From Fractal Audio Systems Wiki
The expander module is sort of a “compressor in reverse”. When the signal level is below the threshold the gain is reduced. The Axe-Fx UltraTM gate/expander block can be used as a gate or expander (as the name suggests).
- THRSH - Threshold control. The Gate / Expander reduces its gain when the input power is below this value.
- ATT - Attack time. Sets how fast the Gate / Expander restores the gain once the threshold is exceeded. Lower values equal faster response.
- HOLD - Hold time. Sets how long the Gate / Expander holds the gate open once the threshold has been exceeded.
- REL - Release time. Sets how fast the Gate / Expander reduces the gain once the signal has fallen below the threshold.
- RATIO - Sets the gain expansion ratio. This is the ratio of input power to output power below the threshold. If the input power is -40 dB and the threshold -30 dB and the ratio 2, the output power will be -60 dB (two dB reduction for every one dB below threshold).
- SCSEL - Selects the sidechain input source. INPUT is the normal setting and selects the block input (sum of the rows feeding the block). The other settings allow isolating a single row as the sidechain input. The other rows are summed as usual. By using a row as the sidechain input you can use the compressor as a ducker or de-esser.
- LOWCUT - Sets the frequency of a highpass filter on the sidechain input.
- HICUT - Sets the frequency of a lowpass filter on the sidechain input.