Whereas the threshold, attack, and release settings on a compressor are all important in affecting the dynamics of your audio, the ratio is the key parameter on a compressor. Let’s identify what is compressor ratio and how it affects the dynamics and overall sound of your audio.
What is Compressor Ratio
To answer what is compressor ratio as succinctly as possible, the ratio is the setting on a compressor which determines the degree to which the peaks of audio are compressor/turned down.
The compressor ratio is measured as a ratio in terms of X:1, or the number of decibels which exceed the threshold which get reduced to 1.
To better explain this, let’s illustrate an example.
Let’s say we set our compressor’s threshold to -14dB. This means that anytime our audio gets LOUDER than -14dB, that excess will get turned down. How MUCH it gets turned down depends largely on the ratio.
Let’s say we set our ratio to 4:1, a fairly average compressor ratio. This means that for every 4dB our threshold is exceeded, it will get reduced to 1 dB (4:1, meaning 4 to 1).
This means that if that threshold is set at -14dB and our audio peaks at -10dB, that 4dB which is going over the threshold will be compressed at that 4:1 ratio.
At a 4:1 compressor ratio, the 4dB gets compressed into 1dB, meaning it gets turned down 3dB in this specific example, and the new peak will be -13dB.
That 3dB is referred to as the amount of gain reduction. We would then turn our output gain up by about 3dB to ensure that our audio is as loud as it was before the compression, making up for the 3dB that we lost in the compression process.
If you used auto makeup gain, it would replace roughly 3dB automatically, as well.
One more example with our -14dB threshold and 4:1 ratio; if we had a -6dB peak, we’d be exceeding that threshold by 8dB. At the 4:1 ratio, that 8dB gets compressed down to 2dB, or 6dB in gain reduction.
Note that you can always just divide the number of decibels OVER the threshold by your ratio to figure out how much you’ll be left with (8dB compressed at 4:1 is 8/4 or 2dB).
Admittedly we’re using nice round numbers for these examples to make the explanation more straightforward, but that’s the basis of what is the compressor ratio and how it works.
The higher the ratio, the more severely any peaks which go above our set threshold get turned down:
As you can see from the above image, the higher the ratio, the more exponential the gain reduction which gets applied.
For instance, a 2:1 ratio literally halves the output (50%) of any number of decibels which exceed the threshold. This makes sense considering it’s dividing that number by 2.
A 4:1 ratio leaves us with 25% (reducing the gain over the threshold by 75%), a 10:1 ratio leaves us with 10% (reducing the gain over the threshold by 90%), a 20:1 ratio leaves us with 5% (reducing the gain over the threshold by 95%), and so on.
As such, there’s a MUCH bigger difference between a 2:1 and 4:1 ratio than a 10:1 and 20:1 ratio.
This can be misleading because at first glance, 10:1 and 20:1 looks like a much bigger gap than 4:1 and 2:1.
If you break down the numbers, though, the 4:1 and 2:1 ratio (75%-50%=25%) feature substantially different degrees to how much they compress your audio as opposed to the difference between the 10:1 and 20:1 ratio (10%-5%=5%).
As you get to that 10:1 ratio and up, you’re quickly approaching that flat line as you could see in the imagine above, which is essentially the principle behind limiting.
As I covered in my comparison of limiters vs compressors, I pointed out that there’s not a huge difference between the two. A limiter is essentially a compressor which is using an infinite ratio, turning the ratio all the way up.
The effect of this is that literally ANY decibels which exceed the threshold are turned down and output to the same level as all the others.
You simply turn a fader (in the case of FabFilter’s Pro-L) to get the level you want and that’s it, making it useful in the mastering stage. Of course you need to pay attention to the attack and release to keep your transients intact and get a natural sounding off ramp, but I digress.
So while the rest of the parameters on a compressor obviously are important, nothing drives the effect or extent of the compression like the ratio.
Speaking of which, check out my overview on the audio compressor settings chart for detailed information on every parameter, and just as importantly grab my free ULTIMATE compression cheat sheet for simple guides displaying how to compress EVERY instrument in your mix to perfection.
