Using two mics - out of phase?

[pure.fusion]

Hey guys,

I'm going to try recording my guitar amp with two mics on the weekend - just for the experience and because someone gave me a condenser mic to play with.

Not to go into specifics about he methods of micing guitar amps, my question is about phase relation between the two mics - I've heard it mentioned many times but know nothing about it other than I have to "beware of it"

Lets see, from my high school physics, depending on the distance from the souce, the signals might be (I assume) anywhere between 0 and 180 drgrees out of phase? Is this correct?

So how do you make sure they're in phase? What's the go? What to I need to do here to make sure I'm doing the right thing?. Is there a standard method that people use in the studio?

Also, If I'm recording this one instrument, do I want to mix the result of the two mics onto one single track, or do I put them on one track each?

Cheers,
FM

 

[TheChikenMaster]

3 to 1 rule. 3 inches apart 1 inch away from source for example. Or the xy have the capsules facing 90deg away from each other. Separate tracks is good. Do you want stereo?

 

[Dogbreath]

3 to 1 rule dude.

Ya place the second mic 3 times the distance from the first mic, as the first mic is to the source.

make sense?

As to the 1 track or 2 tracks...definitely keep em separate. That way you can still apply separate effects, comp, EQ or whatever ya want to the individual tracks.

 

[Dogbreath]

ChickenDude types too fast.

 

[miroslav]

The "rule" is....distance between mics should be 3x the distance of the mics to source but that mainly applies IF the two mics are meant to be eqally distant from the source like in a stereo mic steup (did I say that right?)....
...(yes, I did )....

...but I wouldn't sweat it too much since you can always time-align your two tracks in the DAW easy enough if you are planning on mixing them together and are hearing some phase issues (like a hollow-ish sound when they are played together)...
...though you may actually like the small amount of phase difference as it might give you just the blend you want.

Record each mic to its own track.

 

[pure.fusion]

...but I wouldn't sweat it too much since you can always time-align your two tracks in the DAW easy enough if you are planning on mixing them together and are hearing some phase issues (like a hollow-ish sound when they are played together)...

Wow! Fair dinkum, can you do that? Cool.

Stereo? No. It's just to draw apon a different mic sound to prehaps enrich the tone of the recorded guitar track.

But the 3:1 rule. I was ready for a physics lesson and this is the rule? OK I'll give it a go.

Cheers guys.

FM

 

[DrewPeterson7]

You know, random question that I should already know, but I want to make sure -

the rule is at least 3:1, so as long as the second mic is at least 3x as far away from the first, you should be good to go, right? Or is it exactly, mathematically three times as far away? I always understood it as the former, but then again I don't do this very often, so...

 

[philbagg]

Try flipping the second mic out of polarity.

Move the mic until it sounds HORRIBLY out of phase. Flip the polarity switch back.

Seems silly....but I find that its easier to hear when the mics are really out, than really in.

You know, random question that I should already know, but I want to make sure -

the rule is at least 3:1, so as long as the second mic is at least 3x as far away from the first, you should be good to go, right? Or is it exactly, mathematically three times as far away? I always understood it as the former, but then again I don't do this very often, so...

I think it's exactly 3 times the distance

 

[NYMorningstar]

Here we go again
The 3:1 rule is to minimize bleed and is for multiple sources.

MIC ---------3' ------------MIC
1'--------------------------1'
Source--------------------Source

The distance between the mics uses a ratio of 3 to 1. Because of the distance the bleed become insignificant.

If the mics were 2 feet from the sources then they would need to be 6 feet apart.

You cannot time align your tracks in your daw either if you use multiple mics at different distances when using a single source. You will have comb filtering no matter how you line them up because different frequencies have different wave lengths. The interesting thing about that is the filtering acts like an EQ so you may want to listen while moving the mics around to get the sound you're looking for.

I prefer to EQ after tracking so I usually use one mic, one source but sometimes it doesn't hurt to have an extra track with a different mic but I'll keep it the same distance. The exception of course would be a room mic.

Most DAW's have phase switches you can flip to hear the differences. When you mic a snare on top and bottom you want to check that to see which sounds better. You keep the setting that sounds best to the mix which usually is in phase to give more punch, bite, err whatever but not always. There is no rule for that.

