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What Does the Ratio On Your Compressor Really Do?


After slapping a compressor on a track and fiddling with the knobs, you might be asking yourself, “What does the ratio of the compressor really do?!?”

After all, 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 of the compressor and how it works.

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 of -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.

Audio Compression Formula

If we take the example above and apply it to this formula, we get this:

Audio Compressor Example

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:

Kick Drum Compression

The highest peaks of the kick drum that reached +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.

This Video Helps You Understand Compression

All the compression parameters don’t have to scare you and compression doesn’t have to be hard. You just need to understand what you’re doing and how to use it.

If you’re looking for an in-depth guide on mixing with EQ, compression, reverb, delay, and saturation, check out Step By Step Mixing: How To Create Great Mixes Using Only 5 Plug-ins right here.

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