3 to 1 Phase Rule

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TylerW

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Hello all,

I am aware of the 3 to 1 phase rule. I understand after reading multiple articles and forum posts that when micing different sources that you should keep the microphones at least triple the distance as they are from the source for phase coherency. What I am still confused about is recording on a guitar amp (or just the same source). If I am recording the same source, like a guitar amp, does this rule still apply? What if I like the mic 6 inches off the amp? Do I really need the other mic to be 18 inches away from that mic? If that's the case then the other mic won't be on the speaker anyway, right? Please clarify with any insight. Thanks in advance!
 
It really doesn't apply. The 3 to 1 rule is more about isolation than phase. Having the mic 3 times farther away makes the sound of the other source 9db quieter. It's because the far away source (the one you aren't trying to pick up with that mic) is so quiet in that mic that it doesn't interfere with the source you are trying to capture with that mic.

It has nothing to do with stereo micing.

It only works with near and distant mics as long as you keep the gain setting for both mics the same. But the reality is, you can put the mics anywhere as long as one of them is at least 9db quieter than the other one, you will not have phase problems.
 
It does apply any time you've got the same source in two mics, but doesn't really have anything to do with distance. I prefer to call this "the (at least) 9db Rule (of thumb)".

The idea is that any time you have the same source in two or more channels with a time delay between them - whether it be two mics picking up the same source (deliberately or not) or a DI and miced amp, or whatever - there will be frequency dependent interference. Some frequencies will add to some extend while others cancel. You get comb filtering, or phase issues - basically a set of peaks and notches in the frequency response which might not always suit the sound you're trying to get. The depth of these peaks and notches are totally dependent on the correlation in volume of the two sources to one another: 1 - 1 = 0, but 1 - 0.5 = 0.5. It can be shown that a difference of 9db between the two sounds makes the peaks and dips less than 1db, and therefore barely noticeable. Greater differences in the levels are even better, but 9db is considered the minimum acceptable.

You can get this separation any way you want. Mic distance is one way. Off-axis rejection is another. Moving the faders on the mixer yet another. And all of these can be undone! If you use two of the same mic on a guitar amp, with the same gain settings, with one 3 times further away than the other, then you should be good...

...until you decide that you want to hear more of the distant mic! If you boost that distant channel by 9db to get them to equal levels you'll have a mess. But then boost it another 9db and you're good again.

Now complicate matters by using different mics with different sensitivities and/or different gain settings...

OTOH - Sometimes exactly what we want/need/like about a multi-mic situation is the comb-filter effect.
 
Hm, this is interesting.

So basically if I have more than one microphone on a source I must have one primary mic and then the others should be at least 9dB lower in volume? What if I'm using 3 microphones? Will I need to have a primary, a secondary at -9dB, and then a tertiary at -18dB?

I generally understand most of the recording world, but phase and comb filtering still boggles my mind.
 
ashcat_lt said:
OTOH - Sometimes exactly what we want/need/like about a multi-mic situation is the comb-filter effect.
Click to expand...

Or is this rule more of a suggestion and you have to find the right balance between the two worlds? Obviously it's not actually a rule, but you know what I mean.
 
More of a suggestion really. Finding the way that sounds good, trumps any placement standard. Knowing how this works, is just part of getting there. :)
 
Yeah. It gets confusing. One rule says that, to avoid phase issues you should follow the 3:1 rule then, all of a sudden, we start talking about stereo where we need phase differences to give you a stereo image.

Frankly, I worry a lot more about the so-called 3:1 rule when I'm doing live work. On theatre jobs, I tend to have a row of mics across the front edge of the stage--sometimes 3 mics, sometimes 5 mics, sometimes boundary mics and sometimes cardioids on extension tubes. In these circumstances comb filtering can be a major issue as actors move across the stage, made more noticeable because the relative distances are constantly changing. This means the nasty effects are also changing--a frequency being cancelled on instant is suddenly boosted the next. For this reason, in the theatre I'm pedantic about the 3:1 rule and, even more, mix the mics up and down as the actors traverse the stage.

In the studio though (since things tend not to move much) I can be rather more cavalier and just go by a mix of instinct and ear. Should I hear any phase effects I don't like during set up (rare) a move of a few inches an make a big difference. Or, play with the phase invert button--often helps. Or just listen in the mix and, if it sounds wrong, pull down one of the mics.

But, in the studio, it's not all that much of an issue so long as you listen during set up.
 
All of these being reasons I call it the "(at least) 9db Rule (of thumb)" rather than "the 3:1 Rule"!
 
If you add a third Mic, you have a choice. Either you need to line it up with one of the other two to avoid phase cancellation, or use the cancellation as part of the sound.

Remember, the 3 to 1 rule has nothing to do with stereo Mic techniques.
 
Farview said:
Remember, the 3 to 1 rule has nothing to do with stereo Mic techniques.
Click to expand...

At the risk of confusing everything, it sort of does.

Stereo imagery is caused/created by phase differences between the mics you use (or, if we forget about mics, phase differences between your two ears). With the mics spaced properly (or coincident as in X-Y) you can get spatial info without nasties.

Then, as the mics get farther apart (but still too close together) the spatial info turns into nasty comb filtering.

Finally, at roughly the 3:1 guideline, the phase differences become negligible and no longer a problem.

So basically, they're all just extremes of the same effects.
 
Thank you all for the responses. It all seems awfully confusing but I'm sure the more I work in an actual studio the more I will understand the whole phase thing.
How would the 3:1 rule apply to close vs. distant micing? Should I start with the mic at least 3 times back? I know there will be time differences between the two, but if I nudge the distant mic forward while using the 3:1 "suggestion" will I be close to phase coherent?
 
TylerW said:
Thank you all for the responses. It all seems awfully confusing but I'm sure the more I work in an actual studio the more I will understand the whole phase thing.
How would the 3:1 rule apply to close vs. distant micing? Should I start with the mic at least 3 times back? I know there will be time differences between the two, but if I nudge the distant mic forward while using the 3:1 "suggestion" will I be close to phase coherent?
Click to expand...
The distant Mic can be anywhere as long as it is 9db quieter in the mix than the close one.

That is all the distance does for you, it makes the source quieter in the distant Mic.

Btw, if you nudge the audio of the distant Mic to be in time with the close Mic, all these problems go away. Of course, so does the sense of space. It all depends on why you are using the distant Mic.

If you are using the distant Mic to blend the tones, time aligning them will help

If you are using the distant Mic to get a sense of space, time aligning will ruin it and you will need to make one of the Mic 9db quieter than the other one. Oddly, it doesn't matter which Mic is the quieter one. there just needs to be at least a 9 db difference between them for the comb filtering to go away.
 
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