The first thing is to ask what is the acoustic guitar part? Is it a finger-style piece? Is it a vigorously strummed fast rhythm track? What will the track sit in-rock song, folk song, classical piece, etc? Tools such as compression are used in different ways dependent on the source and style.
Here are some general things to keep in mind about compression.
~The sensitivity setting of the compressor
~The input level in dB when the compressor starts to reduce the signal
Hard Knee and Soft Knee:
~Hard knee compression describes a sudden decrease in gain when the threshold is reached
~Soft knee compression describes a gradual decrease in gain when the threshold is reached
~The difference between the signal level increase at the compressor’s input and the level at the output
~Ratio is a constant value
~A ratio of 10:1 means it takes an increase of 10 dB at the input to cause the output to only increase 1 dB
~A ratio of 1:1 is called unity gain
Attack and Release Times:
~Attack time-the amount of time it takes the compressor to start acting upon a signal or how fast the compressor reacts to a signal above the threshold
~Release time-amount of time it takes the compressor to return to unity gain ¥ (stop acting upon a signal) or how long the compressor continues to act upon a signal when it goes beneath the threshold
~Both are measured in milliseconds
~Attack and release times are the “meat and potatoes” settings of the compressor and are very important to the sound of the compressed signal
Also, another good question to ask is whether or not to compress while tracking or compress during mixdown. Or both. Or neither.
If it is a finger-style piece a la John Renbourn and recorded in stereo (the guitar is the main instrument in the piece) I record with a level of -6 dB or so. In this type of tune the dynamics are crucial to maintain for sure but there are fewer really spiky overs because of the playing approach (unless you're Bert Jansch!) are not significantly louder than the basic playing level. A ratio of 1.5-2:1 is good. The attack time will determine how much "nail" passes as the nail plucking the string is its transient.
A strummed rhythm track has a very different approach based on the responsibility of the guitar in the song. If it's just a rhythm track for, well, rhythm and percussive rhythm then I compress at the input and use a low ratio/low threshold, 2:1 with a -8 to16 dB threshold.
I will start playing and bring the threshold down until the gain reduction meter starts knocking down the peaks. I use a slower release time that sands down the track by acting on the signal pretty much all the time and it's OK for there to be some gain reduction pretty much all the time.
I usually don't rely on the soloed track for reference. I listen to the guitar in context and ask these questions: How does the compressed track sit in the mix? Does the track do it's job? If I get the answers "it sits snugly and it's in the correct context of the song" then I feel pretty good about my starting point for mixdown.
A point was brought up during this discussion that performers enjoy hearing the compression during monitoring. They seem to play/sing/perform into the effect and enjoy the sense of drive and the familiar sound of a compressed signal. I try to give the performer the tune in his/her cans that is reflective of songs mix concept. I will have a mix strategy before tracking so I have a sense of where the tune will go. I've never had someone ask for more compression in the cans but I wouldn't commit the compressor to tape in that situation.
I ask this question all the time-"How do I best serve the song?" That question helps determine everything about the recording process for the entire song including compression strategies.