hi
though i could realize my idea (thanks to cyrille and frank), this was not the solution for my original problem: modelling the snare-sound of the snaredrum of tr-808. i could extract the (original) pure snare-sound by subtracting a soundsample without snare from a sample with both membrans and snare (here: http://www.netpd.org/~roman/sd_snare.wav). when looking at the waveform, it doesn't really look like noise, rather like a kind of filtered dirac, triggered randomly in time. i was not able yet to create this sound in pd (i haven't come even close to it :-( ), though i'm convinced it should be possible, when it was possible to do it the analogue way. if someone has made any efforts in this stuff, i would be very interested ( i'm surely not the first person on the list, who tries to simulate fancy hardware like tr808 and tr909).
for everybody interested: attached is my first try to emulate the 808-bassdrum.
cheers roman