M n S was devised to be phase coherent and therefore fully mono compatable - no worries between these two but they might be out of phase with you spot mic...

Phase is always an issue when using more than one mic, but not always a problem, and rarely a problem that cannot be fixed. The 3:1 rule is often quoted - distance betwee the mics shoud be three times their distance from the sound source.

If you imagine sound waves going up and down moving away from the guitar to your mics arrays, they hit them at different times and at different wave phases right? So if you want them to be in phase you would probably delay front most mic. As sound travels at approx 34cm in 1ms you might want to delay by about 1.5ms but you do this by looking at the waves and lining them up, it therefore helps to have a loud sound at the start to do this with.... but you must use your ears too... there is no such thing as absolute phase coherency because all things are never equal so listen to what sounds best... you might even like it more out of phase than less.

Now, corners are often thought of as being bad, all room nodes terminate in corners, lots of comb filtering, phase issues closest to walls. You might like its accoustic properties tho' or maybe its far away from a noise source, or its the only place there's room for you hmmm... but have a check to see its not giving you problems. Probebly better to have the mic out of the corner than in it!!

jah bless

Your corner approach strikes me as a little odd, but perhaps you've found a sound you like. (It might be that you like hearing yourself this way. The interesting thing about playing acoustic guitar is that everyone BUT the guitarist can hear what it really sounds like....)

I prefer to put the acoustic guitar in the middle of a room, as far as possible from side/reflective walls, with ideally a fairly high ceiling. Sitting down is OK if you're doing guitar-only work, but if it's a folk singer/songwriter situation, I prefer they stand. (A little better separation with the voc & guit mics that way). It also helps to experiment with the flooring - carpeting vs. hardwood vs. stone.

If you're mixing in a DAW, you should be able to compensate for the 50cm-100cm difference in time between the solo and MS mic. I don't think it's going to make TOO much difference overall, but worth a little adjustment if it sounds better to you. The solo mic should give you all the meat and detail, while the MS should let you spread it out a bit in stereo.

You don't need a spot microphone just the MS pair. The "M" is your spot microphone.

MS freak
Ms. Remy Ann David

Thank You all !
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jahtao and JoeH,

This corner thing will make the figure-8 mic get more direct sound. Not bouncing around that much before reaching the figure-8 mic.
I believe M/S is extra sensitive to the room integration. And this corner M/S may not really be "stereo-recording". But art is not just copying reality it may be more interpreting it. (The art of 2-speaker stereo.)

Otherwise this is my listening room with not much more than my speakers and plenty of recording space in the middle.

Maybe I should try some screens and record in the middle of the room ?
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RemyRAD,

No I don't need a "spot mic" but I have tried many setups and just a M/S couple at about 50 cm from the guitar is quite good. A clear center image and some "space".

But this "spot mic" seems to get me a tighter and more front up center image and still more space. (In fact I'm quite convinced of that.)

Now I also have one more parameter in my mix to play with. I may even put some reverb on this center-track (at or just below the A/B threshold).
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Some more questions:

1.)
The guitar is no point source. It is rather the opposite. So will there really be much of phase problems ?

2.)
How to monitor that out of phase. Speakers, headphones, ... ?
(I think it is easy to get a "good sound" i headphones. But not in speakers. I am very picky about speaker sound. It is very hard (maybe impossible) to get a convincing 2.1 stereo sound in speakers.

(5.1 will help a lot. But I am not recording or mixing for that.)

/Goran Sweden

Instead of bringing in a 3rd mic, I'd work with just bringing in your main m-s pair closer. In the end, your phase coherency and depth of sound will thank you. I use M-S for recording guitar all the time and I think it sounds great (it is actually usually my "go-to" method), but I usually mic from the 50-60 cm distance than 100 or more...

What are you recording to? To monitor, you'll need some sort of a matrix. You can either buy them as a seperate analog box, some preamps come with them (like the True P8 and the Amek 9098), or you can decode in a mixer or software...

--Ben

FifthCircle,

I decode in software either manually or by a plug-in (the same result of course).

My heart is with "classical" music. I would never listen to an acoustic guitar record (or pop or jazz). So I don't really know how it should sound.
But I like the sound of an acoustc guitar. But I don't consider it a real instrument. It is folk music. Can be nice. But not more.

Among many other records I have a recording of bach violin solo sonatas and partias. This is more my ideal. But now after listening to my many own different "stereo" recordings I realize that this recording is not really good (but she (it is allways a she) plays wonderful). Maybe this is a spaced A-B (it must be quite phasy it tears my ears apart) and the mics don't seem to be matched either (all high tones left and all lows right). No center image. Strange ! I don't think I can listen to this record any more.

/Goran Sweden

Goran,
feel free to call a fellow acoustic person. My cell phone is 0739-641575 (daytime accountant job phone, most always with me at evenings as well). If you are situated anywhere close to Stockholm, I could bring a few different mics to let you test record with. (No cost of course)

Gunnar.