Dynamics control
Keeping the sound level more or less even is the goal here. Basically the compressor will reduce the peaks and boost the parts below normal volume.
On vocals: most singers, in general, cannot have total control over the volume of their voice, and will naturally sing certain words or phrases louder than others. It is the compressor’s job to give some balance to these fluctuations, without overdoing it.
Another instrument subject to variations is the drums or percussion in general. A drummer who is not too experienced might vary the strength of his hits without noticing it. A consistent drum kit will be the product of good compression.
With a 1:4 compression you will have more than enough. For drums, be cautious, otherwise you will kill the dynamics. With bass, you can use compression more freely, as it doesn’t require too much dynamics.
Another effect to achieve in compression is to give presence to the instruments. After using a compressor, you should be left with the impression that everything sounds more “present” and closer to you.
Become an expert in compression with the recommended book “ Compression in Music Production” (click on the image to go to the link):
To give the mix a punchy, present sound, it is necessary to play a bit with the attack (too fast an attack will kill the transients and therefore the punch effect), and also adjust the release.
Let the sound breathe
A fairly effective trick is to start with the slowest attack, and the fastest release. Increase the speed of the attack until you notice that the transients start to lose strength. Now adjust the release, so that the volume regains its full force right after the initial hit (of a drum kit, for example). It is important to feel that the instruments “breathe”, that is, that they have a way to regain their volume at the right point in time, instead of always being crushed by the compressor.