That's what they do, basically, yes. You set a certain
threshold. Everything that is louder than this threshold is made less loud. But it is not cutting off directly at the threshold. If you have a peak going over the threshold, it will be reduced to a
ratio: A ratio of 4:1 would mean that a peak exceeding the threshold by 4dB would be reduced to 1dB.
So a high threshold, let's say 10:1, would mean that almost everything is pressed down close to the threshold.
So your signal will be less loud in total. To make this up, you usually give a bit of
makeup-gain after the compressor to stay on level. But because of the compressor, everything is getting gain more evenly.
Then you can also set the
attack and
release time, which basically say how fast the compressor reacts. Everything that is now bold-type are the usual suspects you can find as settings on the compressor. That is all that the thing does. It is important to understand how the compressed signal is working with the rest of your setup, because there can be a lot of fun.
You can have a good sound without compresson. You can also turn a nuanced sound of a great bass to rubbish. It will never mess up the sound like a Metal Zone. But you can have more consistency in the level. And if you have gain-applying effects after the compression then you can keep the input gain under more control.
Many do. But in various ways. Most? Hard to do empirical studies. But it is a very, very common thing and you can find it on the pedal board, in the rack or the amp. But I'd bet money that it is at least half the cases you will find one somewhere. At least only to have it just in case.
No! And you will not need an expensive one tbh. And it is not necessarily an effect that is noticed straight away. But it is good to have. You get control on what the drive is getting. You can even out little volume differences from plucking. Sharp peaks should also go. And for a pumping sound you can even use more than one in different places. Just set the ratio relatively high and the make-up gain to a reasonably high level. It will sound a little artificial, but yeah.
If you overdo compression, you will lose dynamics. Some people mix the compressed signal with the dry signal, at least during recording.
Depends on the club.
Small heay metal basement club setting the PA to 11 filled with 50 yelling metalheads? No.
Jazz club where everyone can hear whether you play softly or not? Yes.
Rockmusic somewhere in between? Yes-ish. Might be nice to have a chorus with very consistent bass 16ths (compression) but articulated fills in the verse (not so much compression).
And of course the compression can be used to shape the sound. But the synth guys usually know really well about acoustic modeling and parameters