B
boingoman
New member
giraffe said:
No. It cannot.
No. It cannot.
Reggie
PutsTheLotionOnItsSkin
And the correct answer is:
Ha ha, OK as far as I can tell; 3:1 is a mostly useless, fairly common sense rule for micing 2 or more sources, each with their own mics. Keep the mics three times as far away from eachother as they are from their source.
I don't understand where people got the idea that it meant that if you place a SM57 6" from your guitar amp, then you should place your LD condensor a foot and a half from the 57. Makes no sense!
NYMorningstar said:
Ha ha, OK as far as I can tell; 3:1 is a mostly useless, fairly common sense rule for micing 2 or more sources, each with their own mics. Keep the mics three times as far away from eachother as they are from their source.
I don't understand where people got the idea that it meant that if you place a SM57 6" from your guitar amp, then you should place your LD condensor a foot and a half from the 57. Makes no sense!
M
mixsit
Well-known member
Caution This thread should be considered mildly toxic to Newbies..
hmm..how do you insert a post to the header of a subject..?
hmm..how do you insert a post to the header of a subject..?
C
corban
New member
ding ding ding (applause)
I do believe we've got it.
I do believe we've got it.
B
boingoman
New member
mixsit said:Caution This thread should be considered mildly toxic to Newbies..
hmm..how do you insert a post to the header of a subject..?
*Sigh*
OK I'm changing my post so it doesn't call freak a whiner.
jake-owa
Banned
Ok, but that's the very thing you were fighting so hard for.Reggie said:
giraffe
i love negative rep
Reggie said:
at 6" phase (up) at 12" phase (down) at 18" phase (up)
my head's gonna freaken explode
Reggie
PutsTheLotionOnItsSkin
Come on, keep up...Actually 3:1 is the equivalent in the audio world of the Golden Ratio.
It is relevant in microphone distance, cable length, mono summing, sample rates, phase response, perceived volume, etc.
Reggie
PutsTheLotionOnItsSkin
giraffe said:
Uh, no. if you are talking about keeping in phase, the distance needed between the two mics will be different depending on the wavelength (in other words, impossible to determine unless you are recording simple sine waves or something) .
*pop*
giraffe
i love negative rep
i'll go with *BAM* thank you.
and i'll concede (in retrospect) that the last thing i said makes no dam sense
at all.
and i'll concede (in retrospect) that the last thing i said makes no dam sense
at all.
C
con_ritmo
New member
yes and no. right and wrong.
3:1 rule applies when you are combining two or more mics.
it DOES NOT MATTER how many sources you are micing.
the 3:1 rule applies when you are micing only ONE source.
the 3:1 rule ALSO applies when you are micing MORE than one source.
take a lecturer that is wearing a clip-on lavalier microphone.
everything is fine as his voice projects through the mono pa.
now the lecturer turns on an additional handheld microphone so he can interview somebody. now there are two microphones picking up his voice, the clipon lavalier and the handheld. depending on where the lecturer is holding the handheld mic you will start hearing REALLY NASTY comb filtering (on his voice) in the mono pa.
two mics on ONE source that are combined. 3:1 rule applies.
________________________
the key thing is what each microphone hears. if one microphone doesn't hear what the other one is picking up, you won't have a problem.
this is why you may be able to place narrow hyper-cardoid mics closer together with fewer ill effects (vs. cardioids and omnis which "hear" more).
_______________________
if the mics are hearing the same thing at the same time you won't have as much of a problem either. this is why two mics placed in xy sum to mono pretty well. the sounds arrive at the same time.
when the mics start hearing the SAME THING at DIFFERENT times you start to lose mono compatibility to varying degrees (from very slight to very bad)...
if you just have two mics and you hard pan them for a stereo recording, that's not combining anything...so you don't have any 3:1 issues there (unless again you sum to mono).
______________________
another thing to consider is that we're assuming both mics are at similar volume levels...changing that would affect things.
