phase issues

[ericlingus]

I like to mic my amp with two mics. Right now im using one sm57 and one es57(copy of the original). I like to close mic one( about 3 inches away) and another one about a foot away. I put one on axix(the closer miked one) and one off axis. But I can't get rid of the phasing. I tried changing the mics around a little but it still doesn't work. The only way to get rid of it is if I both put them the same distance from the amp. But I dont want that sound. I play black metal and prefer a little more distance on my guitars. I have a mesa boogie F50,the two mics I mentioned going into a firebox. Cubase LE is my recording software with a pretty high end computer.

[ericlingus]

now that I try it some more, I realize there is a little phasing when they are both the same distance from the amp. Any advice?

[cpl_crud]

You're always going to get phasing whenever two mics are noncoincident from a single source. If you want the distance to hte sound, why not have them both coincident, but at a distance?
And why recording wiht two of teh (roughly) same mics anyway?

[bennychico11]

one rule of thumb you can follow that may help is the 3:1 rule. This rule states that two microphones should be 3 times the distance away from each other as one is from the source. So in your case, the second mic should be 9 inches away from the other mic...or 12 inches away from the cabinet (or the same spot that the first mic is pointing at).

since you are using a DAW, you can zoom in on your wave forms and physically move the wave of the second mic backward in time to compensate for this. Find a nice transient on both of the wave forms and correct it on the second mic's track timeline. I prefer this method because you are actually compensating for the delay in time of the signal. It's like sitting in the front row of a concert versus sitting way back in the lawn seats. Correcting the delay puts you in the front row.

Also, a lot of people like to use the polarity reverse button on their mixers. It may be labeled inverse or have the greek letter Phi. Sometimes this helps, sometimes it doesn't. It only flips the polarity of the wave, it does not correct the phase between the two waves.

[Farview]

Originally Posted by bennychico11

one rule of thumb you can follow that may help is the 3:1 rule. This rule states that two microphones should be 3 times the distance away from each other as one is from the source. So in your case, the second mic should be 9 inches away from the other mic...or 12 inches away from the cabinet (or the same spot that the first mic is pointing at).

since you are using a DAW, you can zoom in on your wave forms and physically move the wave of the second mic backward in time to compensate for this. Find a nice transient on both of the wave forms and correct it on the second mic's track timeline. I prefer this method because you are actually compensating for the delay in time of the signal. It's like sitting in the front row of a concert versus sitting way back in the lawn seats.

Also, a lot of people like to use the polarity reverse button on their mixers. It may be labeled inverse or have the greek letter Phi.

How many times can this myth be repeated on the board in two days?

The 3 to 1 rule doesn't work for and was never meant to apply to a close mic-far mic situation. Ever!!!!

The 3 to 1 rule is about attenuation, not phase cancellation. You would use the 3 to 1 rule when you are micing an acoustic guitar (and both mics are the same distance from the guitar) 1 mic at the bridge and 1 mic at the neck. It also applies to micing two diferent sources.

[ericlingus]

could you explain more to me how to correct the phasing by moving the second mic back in time? Could you explain step by step how to do this?

[ericlingus]

yeah I tried that 3 to 1 rule and it didn't work. I think it made things worse lol. Anyone else have any pointers?

[Farview]

If you zoom in on the waveforms of the two mics, you will notice that one happens first, or earlier in time. Slide the waveform of the later one back (to the left) to match the earlier one.

[cpl_crud]

The timeslip function is the same as using a delay.
To work out how much you have to slip the track, use the following formulae:

t=d/340

where d= distance in meters between the two mics along the axis of the incoming wave.

You have to either delay the signal from the closer mic, or advance the signal from the far mic.

I'm not quite sure what that conversion factor is for your heatanistic "feet and inches" crap, but I'm sure you can work that out yourself.
You'll find that, for the distance of about a foot, the time delay is going to be about 1 millisecond. Or something like that. It's 5am and i'm doing this math in my head.

[ericlingus]

do I open the sample editor? or the audio part editor? I don't know how to move the waveforms.

[Farview]

I don't know what program you are using, but don't you just see all the waveforms on all the tracks in your default view? Just grab one and move it.

[ericlingus]

yeah it does show that. But it says they both start at the same time.

[ericlingus]

I know how to move them and copy paste them and all that but the indicators say they both start and end at the same time. I am using cubase LE btw.

[Farview]

That is the probem, they shouldn't start and end at the same time. If the waveforms (the picture in the object) don't line up, you need to slide one of them over so that they do.

[ericlingus]

okay I see now. So the distant mic waveform should be moved slightly ahead. But how do I tell how much ahead? and how do I move it that slightly ahead?

[bennychico11]

Originally Posted by Farview

How many times can this myth be repeated on the board in two days?

The 3 to 1 rule doesn't work for and was never meant to apply to a close mic-far mic situation. Ever!!!!

