Spaced pair on acoustic guitar - phase problems?

F

Fast Frank

New member
So, was never that great at Physics....

Does anyone know if you can correct the phase problems with a spaced pair of mics on an acoustic guitar by moving the waveforms in a DAW slightly in relation to each other? So, if the recording were to be played back in mono it wouldn't go all horrid and comb-filtery?

Like, really carefully matching the peaks and troughs of the two mono files with each other? Does that make sense?

Anyone got any tips for micing solo ac. guitar in stereo?

thanks
 
Well, yes, you can make phase corrections by stretching/zooming those waveforms out and carefully matching peaks/troughs. If the mics are not reasonably matched, there may still be some interactions that aren't what you were hoping for, but it should eliminate phase issues.

I suggest XY as the easiest way to get a stereo image that you don't have to worry about phase issues. Or MS with a decent figure-8 and cardioid pair, though it's going to accentuate the room more than XY, for better or worse.

You can also use a spaced pair, but there's this rule (of thumb) about 3x the distance so the [same] signals captured by the L mic as the R mic are significantly reduced in amplitude, as to minimize the phase interactions. You can fiddle with angles to also minimize the capture of phase-problematic audio.

Keep flipping the tracks to mono output as you tinker with the phase.
 
The 3:1 rule of thumb doesn't apply to a single source.

Some combination of time shifting and polarity inversion should make it work. Zoom way in and look closely at the waveform to get things close. There are a couple of ways to make the final adjustment by ear. You could just pan them center and listen. You can also pan them center and deliberately make the polarity "wrong" then adjust to maximum cancelation then invert back. Third, pan hard left and right, level match the channels then put headphones on and listen for precedence effect.
 
bouldersoundguy said:
The 3:1 rule of thumb doesn't apply to a single source. ...
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I guess I was thinking that if you're close enough that the guitar is being recorded with a spaced pair, it's not really a single source! Move out enough, then all your "stereo" is the room.

Honestly, I've only done a single guitar as a [pair of] stereo track a few times. I do sometimes mic up with more than one mic if I'm having trouble getting a balance between neck/fingers and body with a single mic (or I'm lazy), but they are going to get balanced and bussed/grouped to a mono track. Any stereo effect is from bleed + verb; but, I digress....
 
I can see wanting to treat the 12th feet and the body as separate sources, but I think they'll have too much in common for that to make sense.
 
bouldersoundguy said:
The 3:1 rule of thumb doesn't apply to a single source.
Click to expand...
Assuming a matched pair of microphones and exactly the same gain settings, having one 3 times as far away as the other would in fact reduce the impact of the comb filtering to where it doesn't matter, but only if you don't turn up the distant mic in the mix. Not sure what good that would do in most cases...
 
bouldersoundguy said:
I can see wanting to treat the 12th feet and the body as separate sources, but I think they'll have too much in common for that to make sense.
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Well, they actually have a pretty distinct sound, timbre, set of overtones, types of technical noises associated with them, etc., so that's why I sometimes like XY because I don't have to get that one mic just perfect. I wouldn't make use them to create a stereo image of a guitar. (Me, I wasn't advocating a spaced pair, just saying if you're gonna do it... I don't know what the OP was trying to accomplish!)

Everybody is so f'n quick to start a little pecking around here sometimes, or maybe I'm just in an annoyable mood... Time to go back and re-read my resolutions list :)
 
I think a key element in stereo recording an acoustic, no matter which mic configuration...is the room. I mean, sure, you can "stereo close mic" an acoustic guitar and get some sort of L/R information, but the stereo acoustic guitar tracks most of is think of as great, tend to be a lot about the quality of the room ambience information that gets recorded with the guitar direct sound.

I prefer doing stereo acoustic on the cheap, 'cuz I don't have the 30' ceiling room with super acoustics...so I'll use one mic, which then makes it just simpler to find that one sweet spot...and then I'll just toss a quality stereo maker plugin on it, and get a much better result than I can without a really great ambient room to do the pair of mics things. :)
 
ashcat_lt said:
Assuming a matched pair of microphones and exactly the same gain settings, having one 3 times as far away as the other would in fact reduce the impact of the comb filtering to where it doesn't matter, but only if you don't turn up the distant mic in the mix. Not sure what good that would do in most cases...
Click to expand...

Right. You could get the same effect by just not using a second mic.
 
This phase problem is the reason I never us a spaced pair. However I do sometimes use Blumlein or Mid/Side if I am recording a solo guitar part. I have also used a close / distant mic setup watching out for phase issues when setting up. Phase issues should be checked before record is pressed not after the event.

Alan.
 
I've never been convinced by the 'phase problem'. Multi-microphones can blend well, or they won't. If you use a stereo spaced pair, then by design - this technique requires distance and the stereo field is created by time and level differences. if you stick two microphones on a guitar, this is not a stereo technique, it's a blending technique where you are recording two areas of the same instrument and blending them together. If they both hear the same sound source, then you will get comb filtering, so you aim them properly and select the mics so as to minimise the effect - perhaps by arranging them so they are each in the null of the other. I'm not sure the 3:1 'rule' is more of a common sense guideline - because it roughly shows how the inverse square law, which is the physics at play for this issue - impacts on the recording.

