Acoustic guitars are one of the most dynamic and expressive instruments used in modern music. They have a broad frequency range covering almost the entire audible spectrum.
The instrument can serve many different roles in an arrangement as a harmonic, melodic, or percussive element. All of these factors will affect how you approach both the recording and mixing acoustic guitars.
The first thing to do is think about its role in the song/mix:
Is the guitar the main harmonic instrument?
Is it supposed to enhance or thicken other harmonic instruments like electric guitars?
Is it enhancing the melody, playing arpeggios or single notes?
Is it a percussive element more like a shaker just to fill out the rhythm section?
Figure out the goal, then you can begin processing.
Depending on the guitar, microphone, pick, player and role in the arrangement, acoustic guitars always require at least a little equalization to fit in a mix.
Cutting of nonmusical sub frequencies (everything below 100 Hz) is a must. A low shelf may be required to tame the remaining low-end. In the midrange, take out any unflattering frequencies causing “boxiness” or anything that might mask the vocals.
A broad, gentle high shelf boost can enhance articulation and an overall more sparkly sound. The general strategy is to remove what you don’t need, then enhance what you do.
An acoustic guitar performance is often extremely dynamic, which is often a problem. A compressor will even out the differences between loud and soft notes and help keep the level more constant throughout the track. It’s easy to take it too far, so keep the gain reduction to no more than 3-4 dB.
The compressor’s attack control will allow you to clamp down on the beginning of the notes, or when set slower (about 20-50ms) will enhance the transient which helps it stand out more in the mix.
At this point you may want to adjust the sound envelope further though use of a transient shaper. This tool can increase or decrease pick attack, and separately adjust sustain.
Sending your acoustic track(s) into a stereo reverb will add space and distance to the sound. The trick here is to not wash out the sound in a ton of reverb, but just to move the move the mics back a bit into an imaginary room.
Usually, a short room type verb with the highs rolled off is the best bet. Try to make the reverb sound like an office or large bedroom – short decay with and a bit of early reflections. The predelay control will separate your source from the room.
And so, following the above strategy you should now have an acoustic guitar track with a balanced frequency response, even dynamics and sense of space around the instrument, (if that was what the track required).
The track is ready to be put in the mix with the other instruments. Tweak the EQ in context with the other tracks, adjust the reverb send amount, and even EQ the reverb if needed.