All About Compressors Part 1
Compression is one of the most fundamental and often misunderstood things about audio today. First thing is, you want to be conservative about it. Compressors do more than manage volume, they add things like harmonics and distortion. Sometimes that's good, sometimes not. Many of the classic compressors are the 1176, LA2A, API2500, DBX160, SSL, Focusrite Red. Each has its own characteristics and can transform your sound in many ways. In the world of software there are many classics and future classics. Too many to name. There are emulations of vintage analog compressors like those named above along with versatile new tools like The Arouser, Presswerk, Kotelnikov, MJUC, and Pro-C2.
Generally, the faster you set the attack and release, the more compression you get. More compression means that dynamic peaks, ie the loudest sounds, get compressed/turned down. There are other controls like threshold, ratio, and input gain that affect how compression is applied. Most all compressors have their own controls and they don't all behave similarly. It's up to you to try them and see what you like. My words here can only tell you so much.
It's good to maybe start with one compressor on a track. You'll usually want to insert it before eqs so that it's not constantly changing with each eq boost or cut. You could also do some eq after compression. It's not a requirement that every track has a compressor. For a vocal, you could try an 1176 set at 4:1 with an attack of 1 and a release of 7 as a starting point and then adjust the input and output. Or maybe you want something comprehensive, so you load up an instance of Arouser and experiment with its filter, attack mod, and saturation. Then on your master fader, you want to tie it all together. First, listen and make sure your mix isn't already dynamically squashed. If the dynamics are lacking or you don't want to adjust them, many compressors have options to turn off compression and just generate non-linearities like harmonics. If you do want to compress, try the lightest settings first and adjust.
If you're new to this, try the compressors that come with your DAW and some free ones like Kotelnikov on your master fader and the free version of MJUC from Klanghelm for some character. There's no rule that says you have to use any plugins on a buss or master fader. One thing you don't want to do is model your learning after modern releases with brickwalled waveforms(see the stevehoffman.tv forums for more info). Another thing to consider is the sample rate your session is set at. If you're working at 44.1khz, you're going to get aliasing that interferes with your mix at times. Aliasing being digital noises created by supersonic frequencies above 44khz which have crashed back into your mix at frequencies below 44khz. It's much safer working at 88.2khz or 96khz(both seem to yield identical results). Generally the more compression you're applying, the more aliasing. At 88.2khz or above, you should be able to push the majority of aliasing out of the range where you can hear it.