Somebody mention polarity so Glenn can get into this

 

[Dogbreath]

I was always under the assumption that it was at least 3 times.

When I put a 57 up on da grill or maybe one inch off, I'll have an LDC or whatever about 3 feet back.
I think it's "at least".

 

[NYMorningstar]

I was always under the assumption that it was at least 3 times.

When I put a 57 up on da grill or maybe one inch off, I'll have an LDC or whatever about 3 feet back.
I think it's "at least".

1', 2', 3', 4', doesn't matter because the rule doesn't work that way. The distance you need for a single source will always depend on what frequencies you want to be comb filtered.

 

[TheChikenMaster]

Here we go again
The 3:1 rule is to minimize bleed and is for multiple sources.

MIC ---------3' ------------MIC
1'--------------------------1'
Source--------------------Source

The distance between the mics uses a ratio of 3 to 1. Because of the distance the bleed become insignificant.

If the mics were 2 feet from the sources then they would need to be 6 feet apart.

You cannot time align your tracks in your daw either if you use multiple mics at different distances when using a single source. You will have comb filtering no matter how you line them up because different frequencies have different wave lengths. The interesting thing about that is the filtering acts like an EQ so you may want to listen while moving the mics around to get the sound you're looking for.

I prefer to EQ after tracking so I usually use one mic, one source but sometimes it doesn't hurt to have an extra track with a different mic but I'll keep it the same distance. The exception of course would be a room mic.

Most DAW's have phase switches you can flip to hear the differences. When you mic a snare on top and bottom you want to check that to see which sounds better. You keep the setting that sounds best to the mix which usually is in phase to give more punch, bite, err whatever but not always. There is no rule for that.

Somebody mention polarity so Glenn can get into this

Very nicely put!
POLARITY!

 

[Sonixx]

the 3:1 rule is not for phase coherent signals such as two mic cab tracking.

FWIW, here's a thread at 3DAudio I started some time back on the issue.

I adjust relative phase post tracking and position mics based on the best tone and not on phase summing. Phase summing can be an excellent EQ.

 

[miroslav]

You cannot time align your tracks in your daw either if you use multiple mics at different distances when using a single source. You will have comb filtering no matter how you line them up because different frequencies have different wave lengths.

Not sure what you are saying here....
If you have say...just a single wave (1kHz) coming from a single point source...and you have one mic facing the source at 1' distance and the other at 2'...
...how does the frequency...wave length change?
It doesn't….if I’m not mistaken.

What changes is the time and amplitude...but the length stays the same...that's what makes it a 1kHz wave.
When you time-align...you are aligning the time difference between the two waves...their single wave lengths will be the same and amplitudes will be a bit different due to the drop-off caused by the different distances, but you can sum varying amplitudes without issues when you align them so they are on the same side (phase).
The only things that will NOT line up...are the initial attack and final tail portions of the waves since any one wave will hit one mic at a different portion of its phase than the second mic....but if you slide one track, you move it's time element, and you CAN get the waves pretty tightly aligned without a heck of a lot of filtering. At least NO more filtering issues than any 3:1 rule will fix for ya!
Don’t align their attacks or tails…align their first complete wave cycles.

the 3:1 rule is not for phase coherent signals such as two mic cab tracking.

FWIW, here's a thread at 3DAudio I started some time back on the issue.

Yes...he makes good points...but I would still use the 3:1 if the two mics were intended as a L/R stereo pair....which is what I was getting at in my first post (the part in bold).

AFA doing mic front/back of a cab or top/bottom of a snare...that's mainly a polarity issue, though once you "flip" either source...you can still time-align if you want the time/length to be generally aligned, as there may be different distances for each mic.
Heck...for drum with both heads mounted it's even more complex as technically it's NOT the same exact source. One mic is on the top head and the other on the bottom....


But all this said…I don’t see that it’s much of an issue when using two mics on a cab…even without ANY special considerations of the 3:1 rule or time-alignment…just blend/mix the two to-taste….s’no big deal.
We have two ears and they don’t follow the 3:1 rule…. that’s how you get depth/distance perception.

