Professional Vocals Don't Have Mono Image!

[boingoman]

Real world example:

I did sound for a band last weekend.

I set my drum OH too high, about 2 feet up.

It was about 3 feet from the singer's head.

When he sang, his voice bled into the OH mic, which made his vocal sound hollow from combing when both mics were in the PA. The filtering came from the delayed signal of his too-loud voice in the overhead combining with his vocal mic signal, which was only inches from him.

I had violated the 3:1 rule. The singer would have to be 6 or so feet away if I had my OH mic two feet high. At two feet from the ride and only three feet from him, his voice was about at an equal volume as the cymbals in the OH mic.

As I couldn't move the singer, I moved the OH mic down about a foot, bringing me within the 3:1 ratio. It was now 1 foot from the ride, and three feet from him.

This made his voice much lower in level compared to the cymbals in the OH mic.

Bring up both channels, no more combing. His voice still bled into the OH, but at such a low relative level to the ride cymbal that it had no combing effect when both channels were in the PA.

 

[con_ritmo]

boingoman, that is just ONE example (i'm refering to your figure 2).
it is not the ONLY example of the 3:1 rule.

the REASON why you see the 3:1 rule talked about in reference to micing multiple sources (as in your example)...is because that's when you're going to have issues with two or more mics picking up the same source in violation to the 3:1 rule.

in the case of your example, when you have two mics-two sources, you will have the opportunity to violate the 3:1 rule TWICE.

1. source one can receive phase cancellation between mic one and mic two.
2. source two can receive phase cancellation between mic one and mic two.

if you have just one source and two mics you still have the SAME problem. again, the problem is two mics picking up the SAME source.

source one (the only source) can receive phase cancellation between mic one and mic two. ...which you yourself admit...and which my lav/handheld mic example illustrates.

you DON'T even need to have two mics to violate the 3:1 rule.
just having ONE mic, ONE source, and ONE reflective surface will do the same nasty phase cancellation. the most common example for this is a podium mic where the speaker's voice will bounce off the podium and into the mic. the combination between direct and reflected sound can cause phase cancellations.
once again, the SAME source being picked up twice.

the solution? make sure the distance of the reflected surface and the microphone are three times the distance from the microphone to the speaker. (3:1 rule)

http://www.recordingeq.com/articles/321eq.html
shows this in figure 11. before you even start yes other figures show two mics/two sources, but the key is figure 3...where you have two mics picking up the same source.

look at the common threads here:

1. your example has two mics picking up two sources. you are receiving phase cancellations when two mics are picking up the SAME source.

2. the example of two mics picking up the same source with the lav and the handheld. you are receiving phase cancellation when the SAME source is picked up twice.

3. the 3:1 rule applied to reflective surfaces...when the SAME source is again picked up twice. first direct, and secondly reflected off a surface.

the key is to PREVENT OR MINIMIZE phase cancellations of a sound source.
it can be one, it can be more than one.

look, after all this pounding of a dead horse into the ground...
if you or anyone else STILL insists on believing that the 3:1 rule is ONLY two or more mics on two or more sources (but not one!?!?!?) go right ahead in that misbelief...and REMEMBER, 3:1 is just a GUIDELINE with many exceptions.

yikes.

 

[boingoman]

Real world example:

I'm recording an amp with two AT4033 mics, about a foot away from the cab.

I decide I want to get a little room sound. I move one of the mics back to three feet away, as I remember something about the 3:1 rule preventing phase problems.

When I listen to the tracks, they sound good. But I want some more of that room mic, and I push up the fader.

All of a sudden, I hear comb filtering, even though I followed the 3:1 rule.

Why?

The 3:1 rule is based on relative levels. As long as my room mic, which is delayed a bit, is 10db less in volume than my close mic when I listen to them both, I won't hear the combing.

As soon as my room mic comes up in level, I hear the combing.

3:1 comes from the fact that if you move three times farther from a sound source, it gets 10 db quieter.

So if you mic a snare drum and a guitar cab, and follow the 3:1 rule, when you listen to both mics, the guitar bleed in the snare mic won't cause combing when it combines with the guitar mic signal, as it will be 10 db quieter. And the snare bleed in the guitar mic won't cause combing when you bring up both channels either, for the same reason.

Hopefully you can see why 3:1 doesn't matter with two mics on a single source, like near and far micing a guitar cab.

Since it is the relative levels that matter, you could have one mic 11 inches away, and one mic 12 inches away, and if you listen to both mics with the farther one 10 db quieter, you won't hear any combing.

And conversely, if you have one mic one foot away, and the other 10 feet away (supposedly well past 3:1), you will hear combing if you listen to both at the same volume.

 

[con_ritmo]

I'm recording an amp with two AT4033 mics, about a foot away from the cab.

I decide I want to get a little room sound. I move one of the mics back to three feet away, as I remember something about the 3:1 rule preventing phase problems.

When I listen to the tracks, they sound good. But I want some more of that room mic, and I push up the fader.

All of a sudden, I hear comb filtering, even though I followed the 3:1 rule.

Why?

The 3:1 rule is based on relative levels. As long as my room mic, which is delayed a bit, is 10db less in volume than my close mic when I listen to them both, I won't hear the combing.

hey you're finally beginning to see the light showing the example of two mics on the SAME (meaning one) source in respects to the 3:1 rule. thankfully.

btw i already talked about the respective volumes of the two mics as an exception. moot point.

thank you, nothing more to see here, move along.

 

[boingoman]

hey you're finally beginning to see the light showing the example of two mics on the SAME (meaning one) source in respects to the 3:1 rule. thankfully.

btw i already talked about the respective volumes of the two mics as an exception. moot point.

thank you, nothing more to see here, move along.

That is what I have been trying to say. Why didn't you post that article right away? It's pretty good.

3:1 by itself doesn't diminish comb filtering with two mics on one source. No magic distance will cancel it out. (Which is what I thought you were saying in your lav example- that 3:1 somehow prevented combing by being some magic ratio)

It is the relative level of the delayed signal to the original signal that determines the audibility of comb filtering. That is why I have been saying that 3:1 doesn't apply to two mics on one source. It won't magically make your phase problems disappear.

 

[Reggie]

I thought pushing the phase button on the preamp magically made phase problems disappear? "....make sure you flip the phase...."

 

[Reggie]

http://www.recordingeq.com/articles/321eq.html
shows this in figure 11. before you even start yes other figures show two mics/two sources, but the key is figure 3...where you have two mics picking up the same source.

yikes.

BTW, check your link a little closer. If you look at what is trying to be shown in Figure 3, they are trying to minimize leakage into mic 2 because it is not intended for singer 1. If you look at the following Figure(5) it shows what happens when you have a strong singer on one mic and a weak singer on mic 2.

I think the big sticking point is people who are intentionally trying to pick up a source with two mics. You don't need to put mic #2 three feet back from the source if mic #1 is one foot in front of the source. Unless it sounds good there.

 

[mixsit]

...So it is relative level that is the important thing. With two mics on only a single source, like a near and far mic on a cabinet, comb filtering happens no matter what. In this case, distance between mics does not matter. No magic ratio will keep comb filtering down. It happens no matter what...

There is one other saving grace we should include. As the second mic crosses into the where the near field and far field are about equal (critical distance) from the source, the random room reflections begin to average out and smooth the direct-path's comb filtering. This lets you get away with what otherwise might be some god-awful phase problems. It also gives a window into why 3:1 doesn't have to relate to a lot of 'stereo and distant-pair mic situations.

Wayne