What is threshold?
Threshold means the boundary level at which a compressor starts compressing. On a typical downward compressor, the part of the input signal that goes above the setting becomes the target for compression. Most plugins display it in dB, and the lower you set it, the more readily it responds to quieter sounds.
The number alone does not decide the result
Even set to the same -20 dB, a quiet input will barely trigger it, while a high input gain will keep it compressing for long stretches. What matters is not the number itself but how far and for how long the sound crosses the boundary. In practice, threshold and input gain together decide how deep the signal drives the compressor.
Combine it with ratio to set the amount
Threshold is where processing starts, and ratio is how much of the part above it is held back. Lowering the boundary does nothing if the ratio is 1:1. Conversely, even a high ratio does nothing if the sound never crosses the boundary. On units with a knee, you can also change how smoothly it moves into compression around the boundary.
How to set it in practice
Start with the ratio at an easy-to-read value, lower the threshold while playing back, and find the point where Gain Reduction moves only where you need it. Adjust ratio, attack, and release after that. With ducking, the processing moves every time the detected source crosses the boundary. For a concrete example, see How to create ducking for EDM (Japanese).
Other effects have one too
Threshold is not a compressor-only term. On a limiter it is the point where strong control begins, on a gate it is the point where the sound opens and closes, and in noise reduction it is the basis for deciding what gets processed. Since some units act when the signal goes above it and others when it falls below, it is important to confirm what kind of boundary it is.
