mid-side micing of acoustic guitar

I've been having a problem in mid-side micing of an acoustic guitar. Following the standard method (below), as the mid mic comes up in the mix, the L/R levels shift, causing the apparent sound source to pan to one side. At first, I thought this was because the sound of the guitar was stronger on one side. But after looking at this more, I think the mid mic is canceling frequencies on one side and summing on the other, because the side mic is phase reversed on one side, and not on the other. If the mid mic were IDEAL (a very narrow pattern), it would only pick up the sound straight ahead, in the rejection point of the ribbon's figure8, BUT with an actual cardiod mic, the mid channel is picking up some of the right and left. This would cause the addition / cancellation with the side mic, I think. The wider the actual cardiod pattern of the mid mic, the worse the apparent panning.
Could this be fixed by copying the mid mic to a second channel, flip the phase, and pan them, like the side mic? Then there won't be cancellation on one side; it would add on both.
Thanks for your help!

MS micing Method:
- Using either a sm7 or re20 for the mid and then a ribbon mic for the side, vertically close together, directed at the 14th fret about 6 inches back.
- copy the side to a 2nd track and reverse phase, then pan L/R

I have to say that I haven't found MS to be so good for close up micing, and prefer a Blumlein pair, or just a couple of spaced mics, for acoustics.

Also, what ribbon are you using? Some ribbon mics do have some cardioid character.

I used a MXL R44 for the ribbon mic. I tested the ribbon mic and it does reject the mid quite a bit, but I didn't measure it. (just an observation from speaking "left, center, right" with the mic). If the ribbon is picking up too much in the center, it would cause the same canceling effect with the mid mic?
I like the sound I'm getting - other than this panning issue.
I would like to try a Blumlein pair since I recently acquired a pair of multipattern condensors. I could also swap the ribbon for the mulipattern condensor in Figure8 and see if I get the same problem.

I have the same issue here using a pair of AKG 414EB's.

I'll try to make sure it's balanced when I mic up the guitar, but inevitably the artist shifts around and gets off kilter throwing the stereo image off center.

If it's tucked into a track, not a big deal.

Solo recording might be more of an issue.

I just live with it mostly, it's part of the fun.  8)

Mark

dmp said:

I've been having a problem in mid-side micing of an acoustic guitar. Following the standard method (below), as the mid mic comes up in the mix, the L/R levels shift, causing the apparent sound source to pan to one side. At first, I thought this was because the sound of the guitar was stronger on one side. But after looking at this more, I think the mid mic is canceling frequencies on one side and summing on the other, because the side mic is phase reversed on one side, and not on the other. If the mid mic were IDEAL (a very narrow pattern), it would only pick up the sound straight ahead, in the rejection point of the ribbon's figure8, BUT with an actual cardiod mic, the mid channel is picking up some of the right and left. This would cause the addition / cancellation with the side mic, I think. The wider the actual cardiod pattern of the mid mic, the worse the apparent panning.

Click to expand...

Wrong assumption. MS works even with an omni.

Could this be fixed by copying the mid mic to a second channel, flip the phase, and pan them, like the side mic? Then there won't be cancellation on one side; it would add on both.

Click to expand...

No. You would completely loose the essence of M/S.
Assuming, the gains are correctly matched, you have a problem either with the S mic and/or with the positioning of the mics.
As zebra says, M/S is not really suitable for close micing, because of the very problem biasrock mentions. Any angular shift is drastically enhanced by close-micing.

I just tracked a bunch of classical guitar this way using Beyerdynamic M130/M160.  Fairly close, maybe two feet away.  No problems.  I have noticed serious attention to mic positioning and matching of mics themselves is key. 

two feet is fairly close for MS, it can work though.

personally, i think of MS as capturing a stereo image of a room or
an ensemble in a room.  at minimum, a half decent sounding one.

as always, positioning of mic or mics and intended target(s) is critical, as
doug mentioned.
and most any close stereo pair means the instrument must hold rather still
in the pickup pattern of the mics, or annoying stuff happens.

dmp said:

But after looking at this more, I think the mid mic is canceling frequencies on one side and summing on the other, because the side mic is phase reversed on one side, and not on the other.

Click to expand...

It is hard to say without actual hearing what you get, but from your description, I had similar cancellation problems on low frequencies with MS matrixing on piano recordings (distant miking). IIRC, the work around was to use low cut on S and making up on M. That also stabilized image and made lows less muddy.

I will look at that recording tonight to refresh things and will get back.

Best, M 

I have had good results with an alesis GT61?(the tube model) for sides and TLM103 for Mid. 2-3 ft away.

I prefer M-S for Acoustic guitars.

I've been experimenting more with this technique, but have come up with very little in the way of best practices.
Can anyone provide tips on evaluating good mics for mid/side?
I tried some experiments last night with the side mic, thinking that perhaps it was hotter on the back than the front, which would through off the stereo field (some ribbons have more pop protection on the front I think). After the experiments, however, I wasn't sure if I'd actually learned anything  .
Also, in terms of positioning, the mid / side pair is always in the same relation to each other, correct? It would be adjustments of aiming and distance for best sound / stereo field?

Here's a good overview of MS techniques.

Dry as hell, but it's all there.

http://www.youtube.com/watch?v=Da7A0fbA754

Mark

try summing to mono  See what happens. Could be revealing.  Not sure as i usually use 1 mic on acoustic. mute the phase reversed and sum.  Are you processing the signals differently?  Could latency be an issue?

What are you using for the phase reverse? 
If its a plugin, make sure theres no latency on the track with the plug, the latency may be causing the phase problem (another way is to just put the same plug on the non phase rev channel and the center channel and just leave it alone so the other 2 tracks have the same plug latency)  again, this is assuming youre using plugins to do this.

try summing to mono

Click to expand...

The sides cancel completely, as they should

What are you using for the phase reverse? 

Click to expand...

Phase switch in my DAW - has no latency

Also, in terms of positioning, the mid / side pair is always in the same relation to each other, correct? It would be adjustments of aiming and distance for best sound / stereo field?

Click to expand...

That's right, the figure-of-8 should be 90 deg to the mid. With the usual caveat of "what sounds good is good."

From what I've read, I don't think you actually have a problem with your method, just that one side of the guitar is louder - if you're pointing the mid at fret 14 then the 'body' side is likely to be louder than the neck side.

With an acoustic, I've usually found it easier to record myself than to record others, as I can listen on phones & move myself around the mic until I hit the sweet spot.

Anyway, have fun - you'll learn more through your experiments than reading forums.

Stewart

Just another note... I have a guy in the studio right now doing just acoustic work.  We have a couple of room mics, a ribbon and a condenser set up.

I missed the part of you aiming the pair at the 14th fret.  Usually when doing M-S Ill put em right infront of the sound hole (because the body is where most of the sound is coming from as previously stated by zebra50).

Usually, though, I have the ribbon on the sound hole to suck up the bottom end and the condenser near the 14th for the high end (obv. not using the M-S technique) the room mics for ambiance... which also gives a nice stereo field.

Before tracking, we A/B'd MS and the setup mentioned above, and the later won.  MS has nice benefits later on, but with the room mic's spreading things out, there just was no comparison.

Try changing the position if M-S is the way you want to go.  If you can, even have the person playing move while recording to see where things sound best.  (the further back the wider it seems to get but the more distant it sounds...)  Experimenting is the best way to learn and discover new things.