In zita-rev1, the damping filter for each delay line consists of a low-shelf filter ,4.19in series with a unique first-order lowpass filter that sets the high-frequency to be half that of the middle-band at a particular frequency (specified as ``HF Damping'' in the GUI). Since the filter is constrained to be a lowpass, for , i.e., the decay time gets shorter at higher frequencies.
Viewing the resulting damping filter as a three-band filter bank (§3.7.5), let and denote the desired band gains at dc and ``middle frequencies'', respectively.4.20 Then the low shelf may be set for a desired dc-gain of , and its input (or output) signal multiplied by to obtain the resulting filter
The lowpass filter is also first order, and to provide half the middle-band at the beginning of the ``high'' band, the lowpass should ``break'' to a gain of at the ``HF Damping'' frequency specified in the GUI. A unity-dc-gain one-pole lowpass has the form 
Spatialization of Reverberant Reflections
Multiband Delay-Filter Design