In Part One we covered how compressors work. Now we'll cover how and where you use them.
The most straight forward way to use any piece of gear is the simple in and out. You could use a compressor on your main mix by coming out of the board, into the comp, and then out to your amps. This method is used for more than just compression though. It's the way you'll see a loudspeaker management box plugged in this way. In the old days there was a longer chain of individual pieces of gear like EQs and such that made up your "drive rack", hence the name of the piece of gear by DBX that does all of that in one box.
But what about using a comp on just one channel? That's what the inserts are for. Inserts could easily be a post on their own, so let's just say that they're a way to make a path like an effects loop on an individual channel. They let you plug in a comp and have it work just on the signal from that channel. This is pretty useful if you've got an input with a lot of dynamic range. But there is a catch. Most consoles don't let you select if the processing affects just the signal to the main mix or all the aux mixes too. So now you've got compression on the signal to the monitors. That could be a help to a singer but it also raises the noise floor and can make it easier for feedback to occur, so use wisely. The other problem is that if you've got a high channel count it takes a lot of compressors to deal with all those inputs.
So the next way to use them is on the inserts on the sub groups. This lets you do things like route all your vocals to a single compressor. It takes a little finesse but it can be a great way to work fast and might be all you can do if your resources are limited. You have to set it up so that when just one person is singing they're barely making it work. Then as back up singers come in, it starts to compress, so instead of getting drastically louder it only gets somewhat louder but a lot fuller. With a four bus console and four channels of comps, you can take care of bar or small festival stage easily and hardly think about the comps.
To make things a little more hectic, a lot of compressors have a side chain on them which is another insert on the compressor itself! What? Here's what it's for. If you split an input and put it up on two channels, you insert the comp on one channel, then plug a signal from the other channel into the side chain. Now the compressor is processing the first signal based on what the second channel sounds like. But they're the same right? So you make the second channel different. If you crank the highs, the comp will act when a hot S comes through. You just built yourself a de-esser. Or, you could insert the comp on a channel with your background music on it, and feed the side chain signal from an announcer's mic. Work the settings a little and you can make it lower the music volume when someone is talking. This is called "ducking".
As usual Brethren of the Knob and Fader, we've just scratched the surface here. If you're perplexed, good. Go out and seek knowledge. Stock up on it because knowing how to make the most out of a little dynamic processing is what separates the sheep from the goats in this business.