The mystrey of Phase. Fell free to help

  • Thread starter Thread starter chadsxe
  • Start date Start date
C

chadsxe

New member
Well I would be lying if I said I truly understood what phase was, so I am posting this in attempt to better understand it.

First off what is phase and how does it effect mic placement? As far as I can gather it has to do with the sound reaching one mic at a different time then the other. This some how cancels some frequencies. From personal experience I think I discovered this when cloning a track and bumping it forward to much. When playing them back (panned hard L/R) they sounded tiny.

I hear a lot of talk about the 3:1 ratio when placing mics. Does this mean that the second mic should always be placed three times as far away from the sound source that the first mic is. What happens if it is closer the 3:1? What happens if it’s farther? I hear people saying that they sometimes mic and amp with one mic up against the grill and another mic 5,6,7,8,9,10 feet back. How does that not have phase problems?

When recording an amp (4x12) would I have a problem placing a mic on one speaker and another mic on a different speaker. I want to get more then one option on track so I am not locked in during post.

Well I am sure any response will yield more question form me.

Thanks
 
chadsxe said:
As far as I can gather it has to do with the sound reaching one mic at a different time then the other. This some how cancels some frequencies. From personal experience I think I discovered this when cloning a track and bumping it forward to much. When playing them back (panned hard L/R) they sounded tiny.
Click to expand...

That is correct, although hard panning should have minimized the problem.

I hear a lot of talk about the 3:1 ratio when placing mics. Does this mean that the second mic should always be placed three times as far away from the sound source that the first mic is. What happens if it is closer the 3:1? What happens if it’s farther?
Click to expand...

No, that has to do with two (or more) mics on two (or more) sources. Each mic must be three times further from each other than the other source. This means that each mic's source will be around 9 dB louder than the other source, which minimizes phase cancellation. If they are even farther apart, there will be even less cancellation.

For two mics on a single source, you need them to be equidistant from the source to be in phase.

I hear people saying that they sometimes mic and amp with one mic up against the grill and another mic 5,6,7,8,9,10 feet back. How does that not have phase problems?
Click to expand...

It does. They might be after that sound, they might pan the tracks apart, or they might correct the phase relationship with a delay on the close mic (or by moving up the distance mic).

However, you cannot easily correct phase cancellation from multiple mics on multiple sources with delay, EQ, or polarity reversal. This can be an issue with drum overheads, thus, you either use coincident mic placement (XY), you hard pan the overheads, or you obey the 3:1 rule.
 
mshilarious said:
For two mics on a single source, you need them to be equidistant from the source to be in phase.
Click to expand...

Can you explain this a little bit better. Say I have one mic on the bottom left of speaker of the cab, and it is in the middle of the speaker pointed at the left side of the cone. I have another mic on the bottom right speaker, and it is on the right edge of the speaker straight one. If I am understanding this correct I should have no problem with phase and yeild two diffrent sounding tracks. Correct?


It does. They might be after that sound, they might pan the tracks apart, or they might correct the phase relationship with a delay on the close mic (or by moving up the distance mic).
Click to expand...
With that said is the a mathamatical way of figureing out how much a track needs to be moved in milaseconds per distance.


This can be an issue with drum overheads, thus, you either use coincident mic placement (XY), you hard pan the overheads, or you obey the 3:1 rule.
Click to expand...

So hard pan L/R would elimante any phase issues with over heads because there no longer interacting with each other?
 
chadsxe said:
Can you explain this a little bit better. Say I have one mic on the bottom left of speaker of the cab, and it is in the middle of the speaker pointed at the left side of the cone. I have another mic on the bottom right speaker, and it is on the right edge of the speaker straight one. If I am understanding this correct I should have no problem with phase and yeild two diffrent sounding tracks. Correct?
Click to expand...

Not sure, I don't have a lot of experience with guitar cabs.

With that said is the a mathamatical way of figureing out how much a track needs to be moved in milaseconds per distance.
Click to expand...

At sea level, sound travels pretty close to 1 ft/msec. But it's easiest to line up a single peak on the two tracks.


So hard pan L/R would elimante any phase issues with over heads because there no longer interacting with each other?
Click to expand...

I wouldn't say eliminate, because it would depend on your monitoring environment. Headphones, for example, wouldn't be a problem.

Understand that certain stereo mic techniques are designed to capture some amount of phase cancellation, because that enhances the stereo imaging. If there was no phase cancellation from hard-panned near-coincident stero mics (ORTF, for example), it would sound pretty much the same as hard-panned XY.
 
mshilarious said:
At sea level, sound travels pretty close to 1 ft/msec. But it's easiest to line up a single peak on the two tracks.
.
Click to expand...

If you line up peaks does that elminate it 100%. Sorry I am asking really basic question but I just want to make sure.
 
No, I don't believe so. Peaks I think are amplitude based, and phase is more fequency based, so lining up peaks is just like lining up where the loudest part came through the mic, but in all honesty I do this a lot with my kick drums and it works pretty well if you want to kick to be more clear.