 

[NYMorningstar]

Not sure what you are saying here....
If you have say...just a single wave (1kHz) coming from a single point source...and you have one mic facing the source at 1' distance and the other at 2'...
...how does the frequency...wave length change?
It doesn't….if I’m not mistaken.

.

You'll never have a single wave and I'm not saying the length of the wave will change. What I mean is there are various lengths and you can't get them all to line up at the same time so it causes comb filtering.

 

[Sound & Vision]

Wow! Fair dinkum, can you do that? Cool.

Stereo? No. It's just to draw apon a different mic sound to prehaps enrich the tone of the recorded guitar track.

But the 3:1 rule. I was ready for a physics lesson and this is the rule? OK I'll give it a go.

Cheers guys.

FM

Don't sweat the 3:1 rule. You will never keep all of your guitar frequencies in phase with 2 mics. If you played an "A" note (fundamental) at 440Hz you could set up you mics to be perfectly in phase for the 440Hz and its harmonics. The only problem is that when you go to play a different note the two microphones would be out of phase with each other since the wavelengths are different. The 3:1 rule does not get around this! The phenomenon happening at various frequencies is called comb filtering as NYMorningstar said.

So, what to do? Have someone play guitar and move your second mic around in a straight line from the amp and listen to them together. You will hear the filtering effect as a "swashing" sound. Get it to a point that sounds good to you and leave it!

The is nothing terribly wrong with sounds being out of phase. Remember our ears are spaced apart and our brain uses time delays and phase information to determine distance and direction of sounds.

 

[bennychico11]

When you time-align...you are aligning the time difference between the two waves...their single wave lengths will be the same and amplitudes will be a bit different due to the drop-off caused by the different distances

not necessarily. Because of different room reflections/modes (and microphone frequency responses) you will see different representations of wave lengths in your DAW. They will look similar, but will not be 100% identical.


the funny thing is, I think we're over complicating this. If the OP's only goal was to try two different mics on the cab to see what different tonal sounds he could get...all he needs to do is place them coincident and any polarity/phase issues will be moot.

 

[miroslav]

....you can't get them all to line up at the same time...

OK....time for some examples.

I set up a simple 2-mic rig as you can see in this image. One is up against the grill and the second mic is directly behind it 2 feet back.

Here you can see the waves from the two mics Nonaligned (blue) and Aligned (red).
(The upper track for each color is the near mic and the lower is the far mic.)

I simply slid the lower red track (far mic) forward so it is time-aligned with the upper red track (near mic).
You can see the alignment where the cursor falls across them all.

Here is more toward the end of the waves...and you can see that the two top ones are still Nonaligned but the two bottom ones remain pretty Aligned even as the wave continues toward its end.

Here are the audio files...you can easily hear the comb-filtering in the Nonaligned waves:

PhaseTest_NonAligned.WAV



And how much better they sound once I have time-aligned them:

PhaseTest_Aligned.WAV


So...I'm not sure why you say you can't time align to compensate for phase because it will cause comb filtering issues…?
It's MUCH more of an issue when you DON'T time align...don't you agree?

 

[miroslav]

They will look similar, but will not be 100% identical.

I never suggested they would be 100% identical...only that they would be similar enough for time-alignmet to work.

 

[bennychico11]

I never suggested they would be 100% identical...only that they would be similar enough for time-alignmet to work.

oh I adjust the waves to time align all the time. And I don't think NYMorningstar was saying it's physically impossible to move them in the time line and get it to sound good. I think he also was saying you won't get a perfectly, 100%, identical, in phase signal. As you can see from your own pictures, they aren't 100% identical...even if you time aligned them. The different responses of the microphones alone prevents them from being perfect. So even if you adjusted them in the time line to occur at the same time, there still would be comb filtering due to this. Of course, there would be extremely minimal filtering. In fact, the resulting signal may sound amazingly perfect! Not all comb filtering (or phase misalignment) is bad. Audio is never perfectly in phase in the real world. In fact, the minimal filtering can be a good thing. It's what gives us that different tonality we're looking for when using two separate spaced mics. We just want to minimize BAD phasing...when the comb filtering filters out frequencies we LIKE instead of amplifying them.


again, I think that's what NY was saying.