On acoustic instruments it's absolutely vital to sort the mic placement out BEFORE you record anything. In fact, if you are serious about recording in stereo, you need a pair of speaker, not just headphones - or you really cannot hear what is happening. Listen on headphones and this cancellation doesn't happen - on speakers it does!

I'm really not sold on the idea of fixing things in the mix - fix it before you record anything. it really is that critical.

The delaying of one channel against the other can indeed reinforce or cancel, but it's frequency specific. If you sort one cancellation, something else pops out, or if you want the popping out, you'll get cancellation somewhere else.
 
I have an artist that I've produced 20 albums for over the years. We always record guitar and vocal together, as that is the way he wants to work. He is a folk artist, and some of the recordings are just guitar and vocal, but most have additional instruments including bass, sometimes drums and percussion. mandolim extra guitars, background vocals, etc. We always record the guitar in stereo with a spaced pair of SM-81s. One pointing dowm somewhere over the 12th fret, pointing down to minimize vocal pickup. the second SM-81 is pointing at the bottom of the bodyseveral inches below the sound hole and towards the edge of the body. Both mikes are 8-10 inches away from the guitar. His vocal is recorded simultaneously with a CAD E200, which tends to hyper-cardioid. There is some bleed into the vocal mike.

This setup has worked extremely well which is why we've used it for 30+ years. We've received lots of compliments on the sound and other engineers have copied the guitar miking technique. I've really never made any attempts to correct phase and I always check my mixes in mono to be sure there are no cancellations. The rooms we've recorded in are treated, but not what you would call acoustically "great" spaces, but the mikes are close enough that unless the room is terribly reverberant, it shouldn't be a problem.

For most of my career, phase matching wasn't a problem because you couldn't see it or fix it other than moving a mike or reversing its polarity. But, try it! See if it improves the sound or does anything to the mono mix. Learning engineering is a lot about trial-and-error and experimentation. There are a lot of paths to a good recording, and everyone develops methods that work for them and their artist.
 
Or just record the same thing twice, once with each mic. Not really stereo , but just as effective pan one left the other right.
 
bouldersoundguy said:
Right. You could get the same effect by just not using a second mic.
Click to expand...

I'd hardly call myself an exceptional engineer, certainly compared to some of the regulars around here...

...but over the years I've gradually gone from just by default starting with some sort of stereo spaced pair (X-Y is a great starting point) to, for anything I'm recording in a mix, just going with a single mic, and either leaving it mono and panned to one side in the mix, or double-tracking if it's a strum-y sort of part that lends itself to this if I really want stereo.

For a solo performance where you want it in stereo, absolutely... But, as long as you check mono compatibility while tracking and don't move around much.... Idunno, I feel like phase issues are often made out to be a bigger deal than they really are, in tracking. I'm careful when setting up mics, so maybe that's a large part of it, but I also have never collapsed a performance to mono and thought, "dear god, what happened here?!?"
 
I've had "Great" results lately with single guitar in stereo. I'm miking at at the 12'th fret feeding a mono track panned 80% to left, and feeding a 80% to the right mono track with my guitar pickup. The differences in processing time and strength of individual notes are sweeping and syncopating L/R. I'ts also leaving a good spot in the middle for vocals.. This has worked well with my Takamine and and Godin nylon guitar. Both gtrs. have good electronics.. I'm done with duel mic on acoustic guitars. Mark ..
 
Two mics on an acoustic guitar are valid enough, if, they are really doing different jobs. If you have a nice round tone that iOS what you want from a sound hole focussed mics, but the playing style has lots of left hand finger work that needs capturing too - then another carefully aimed mic does the trick. On other guitars, this doesn't work. Multiple mics is a perfectly valid techniques - few record grand piano with just one - and they way they interact is not automatically bad. It can be of course, but you have to try.
 
mark skinner said:
I've had "Great" results lately with single guitar in stereo. I'm miking at at the 12'th fret feeding a mono track panned 80% to left, and feeding a 80% to the right mono track with my guitar pickup. The differences in processing time and strength of individual notes are sweeping and syncopating L/R. I'ts also leaving a good spot in the middle for vocals.. This has worked well with my Takamine and and Godin nylon guitar. Both gtrs. have good electronics.. I'm done with duel mic on acoustic guitars. Mark ..
Click to expand...

That sounds like it could be an interesting effect. As in are you getting movement or shift in the stereo field in playback?
There is the first arrival (precedence effect) that would draw the image to the earlier side. But I'd expect that shift to be a stable one.
 
Hello... The stereo field sounds stays very wide with this setup . It seems as if the higher pitched notes sweep more from left to right . The pickup is probably boosting them more . I did minor eq changes to make sure the tracks were a little different. The Godin nylon guitar has a saddle pickup and a built in mic. The built in mic put the tones of the two tracks pretty close together , but everything stayed wide and moved around as well as the steel strung gtr. Even the notes not sweeping are staying hard L/R instead of moving to the middle. I'm doing tracks now using both guitars , with the tracks panned narrower on 1 guitar and using the same setup on a mandolin. Still leaving a big hole for vocals at center. MS..
 
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