 Compression is a complex subject. Everyone has an opinion on compression and everyone uses it differently. But what do some of the parameters on a standard audio compressor do? In this post, we’ll be looking at the ratio. The ratio is where you determine how much compression you are going to apply to a signal that goes over your threshold. For every signal that goes over the threshold, it gets compressed according to a certain ratio.

For example: A compressor with a threshold at -10dB and a 3:1 ratio is a nice starting point for vocals. If you have a semi-constant level of the vocal at -1dB it will become compressed so that it only reaches -7dB.

Why?

Because after going over the threshold the vocal reaches its peak 9dB after -10dB, or at -1dB. We take those 9dB and divide them by three, since the ratio is at 3:1. Out of that we get 3dB which we add to the threshold at -10dB. A compression of 6dBs reaching its peak at -7dB. Let’s illustrate this with a simple formula:

In this formula you can see the basics of calculating the output of a compressor. If we take the example above and apply it to this formula, we get this: So you see, that if we have a higher ratio, we compress the signal more resulting in less signal at the output. Say we have an example of a loud kick drum that’s peaking at +4dB but we have a threshold at -20dB and a ratio of 8:1. That’s a lot of compression but serves to illustrate a point.

We have a dynamic range of 24 dBs, from -20dB to +4dB. We are compressing everything that goes over -20dB by a ratio of 8:1.

Let’s plug those numbers into the equation: The highest peaks of the kick drum that are reaching +4dB before are now only reaching -17dB! That 24dB dynamic range we had from -20dB to +4dB has been reduced to 3dB. Talk about over-compression!

Even though these formulas don’t have much to do with how you compress – since it’s an artistic process that relies more on ears than mathematics – it does serve a purpose in explaining the underlying principle of the how a compressor works.

(source: http://www.audio-issues.com/music-mixing/what-does-the-ratio-on-your-compressor-really-do/)