Edit: Just read that link above and disregard my post.
 
Light said:
THIS SHOULD HELP.


Light

"Cowards can never be moral."
M.K. Gandhi
Click to expand...


Well after reading that I am 100% understand the tech side of it. Thanks

So I am guessing with my situation listed above about micing two diffrent speakers on one cab, that the phase problems would be really small. Correct?
 
phase is a time relationship,check out the other post on here about phase.
the 3 to 1 rule is for spaced pair mic's,the distance from one mic to the other should be 3 times the distance as that from the sound source.every time you use multiple mics you introduce phase issues,not necessarily good or bad,it depends,if the sound seems hollow,tiny as you put it,that could be a phase problem.the only thing that corrects phase is moving the mics or time based processing (delay).a good example for hearing phase problems is listening to your stereo while moving one speaker closer or further away than the other one,you'll hear certain frequencys drop out (cancel),also if you reverse the wiring to your speakers you'll hear the difference,wich is because of phase.
bigger consoles have "phase reverse" buttons,wich is kinda incorrect because phase is a time relationship and polarity inversion (wich is what they do) has nothing to do with time.they can help,or not,they invert the waveform,turn it upside down.they are useful for double micing snares,reversing polarity on the top or bottom mic because the mic on one side of the snare will be picking up an upside down waveform relative to the other one,so reversing one of those mics will correct that,sometimes polarity reverse is good when using room mics and having problems with the time difference from the room mics relative to the direct mics (phase).when i record i try polarity reverse on each mic,if it sounds better that way i leave it on,but since they only exactly invert the waveform there's no variability with it,it's either exactly upside down or off.
the best way to correct phase problems is by moving the mics,this way you have some control over how much you are correcting phase problems,if pushing the so called phase reverse button doesn't work.
phase shift is a term that describes one waveforms lead or lag in time with respect to another.
 
Well, the point of lining up peaks is that they give an easy graphical reference to help compensate for the difference in time that the sound took to reach the two mics.

Getting two mics in different placements completely in phase is pretty much impossible, as different frequencies will be in or out of phase at different times - which results from the fact that different frequencies had different time periods it takes for a cycle.

And if it's hard panned, it should be minimized, because where the problems from things being out of phase come in is when they overlap, being added together - such as both down the center or mono (or they dont have to be right down the center, but just coming out of the same speaker at the same time).

Because they reach their peak and trough at different times, sometimes you can end up with the two cancelling out either completelyu or partially (when one thing is "up" while the other is "down") - destructive interference - or adding up - constructive interference.

So you could easily adjust things so that certain frequencies are in phase, but other frequencies will be varying on different time intervals in some situations, so a lot of times the best you can do is to minimize phase problems, or work with them until they're happening at the least undesirable frequencies.

For example, if you have two of the same signal, and you sweep them in and out of phase by adjusting the time, you'll get sort of a phasing (which is where the term phaser comes from) effect, as you hear the phase cancellation sweep through the frequency spectrum, as the amount of time needed to be completely out of phase is much greater for lower frequencies than higher frequencies (because their cycle is spread over a longer period)
 
chadsxe said:
Well after reading that I am 100% understand the tech side of it. Thanks

So I am guessing with my situation listed above about micing two diffrent speakers on one cab, that the phase problems would be really small. Correct?
Click to expand...

You want the diaphragms as close to the same distance from their respective speakers as possible. You just need to know where in the mics the diaphragm is located.


Light

"Cowards can never be moral."
M.K. Gandhi
 
I am guessing the majority of the phase issue would be coming from a mic picking up the sound from the other speaker that is miced.
 
that is correct.

Remember what mattamatta said, Avoiding phase is necessary to have a good mono mix. And having a good mono mix usually leads to having a great stereo mix.

If you record your two cabinet speakers, and pan them Hard L&R, there will be no concern for phase because the two sounds are not overlapping in one speaker.

It is when you check your mix in mono that you will find....HEY! Everything just got smaller.....Things panned center will appear much louder and the guitars disappear.

If you can balance everything well in mono using the panning and signal delay talked about above, when you switch back to stereo it will only sound better.
 
chadsxe said:
I am guessing the majority of the phase issue would be coming from a mic picking up the sound from the other speaker that is miced.
Click to expand...
All phase issues in this context are from combining sound that is out of time alignment for any reason -mic positions, reflections, anything that causes delay on one relative to the other.
The amout of time difference (not the distance from the source) says how and which frequencies will comb.
Their relative volumes say how deep the comb filtering will be.
A quickie re-cap. ;)
Wayne
 
mattamatta said:
... Getting two mics in different placements completely in phase is pretty much impossible, as different frequencies will be in or out of phase at different times - which results from the fact that different frequencies had different time periods it takes for a cycle.
Click to expand...
If you have a simple two-path difference, they can be brought back into alignment. It's once there are any other late arivals that goes out the window.
:D
Wayne
 
You must log in or register to reply here.
Back
Top