FabFilter Pro-G Help
The section in the top-left corner of the FabFilter Pro-G user interface is where the conventional gate/expansion settings can be found: threshold, ratio and range. These determine the dynamic behavior and the amount of expansion.
The Threshold knob sets the threshold at which the gate/expander will open. With a lower threshold, the gate/expander will open earlier. Finding a good setting always depends on your audio, and the real-time display and metering will help you set the right level.
The Ratio knob sets the amount of expansion when the signal level drops below the threshold. At a ratio of 4:1, every dB under the threshold results in an target reduction of -4 dB. If you set the knob to 1:1, no expansion will take place at all. If you set the ratio to values larger than about 5:1, Pro-G will starting acting more and more as a gate.
With the Range knob (often also called floor), you can specify the maximum expansion range of Pro-G. For example, when you set it to 20 dB, Pro-G will reduce the signal level with a maximum of 20 dB. This way, you can choose to suppress unwanted background noise only a bit, or expand only a specific area of the dynamic range.