_______________________
the bottom line is that 3:1 applies any time that you are COMBINING mics.
it is just a general guideline to help minimize phasing and comb-filtering issues. if you get the placement just right you can be in violation of the 3:1 rule and still be ok.
just look at the multi-mic techniques for cab micing. what are one of the things you immediately check out for? ideal spots where phasing/comb filtering is at its minimum and/or is most pleasing...
why do you have to reverse the polarity depending on where you mic the kick drum? same deal....
Last edited:
MadAudio
Damned if I do
Now THAT makes sense. Thank you, con_ritmo.
Hmmm. "with rhythm." Are you a drummer?
Hmmm. "with rhythm." Are you a drummer?
MadAudio
Damned if I do
This thread actually inspired me to dig up my notes from "audio school." While fiendishly scouring the pages for the 3:1 mic rule, I found the little gem.
SIX MICROPHONE RULES
1. Never use 2 mics when 1 will do the job.
2. Beware of gobos, sound absorbers and hand usage when using cardiods. (Too much blockage causes a cardioid to act like an omni.)
3. Take proximity effect & off-axis coloration into consideration.
4. Avoid overload.
5. Beware of phase reversal in cables, and of shielding problems.
6. Beware of wind and vibration noises.
and.... LISTEN!
SIX MICROPHONE RULES
1. Never use 2 mics when 1 will do the job.
2. Beware of gobos, sound absorbers and hand usage when using cardiods. (Too much blockage causes a cardioid to act like an omni.)
3. Take proximity effect & off-axis coloration into consideration.
4. Avoid overload.
5. Beware of phase reversal in cables, and of shielding problems.
6. Beware of wind and vibration noises.
and.... LISTEN!
Last edited:
Rokket
Trailing Behind Again
The only thing this thread has done for me is give me a headache and lay bare how much I don't know.... It is beer-thirty yet?
*looking at clock on the wall*
*looking at clock on the wall*
C
corban
New member
It's twenty-to-beer-thirty over here. Con-ritmo, what you're saying makes sense except that the 3:1 rule works with two sources by making sure that very little signal from one source gets into the mic that's supposed to be picking up the other source. By the inverse square law, the amount of signal that reaches it will by 3*3=9 lower than the signal from the source it's supposed to be picking up, which makes the comb filtering of phase issues inaudible.
The only way that applies if you're micing one source is then if your second mic is going to pick up 9 times less of a signal as the first mic Like in your example, if the handheld mic is picking up a lot less signal than the lapel imc because it's further away. However, the principle does not work if you're just going to boost the second mic's gain to receive the same signal anyways. So if you're depending on the 3:1 guideline to control your phasing issues, you can only use the second mic as an ambiance mic to pick up very little signal.
So it's pretty much useless.
Yes, you can move that mic around to find the spot with the least phasing and then turn it up.
But that is not the 3:1 rule.
The only way that applies if you're micing one source is then if your second mic is going to pick up 9 times less of a signal as the first mic Like in your example, if the handheld mic is picking up a lot less signal than the lapel imc because it's further away. However, the principle does not work if you're just going to boost the second mic's gain to receive the same signal anyways. So if you're depending on the 3:1 guideline to control your phasing issues, you can only use the second mic as an ambiance mic to pick up very little signal.
So it's pretty much useless.
Yes, you can move that mic around to find the spot with the least phasing and then turn it up.
But that is not the 3:1 rule.
B
boingoman
New member
Yes, it does matter how many sources there are, as the 3:1 rule is for multiple sources, with a mic on each one.con_ritmo said:
It is to make sure that when you have many sources, and one mic on each source, that each mic hears only one source.
It prevents bleed from your guitar, for example, from being to loud in your snare mic and causing comb filtering from the delay.
http://www.prosoundweb.com/install/spotlight/bartlett/phase/phase.shtml
3:1 is for multiple sources and mics, and it is to prevent comb filtering from mics combining during playback, on tape or in a PA. It ensures that on playback, you only hear one source in each mic. This prevents comb filtering from being audible.
