I read:
and: i think there is a legal definition of how much of the original sound has to be preserved/discarded to (not) make a new piece out of an another one with(out) violating others' copyrights.
yes there is but actually legal definitions sometimes are a bit - uhm - weak. The up-to-n-bars stuff is a myth. Usually the RIAA's lawyers are better than yours (and you hardly can afford losing the case), there is usually a parapgraph about recognition (so 2 notes from the knightrider theme that virtually everybody could have come up with are 2 notes too many to sample, actually even to 'cover').
if in doubt just don't sample, and allow others to sample your stuff and feel happy about the next robbie williams album using your bassdrum.
regards,
x