Interlude. Why the Compressor Makes the Quiet Loud and the Loud Quiet

In the seventies an engineer at a mix never took his hand off the vocal fader. The singer sings very quietly in the verse and breaks into a shout in the chorus, and if you leave the fader alone, the quiet words vanish behind the guitars and the shout hits you in the ears. So the engineer rides the fader after the voice, lifting it on the quiet words and pushing it down on the loud ones, and the better he knows the song, the less noticeable his work. A compressor does the same thing, only on its own and faster than any hand.

It is built simply. The device listens to the signal all the time, and as soon as the signal gets louder than a set threshold, the device starts pushing it down. How hard is set by the ratio: at two to one, whatever goes two divisions over the threshold is let through as one. Two more knobs decide how quickly the device grabs a loud sound and how quickly it lets go. After that the whole signal is raised back up, and the result is that the loud places have become quieter, while the quiet ones, which the device did not touch, have become louder in the overall mass. The difference between quiet and loud is compressed.