In your lav example, one of two things was going on.
1) The mics were both picking up his voice, but were spaced so that comb filtering was minimized or shifted out of the audible range (not the 3:1 rule)
2) The handheld finally got far enough away so it was -10 in level from the lav, and the comb filtering was reduced in level.
Of course, this means that you basically couldn't hear the handheld anymore, and were just listening to the lav.
(This phenomenon forms the basis of the 3:1 rule- it helps maintain one mic for one source.)
It was found that if you have two sources, with a mic on each, that if they were 3 times as far apart as the distance they were from their sources, the bleed would be about -10 db.
Hence, the 3:1 rule for multiple sources, each with their own mic.
C
con_ritmo
New member
con_ritmo said:
corban, you are correct about what can happen when you turn up one mic in relation to the other. that's why i included the above.
madmax, i am a violinist...
as rhythm is one of the two universal laws of music...it's very important!
C
con_ritmo
New member
boingoman said:
perhaps you were confused on my lav example.
the more clear answer is
3) the lectuer has both the clip-on lav and the handheld turned on
when the lecturer holds the handheld mic far away from him, the 3:1 rule is being adhered to and the phase cancellation is minimised.
however when the lecturer brings the handheld closer to his body, the 3:1 ratio is NOT adhered to, and you will probably hear NASTY PHASE CANCELLATION.
it's that simple. it happens all the time in live sound.
i repeat:
the 3:1 rule applies when you are combining two or more mics.
the 3:1 rule applies when you are micing only ONE source.
the 3:1 rule ALSO applies when you are micing MORE than one source.
it DOES NOT MATTER how many sources you are micing.
it is just a general guideline to help minimize phasing and comb-filtering issues. because of this there are many exceptions which i have already discussed...including the volume of the individual mics, precise placement to minimise comb-filtering, pickup pattern, etc.
C
con_ritmo
New member
boingoman said:
Boingoman, here is a quote from the link YOU provided.
notice it says "SAME source". as in ONE. not MULTIPLE sourceS.
sorry, but again...
the 3:1 rule applies when you are combining two or more mics.
it DOES NOT MATTER how many sources you are micing.
it could be ONE source...it could be many sources...
B
boingoman
New member
con_ritmo said:
Yes. That statement is part of the basis 3:1 rule. It is not the 3:1 rule.
con_ritmo said:
The number of sources has to be more than one to apply the 3:1 rule.
"The distance between mics should be at least three times the mic-to-source distance (Again, see Figure 2). For example, if two mics are each 4 inches from their sound sources, the mics should be at least 12 inches apart to prevent phase cancellations."
Look at figure two. It can't get any clearer. Two mics, two sources.
On one source, there is no distance between mics that will eliminate comb filtering if you listen to both mics at the same level.
"How was the 3:1 rule determined? It started with the following fact:
When you add a signal to its delayed replica at equal levels, you get severe comb filtering with deep notches. But when you mix direct and delayed signals at different levels, you get less deep notches.
Specifically, if the delayed signal is 9 dB less than the direct signal, the comb-filter notches are only +/- 1 dB, so for all practical purposes they are inaudible.
How do we make sure that the delayed signal, picked up by a distant mic, is at least 9 dB below the direct signal picked up by the closer mic? Put the distant mic at least 3 times farther from the source than the close mic is. Due to the inverse square law, the level drops 9.54 dB when the distance to the source is increased 3 times."
If you listen to two signals, one -10db from the other, the filtering will essentially be inaudible.
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.
It is the relative levels when you listen to both that matters.
Keep the far mic -10 db from the near mic, and you won't hear combing. But the far mic will be useless at that level, most likely.
So turn it up, and deal with the comb filtering, and use placement to minimize it.
Last edited:
