Phase Question

J

jonmmartin

New member
(I originally posted this in the rack forum by mistake)
I have 2 unmatched small condenser mics. For recording in stereo, is it better to use the phase reverse button on the mic pre or to move the mics using the 3 to 1 rule when phase issues exist. The question is...does engaging the phase reverse button degrade the sound.
Also, can someone explain the technique for checking phase problems when using multiple mics on one source.
Thanks for the help.
 
first, only use the phase button if you have a phase problem, and even then it will only help if the phase is out a full 180.

much love to the 3-1 rule, but sometimes you want the mics equidistant from the source, depending on the situation........ (actually, a little context would help)

the magic bullet here is just placement/trial and error.
if when both sources are paned center you notice some hi's, mids bass.... whatever missing (compared to hard paned) then you have a phase problem.

easier than that, have source # paned center, when you put #2 in, do you lose some hi's bass mids?
if so, one of the mics probably needs to be moved.
hope that all makes sense.
 
A phase reverse switch shouldn't affect the signal quality, there isn't that much circuitry for it to go through.
Simplest way to tell if you need a phase reverse in oyur mix is to listen with it out, then punch it in and listen for the difference. If, with the reverse in, it sounds muddier/quieter, then your original phasing was correct.
 
like the one guy said, the phase reversal switch will only help when both mics are 180 degrees apart...like when both top and bottom mic'ing a snare, you must put 1 of the mics out of phase. flipping the phase on one mic when you are experiencing cancellation will only serve to reverse the frequencies that are being cancelled, which in some cases may make things worse.

the 3-to-1 rule is really your best bet to avoid phasing...if that doesn't work, try some acoustical treatments in the room - reflections in the room can cause all sorts of phasing havoc
 
Ironklad Audio said:
the 3-to-1 rule is really your best bet to avoid phasing...if that doesn't work, try some acoustical treatments in the room - reflections in the room can cause all sorts of phasing havoc
Click to expand...
The 3 to one rule only applies to 2 mics on two different sources. Not 2 mics on the same source. It has nothing to do with stereo micing either.
 
  • Like
Reactions: XLR
Farview said:
The 3 to one rule only applies to 2 mics on two different sources. Not 2 mics on the same source. It has nothing to do with stereo micing either.
Click to expand...

i was under the impression that it applied to a single source as well...keeping the mics equidistant from the source, and making sure that distance between the mics is at least 3 times the distance from the source
 
this thread is on a wild train to wrongsville...

think about what you're tryingto do with the phase switch- you're invertingthe waveform that the microphone is picking up.
WHy would you want to do this?
Well, it's because if two similar waveforms that are out 180 deg out of phase are added, then they will totally cancel each other out.
This is basic superposition of waveforms, and if you don't know what that's all about, I strongly suggest you go read something about it before you even think about a phase reverse switch. Try wikipedia- it has moderators that make sure that only "good" information is posted there, and on a topic as wide-known as superposition, there should be a lot of accurate info there.
Becuase people fail to actually understand what they are doing, things like phase reversal become superstition in home recording circles- myths perpetuated by peopel who think they are helping, but are using bad info.

The 3-1 rule can be applied to a single source. By placing the second mic so that an odd interger multiple of the distance of the first mic exists, you're making sure that each wave is arriving at the same phase-point as it does to the first. The reason you take the second mic to thrice the distance of the first is that, in the extreme example where 1=1/lambda, you're back in phase with the original signal. If you only went to double the distance, you'd be 180deg out of phase with the cirtical wavelength. However, remembering that different frequencies have different wavelengths, you're still going to have phase issues if you use the 3-1 rule.


The best way to avoid phase problems is proper mic placement. The best way to do it is have the mic's capsules equidistant from the source. That way, wavefronts will reach both mics simultaneously, i.e. they will be in phase.

If you have to place the mics in some non-equidistant configuration, try to use an odd integer multiple of the original distance.

Also, remember that this is only covering direct phase- not other effects like combing, which can also effect your stereo signal.

If you really don't want to do much reading, at least look for examples of stereo micing techniques (X/Y, A/B, Tree etc) and follow those to the letter. They are all tried and true, and are based on principals of good phase relationship.

Now, quickly onto another pet hate of mine- flipping the phase on under-drum mics.
Yes, you have to flip the phase, but this technique will only work when the mics are equidistant from the skins. Why? Well, because when the top skin is in compression, the bottom skin is in rerefaction- ie 180 deg out of phase, hence the mics (if they are equidistant) will be out of phase. Once again, it comes down to proper mic placement.
 
Last edited:
Farview said:
The 3 to one rule only applies to 2 mics on two different sources. Not 2 mics on the same source. It has nothing to do with stereo micing either.
Click to expand...
This is true. 3:1 rule is for mic's each on a different source, when the mic's are intended to be summed to a single channel.

Re your question about dealing with phase issues between two mic's on one source, here are a few things I do, FWIW.

1. use a coincident stereo config (XY, MS) if I want them to be fully mono compatible

2. use a near-coincident config (ORTF) if I want a little more L/R "width", keeping the mic's hard panned L/R. Doesn’t get too phasey/funky if I want to sum the channels later.

3. use two mic's close to a source, each picking up a different part of its sound (for instance on ac guitar, mic's at neck/body joint and bridge, a foot or two out) and keep the mic's hard panned L/R. Gives a very wide sound. Can get phasey if summed to mono, so if I need mono, I’ll just use one of the mic’s.

4. 2 mic's at different distances, in line with one another going out from the source. Any distances can work... just record them to separate tracks and later slide the waveforms to time-align them and they're usually very mono compatible.

Tim
 
Last edited:
Ironklad Audio said:
i was under the impression that it applied to a single source as well...keeping the mics equidistant from the source, and making sure that distance between the mics is at least 3 times the distance from the source
Click to expand...
That would mean that if you were 3 feet from the source, your mics would have to be 9 feet apart. Mr. Lawler explained it better than I.
 
cpl_crud said:
The 3-1 rule can be applied to a single source. By placing the second mic so that an odd interger multiple of the distance of the first mic exists, you're making sure that each wave is arriving at the same phase-point as it does to the first. The reason you take the second mic to thrice the distance of the first is that, in the extreme example where 1=1/lambda, you're back in phase with the original signal. If you only went to double the distance, you'd be 180deg out of phase with the cirtical wavelength. However, remembering that different frequencies have different wavelengths, you're still going to have phase issues if you use the 3-1 rule.
Click to expand...
You still have phase issues using the 3 to 1 rule in this manner because the 3 to 1 rule doesn't apply to multiple mics on the same source.
 
If I were using two mics that I couldn't get coincident, I would be using an odd-interger rule. Yeah, there's going to be phase issues, but so long as you choose an appropriate intergral distance, then you'll be fine.

To be totally honest, something as vauge-sounding as a "3-1" rule is more than likely to have ambiguosity in its interpretation.
My source:
http://humbuckermusic.com/acguitrectec.html
Sure, it's a webpage, but, hey, this is how ambiguity happens. If you can show me a textbook source that defines "3-1" rule as a mutlipe-mic technique for different sources, then I'll be leading the campaign to burn this site down.
However, I'll think you'll find that, logically, the 3-1 rule is something that can apply both to phase cancellation from a single source AND noise rejection from other sources that you're recording at the same time.
Just apply a bit of brainpower here- why would something ambigious show up in both examples?
It's because the examples are linked.

When you're talking about noise rejection, it's because the drop in power over triple the distance is a factor of 9, which, by my dodgy 0300 maths means an approximate drop of something like 10dB. Also, 3-1 is kind of a nice number, they're both part of the fibbonacchi (i can never spell that right) series, and a seperation of 3-1 isn't going to take up a massive amount of space.

The 3-1 rule in realtion to phase cancellation is simple- when multiple-micing an instrument, as in the top picture on the above link, you're going to want different signals going into the mics, but you're going to want them in phase, right?
The "1" part of the rule keeps both signals in phase, and the "3" part deals with the noise rejection from what are, essentially, different sources.

