Empress Effects ECM-519 compressor - Manual (Page 3)

The Basics of Compression
Compression works by narrowing the difference between high
and low audio levels by reducing the gain of any signal over the
threshold.
By doing this at specific time constants, compression
can add consistency, sustain, punch, detail, definition and
personality to your tracks.
The ratio determines how
much gain reduction will be
applied to the signal once it
crosses the threshold. At a 3:1
ratio, for every 3dB of input
signal above the threshold,
there will be 1dB of output
signal. A 12:1 ratio would
output 1dB for every 12dB of
input signal, etc.
Attack controls how quickly the gain reduction will begin after a
signal has crossed the threshold. Release controls how long it
takes for the compressor to return to its initial level after the
signal drops below the threshold.