Although I understand the words & their meaning, I have no idea how to start implementing such a filter.
I've found a 1st order algorithm on musicdsp: http://www.musicdsp.org/showone.php?id=257
Since its second order I understand it needs the previous 2 outputs.
Critically dampened implies that the "factor" of influence of the previous samples is very close to 1 ( 'a' in the algorithm linked )

How is the "rise time" determined here?
Noobie questions :) -Harry