So, how's about we lay this argument over symantics to rest, and help the OP with his question about the phase reverse switch? As I said in a post somewhere else, you should consider reading a bit into wave theory and what exactly you're doing when you're reversing phase etc.
This thread should give you good grounding into the basis of how to keep things in phase, but you should at least look up the superposition of waves and basic phase shifts somewhere.

http://en.wikipedia.org/wiki/Phase_(waves)

that might be a good place to start. It also shows the proof behind Timothy Lawler's point #4- a shift ina tme is the same as a phase shift.
 
you'd have to know the length of the wave to calculate proper distance between mics (for coherence) based on a mathematical formula.
 
The best way to avoid phase problems is proper mic placement. The best way to do it is have the mic's capsules equidistant from the source. That way, wavefronts will reach both mics simultaneously, i.e. they will be in phase.
Click to expand...
With regard to the 'two spaced mics' examples (like in the link), being equidistant is agreeably your best shot at having things in phase. What I don't understand is the point of the 'odd integer' regarding favorable ratios. Would this have anything to do with which frequencies are in phase? Seems like it's still simply a mater of attenuating whatever differences there is in the cross bleed from the source, not frequency related.
Wayne
 
there could be an entire foroum devoted to the threads that just deal with the 3-1 rule.

it's a bit like beating your head aganst a wall, but i have learned from them.
 
giraffe said:
there could be an entire foroum devoted to the threads that just deal with the 3-1 rule.

it's a bit like beating your head aganst a wall, but i have learned from them.
Click to expand...
:rolleyes: It's obviously a tough one to get our heads around at first for sure. But I think a lot could be made simpler by keeping it clear up front that 3-1' is about attenuation of the cross feed you don't want to hear, not about fixing or the setting of frequencies (at least pending what cpl_crud' was getting into...? :confused: ). Often the examples don't say 'why, or worse, imply it does apply to 'frequency.
 
cpl_crud said:
If I were using two mics that I couldn't get coincident, I would be using an odd-interger rule. Yeah, there's going to be phase issues, but so long as you choose an appropriate intergral distance, then you'll be fine.
Click to expand...

Sorry, that's wrong. If you mic a guitar amp with an SM57 up close, and with another mic far away (very common technique), there is no magic ratio to make sure phase problems are minimized. You have to listen. Some calculating can tell you what freqs will be affected, but the 3:1 rule won't do anything for you in that case.

To the original poster- to check for phase problems when using two mics on one source, listen to them together.
 
  • Like
Reactions: XLR
Phase cancelation is when you have the peak of one wave with a trough of another - so the sound cancels out. To avoid this you can set up mics so that you dont get this cancelation. Rules like the 3 -1 rule.

BUT as every freq will have a diffrent place where phase cancellation will occor due to the fact that the wavelength of every freq will be diffrent the 3 -1 will not make EVERY freq have no cancelation.

This is why you need to listen. Find the place where phase problems are the smallest.
 
  • Like
Reactions: XLR
drummerdude666 said:
BUT as every freq will have a diffrent place where phase cancellation will occor due to the fact that the wavelength of every freq will be diffrent the 3 -1 will not make EVERY freq have no cancelation.
Click to expand...
Which is what makes the 3 to 1 rule useless in the close mic-far mic situation. It was never meant to apply to that. EVER!
 
drummerdude666 said:
Phase cancelation is when you have the peak of one wave with a trough of another - so the sound cancels out. To avoid this you can set up mics so that you dont get this cancelation. Rules like the 3 -1 rule.
Click to expand...
Farview said:
Which is what makes the 3 to 1 rule useless in the close mic-far mic situation. It was never meant to apply to that. EVER!
Click to expand...
Exactly. Think about it. 3-1' on it's own doesn't predict 'which frequencies'. Even if you were only concerned about a particular pitch, '3-1 with '1' starting at 2" is a different set of frequencies than '3-1 with '1' starting at 3".

This could end up being harder to crack than 'track digital hot' was.

'3-1 is about attenuation.'
Stir, heat, repeat, apply untill thick and phat.
:rolleyes:
 
You must log in or register to reply here.
Back
Top