recording acoustic songs (mono or double track)

You can track the guitar once with a stereo micing setup. I like to use two different mics in an XY pattern, centered where the neck and body on the guitar meet, about 18-24" from the guitar. One mic points straight at the neck and one at the body. You might have to move the mics around depending on the room and the guitar.

Pan them wide and the differences in the sound/positioning of each mic will give you a bit of stereo spread, but it will also be mono-compatible.

You can also use spaced pairs or one closer/one distant mic if you're not worried about the mono compatibility. As long as the sound varies between the mics, you'll get a nice stereo feel with a single performance.
 
You can track the guitar once with a stereo micing setup. I like to use two different mics in an XY pattern, centered where the neck and body on the guitar meet, about 18-24" from the guitar. One mic points straight at the neck and one at the body. You might have to move the mics around depending on the room and the guitar.

Pan them wide and the differences in the sound/positioning of each mic will give you a bit of stereo spread, but it will also be mono-compatible.

You can also use spaced pairs or one closer/one distant mic if you're not worried about the mono compatibility. As long as the sound varies between the mics, you'll get a nice stereo feel with a single performance.

+1, good post, I definitely wouldn't want two separate takes all the way through for an acoustic tune! (maybe some overdubbing at key points, but even then, keep it subtle I'd say)
 
double tracked acoustic sounds pretty wide, but watch out for ping-ponging.
Also beware of proximity effect

I like to mic over the shoulders, spaced pair personally. Sounds the most natural for the performer. You can't do vocals at the same time like that though, it's really tough to manage the bleed with this position.

Check out the first mix on my site, http://epicsounds.ca/ Ania Z. someone else tracked it and I mixed. I have no idea what the mics were, but there was one on the neck, one on the body and one a few feet back. I panned the neck and room mic L and R and balanced them out, then brought up the body mic for a fuller sound. All the mics have a bit of reverb too.
If you check out the Jay Scar track on there you can hear the 'over the shoulder' method. It's actually a small diaphragm on the left (neck) and AT2020 on the right (pointed at bridge). The mics don't have to be matched.

There's a billion ways to do it and have it sound right.
 
I found this article at Sound on Sound pretty helpful when I first started recording acoustic stuff - it covers pretty much what others have posted plus other options and methods.

As for the specifics of mic positioning, a common approach is to set up the mic around 40cm from the guitar, with the capsule aimed at the point where the guitar's neck joins the body. This will usually produce a well-integrated sound -- the levels of direct and reflected sound will be about right, and the sound hole's contribution will be controlled because the mic doesn't point directly at it. If you have a pair of enclosed headphones, then you can easily experiment with tweaking this mic placement while listening for the best sound. If you find a promising sound in this way, remember to check it out on your monitors before committing yourself -- headphones can sometimes be rather misleading. As a general rule, moving the mic further towards the neck will brighten an excessively bassy sound, while moving closer to the sound hole will bring more warmth and fullness to the sound. Moving the mic further away from the guitar will increase the proportion of room ambience overall, while moving in further will dry the sound up. Alternatively, if you like a closer-miked sound, but would prefer more room ambience with it, try using an omni-pattern mic instead of a cardioid, if you have one.

Even though the basic mic placement described above is by far the most commonly used, it doesn't always produce the best results. For example, if you're after the sound that the guitarist hears, then a single mic or a pair of mics set up to look over the player's shoulder at about head height can often capture a convincing tonal balance, particularly when using a large bodied guitar that is excessively boomy miked from the front. It can also be educational to point the mic in even less obvious directions, such as at a nearby reflective surface, or even at the underside of the guitar. Such alternative placements are often quick to try if you're wearing headphones, and can sometimes turn up a brilliant sound that no amount of theory would have predicted.

Sound on Sound is one of my favorite magazines and I find myself going back issues all the time for hints and tips. Best subscription I've ever made - plus I like that I was able to buy the back issues on CD-ROM (yearly bundles) all the way back to 1998 if I recall.
 
I really like the sound of double tracked acoustic guitars... I've had great results recently using a RODE NT2A aiming where the guitar's neck joins the body...

I know it's not metal, but I love the production of Johnny Cash on American III (Rick Rubin)... maybe I'm wrong but it sounds like they tracked 3 guitars (LCR)... I gotta try this the next time! :)

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