What is ratio?
Ratio means proportion, and on a compressor it expresses how much of the change in the input above the threshold is kept at the output. It is shown as 2:1, 4:1, 10:1 and so on; when the number on the right is 1, a larger number on the left means stronger compression. In Japanese it is also called the compression ratio.
How to read 4:1
At 4:1, when the input rises by 4 dB above the threshold, the output rises by only 1 dB. That works out to 3 dB of the excess being held back. It does not mean the whole signal is reduced to a quarter of its level; it is a ratio applied to how much the part above the boundary changes. Sound below the threshold is basically outside this calculation.
Roughly what different values do
At 1:1 there is no compression. Around 2:1 it is easy to even out level differences naturally, and from 4:1 up the change becomes obvious. Very high ratios behave much like a limiter. Note that Knee and the circuit modeling of a given unit change how it acts near the boundary, so the same number will not sound identical on every compressor.
Ratio alone does not set the depth
Even at a high ratio, if the sound only just crosses the threshold, the amount of compression stays small. Conversely, even at a lower ratio, Gain Reduction increases if the signal keeps going well past the boundary. Attack and release also change how the signal is actually held back, so think of the values as a combination rather than one at a time.
Things to watch when comparing by ear
After compression you usually raise the level with Makeup Gain, and something can sound better simply because it is louder. Match the perceived level before and after processing, and compare the attacks and the sense of stability. You can see a practical example of using ratio in a sidechain in How to create the ducking used in EDM (Japanese).
