What makes an acoustic guitar a wonderful instrument is that it produces so many sounds from a single source. Capturing all of those sounds and nuances effectively in a recording is the challenge.
According to Fett of the Performing Songwriter, multiplicity is one of the best tools. It means capturing the guitar’s sound through more than one method and then combining the results in the mix.
The most obvious way to capture an acoustic guitar’s sound in different ways is to use more than one microphone at the same time and record them onto separate tracks. An entirely different result can be achieved by using another technique. This is done by using one or more mics on one set of tracks, and the guitar’s internal pickup on another. Most acoustic guitar pickups give you a distinct emphasis in the midrange that you’ll never hear a microphone. Sometimes that little bit of “squawk” in the mids is just what you need to add to a particular track to make the acoustic sit where you want it in the mix.
Another way to achieve a fuller sound through multiplicity is to combine dry version tracks (either pickup or mic) with copies of those same tracks with one or more effects added. A dry track panned left and a wet track with a 60-millisecond delay panned right, for example, will give you a feeling of room dimension in an acoustic guitar. An example of a device that will give you an excellent combination of internal pickup and processed sound in a single track is The Fishman Aura.
Using multiple acoustic guitars on different tracks is another method of beefing up an acoustic guitar sound. Even if both guitars are playing the exact same part, the natural, subtle differences in playing will give you a fullness that no processor can quite achieve. You may try playing the same part twice, but on two totally different sounding guitars. You’ll find that you get a wider spread across the tonal spectrum because each guitar naturally emphasizes different frequencies. For even more variety, play one guitar in an open-string position and the second guitar with a capo way up the neck, in a different inversion. The technique will yield full, open, 12-note chords that no single guitar (even a 12-string) could ever produce.
Any of these techniques can be combined. For example, an open-string dreadnought recorded with a mic with no effects and panned hard left, against a capo’d grand concert recorded with an internal pickup with a 60-millisecond delay panned hard right, will definitely give you a big acoustic guitar sound that will leave your listeners in awe of the final output.