Originally Posted by qervo
I understood the principles but I wasn't to sure about ratios.
The ratio dictates the amount of compression a note experiences once it passes the threshold. For instance, a 2:1 ratio means that for every 2 dB a note is above the threshold, the volume only goes up 1 dB.
That's pretty gentle compression and with a slow attack it would be a very subtle change to your sound.
As the ratio gets higher and higher the compression really squeezes any notes that get above the threshold so that they don't get much louder than where the threshold is set which makes for VERY noticeable compression when the ratio gets to 10:1, 20:1 or even infinity:1 where other than the initial spike that the compressor doesn't catch no notes are higher than the threshold.
Since the compressor is basically decreasing your average volume (since the loud spikes are now quieter) compressors have a gain knob (or automatic internal control) to raise the average volume back up. This is what makes your softer notes louder when using a compressor. It's also why compressors enhance existing noise since you're essentially turning up the volume on everything.