iZotope Pro Audio Essentials: Audio Compression Ratio Explained
The ratio tells us how much gain reduction is being applied when the compressor starts working. For example, a three to one ratio would mean that if the input goes over the threshold by three dB, the compressor would reduce the output to one dB.
If it goes over the threshold by six dB, the compressor would reduce it by a factor of three, so you would get two dB at the output.
Here, I have the threshold set to minus twelve dB, and watch what happens. With a three to one ratio, and the input rising to minus nine dB, in other words, it is now three dB over the threshold, the output is being reduced to minus eleven dB.
As you increase the ratio, you get more gain reduction, so in order to hear the effect of the ratio, make sure the signal is higher than the threshold, and observe what happens when the ratio is increased.
Also, to observe the true impact of the compressor, be sure to have the automatic makeup gain turned off.
Remember, if the signal isn’t reaching a level where the compressor is triggered, it doesn’t matter how strong or weak the ratio is, the compressor will do nothing. The moment the compressor kicks in, the ratio determines how much gain reduction is taking place.