiorulez's edification for when he said, "They're not miking the same source."
G.
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