The perfect ratio for success
Just about any music you hear has gone through a compressor several times. Each individual instrumental line, each vocal, the entire mix, and again at the mastering stage.
Obviously compression is important, but it's tricky to get the hang of the controls at first.
Compressors really should not come with preset settings, although some do. Each situation will be different and the engineer needs to use his or her ears.
The best place to start is with the 'ratio' control. This is the main control governing the 'strength' of the compression effect.
A ratio of 1:1 means no compression at all is happening. Not much point in that then. At the other end of the scale, a ratio of 20:1 means that no matter how much you keep on increasing the input level, the output doesn't get any louder, at least hardly so.
One of the most frequent uses of the compressor is for vocals, simply because compressed vocals sound nicer. Here, you want the compression effect to be noticeable, otherwise there would be no point.
A setting of around 3:1 or 4:1 is a good place to start, and probably you will be able to leave the setting there.
A lower setting of around 1.5:1 or 2:1 is suitable if you have a recording that is too quiet in some parts, too loud in others. A low ratio should make the compression effect hardly audible but will balance out the levels nicely.
A full-on ratio of 20:1 or more creates 'limiting'. This is used so that you don't go into the red and spoil your recording through distortion. However, the limiting effect is clearly audible, so you would just use it as a standby and aim to keep your levels below the limiting threshold.
A ratio of around 3:1 to 4:1 is a good starting point for most types of recording.