The threshold is the level above which a compressor starts reducing gain

The threshold sets the level at which compression begins. Set it higher than the loudest peaks in the audio and no compression takes place at all; pull it down and the compressor starts catching the loudest parts first, then progressively catches medium-level material as it drops further. On a transfer graph (input on the x-axis, output on the y-axis) the threshold is the corner where the diagonal starts to bend. Pulling the threshold down therefore controls both how often compression engages and, indirectly, how much of the signal is affected — but not the steepness of the reduction, which is the ratio’s job.

Examples

Start with the threshold above every peak: the gain-reduction meter never moves. Lower it until only the loudest vocal consonants trigger a dip, then lower it again until quieter syllables are caught too — the meter now dips almost continuously.

Assessment

What happens to the gain-reduction meter as you lower the threshold from above the peaks down through the signal? Where does the threshold sit on the input/output transfer graph?