Rocktron HUSH Super C - Manual (Page 11)

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As the input signal drops further below the threshold point, downward
expansion increases. A drop in the input level by 20dB would cause the
output level to drop approximately 40dB (i.e., 20dB of gain reduction). In
the absence of any input signal, the expander will reduce the gain so that
the noise floor becomes inaudible.
This means that if a signal is present at the input which consists of
primarily bass components, the dynamic filter will reduce mid or high band
noise. If no mid or high band information is present, the filter will close
down to a pre-set cut-off point of 1kHz (allowing only frequencies of 1kHz
and below to pass). However, if the input signal has high frequency
components present, the dynamic filter will open fully to pass the signal
and eliminate the possibility of a loss of high end frequency response.
Downward Expansion
The second half of the HUSH ® process incorporates downward
expansion. The low level expander of the HUSH ® system operates like an
electronic volume control. The HUSH ® system utilizes a voltage-controlled
amplifier (VCA) circuit which can control the gain between the input and
the output from unity to 30, 40 or even 50dB of gain reduction. When the
input signal is above the user preset threshold point, the VCA circuit
remains at unity gain. (This means that the output signal level is equal to
the input signal level.) As the input signal level drops below the user
preset threshold point, downward expansion begins. It is at this point that
the expander acts like an electronic volume control and gradually begins
to decrease the output signal level relative to the input signal level.