| Oh yeah... those vibrations are travelling everywhere. You have to 'baby-sit' all the strings because of that. Try bangin' just a couple notes on a piano with the sustain pedal down - you'll hear a hundred other strings sympathetically vibrating. Especially in the case of playing the D and G strings, an un-tended E or A will very adversely muddy-up your sound (and the sound of the whole band you're playing with) - you don't want to leave those un-tended!
I play a 5-string. When I'm playing the D and G I use the 'thumb-trailing' method (for normal finger style playing), where my thumb rides down along the strings, damping the ones that are above the one I'm playing. Of course you usually don't have to damp the one right above the one you're playing because your fingers stop against that one, thereby intermittantly damping it.
You really need to pretty conciously damp unused strings, but it's not that bad. Strings that are physically below the one you're playing can be pretty well taken care of with some intentional 'slop' in the left hand - touching the unused high-strings. For instance in the same way the string above is sort of automatically damped between plucks with the plucking hand, the string below is automatically damped with the finger that's fretting the string above it. the G-string is easily damped by just letting the meat of one or more fretting fingers touch it where they come around the bottom of the neck.
I'm tellin' you: you can play supposedly fast or otherwise 'impressive' stuff, but if you're not exhibiting clean damping, it'll have that cerrrrrtain 'amateur'-sound!
Joe | 