The 3 to 1 rule is about attenuation, not phase cancellation. You would use the 3 to 1 rule when you are micing an acoustic guitar (and both mics are the same distance from the guitar) 1 mic at the bridge and 1 mic at the neck. It also applies to micing two diferent sources.

sorry never noticed that someone else posted this.
phasing does result in attenuation, though. When the out of phase signals (out of time signals) are summed together certain frequencies cancel creating a comb filtering effect. The frequency response of this comb filtering creates peaks and dips in our sound, attenuating the sound at that moment in time. Separating by AT LEAST 3 times the mic-to-source distance creates a level difference of 9dB or more in turn reducing the comb filtering dips to an inaudible 1dB or less....we aren't trying to completely eliminate the comb filtering, just trying to lessen the effect. Also at this distance (some suggest a 4:1 rule) the second mic can pick up enough room ambience to minimize any phase issues.
Using the 3:1 with a guitar when micing the neck and body is the same thing as micing the cabinet with a close and far mic, in my mind. In the acoustic guitar example, the neck mic is the far mic in relation to the body. It's still a further distance away from that sound source than the close mic is. You just want to eliminate any phasing from body to close mic and from body to far mic. True, 3:1 is better used when recording two different sources and you want to minimize bleed from other sources (ie. snare mic and tom mic).

However, we don't live in a world with perfect sine waves. The wave form bouncing of the ground, wall, mic stand of the first mic, etc. change the way it sounds in the second mic...which is one of the reasons why we use a close mic and a far mic. It's the same sound coming from the amp, but the second mic might give us a better result after the wave form has had time to develop. Don't treat each of the mics as if they are recording the same source, treat them as if they are recording two different sources...because they actually are capturing two diffrent sounds.

I did say that the 3:1 may not always work for what you are recording, depending how much of the sound and from where the reflections are coming from. Use you ears. It sounds like it's not working for you, so try the sliding of regions thing I suggested....I prefer that method anyway. 3:1 has worked for me, but moving the wave form is the only way you can actually correct the phase of the two waves.
The 3:1 has never been an aboslute rule. It was mainly used in situations where you had to quickly setup two mics or wanted to see if maybe two microphones would work well for what you might be recording.

http://www.crownaudio.com/mic_web/tips/mictip6.htm

[ericlingus]

okay I figured everything out but how much to move it. There is no way I can do that mathematical formula. Any other way to find out?

[bennychico11]

Originally Posted by ericlingus

okay I see now. So the distant mic waveform should be moved slightly ahead. But how do I tell how much ahead? and how do I move it that slightly ahead?

find a reference point. peak transients are good for that...you should see it on maybe a downstroke of your guitar playing where the wave all of a sudden compresses upward. You should see a similar movement in the other track....line those two up (move the second mic...not the first mic, otherwise you're just inducing more delay into your mix).

[metalhead28]

Hey, if you're using 2 57's basically, what's up with the "off-axis" thing for the distant mic? That seems like it would sound wierd anyway....just a thought.

Anyway, whenever you're using two mics there is going to be some sort of phase cancellation. If you are really meticulous in placing the mics, sometimes you can make that part of the sound work for you.

I've found that whenver I've tried to move waveforms to line them up, no matter what I do it doesn't really get alot better....it just gets different. I suppose you could find a magic spot where you liked the sound....who knows.

[Farview]

Originally Posted by ericlingus

okay I figured everything out but how much to move it. There is no way I can do that mathematical formula. Any other way to find out?

Look for a transient. (the beginning of a note) once you find that on both tracks, line them up. You have to zoom in almost all the way.

[bennychico11]

Originally Posted by cpl_crud

t=d/340

what's the 340 for? just out of curiosity?

[ericlingus]

see I can't figure out how to do that. I cant see both waveforms up close together. One or the other is on the screen.

[bennychico11]

Originally Posted by ericlingus

see I can't figure out how to do that. I cant see both waveforms up close together. One or the other is on the screen.

then do this, is there a way to nudge the wave form by like 10 samples at a time or maybe a little more than that? just nudge the second one slowly back until it sounds the way you want it. or find a reference transient point in the time code on the first track, write that down, and place the same transient on the second track at that time code

[Farview]

[QUOTE=bennychico11]sorry never noticed that someone else posted this.
phasing does result in attenuation, though. When the out of phase signals (out of time signals) are summed together certain frequencies cancel creating a comb filtering effect.[quote]
That isn't the attenuation I'm talking about. You want the mics 3 times farther apart than they are from the source. It helps with phase cancelation because the two mics are not hearing the same thing.

Originally Posted by bennychico11

Separating by AT LEAST 3 times the mic-to-source distance creates a level difference of 9dB or more in turn reducing the comb filtering dips to an inaudible 1dB or less....

But if you turn up the distant mic to be as loud as the close mic, you just lost the advantage.

Originally Posted by bennychico11

Using the 3:1 with a guitar when micing the neck and body is the same thing as micing the cabinet with a close and far mic, in my mind. In the acoustic guitar example, the neck mic is the far mic in relation to the body. It's still a further distance away from that sound source than the close mic is.

No it isn't, the neck is a different sound source. The sound doesn't come from only one spot on the guitar. The mics will be the same distance from their sources.

[ericlingus]

alright I have the audio part editors open for each track, then I just minimized the windows for both and put them under each other. I zoomed inall the way. I see a transient where the second track is further ahead. I just can't seem to figure out how to move the waveform. I know how the move the whole track. But not just the waveform if you know what I mean. The blocks I can move but not the inside of it. If anyone could teach me how to move the waveform back slightly while in the audio part editor i'd be grateful Thanks so far though guys.