I want to apply sound insulation to an internal soil pipe, which runs
vertically from ceiling to floor. I've built a timber frame around it,
which I'm going to cover with two layers of 12.5mm plasterboard and have
skimmed. I know the ideal way would be to fill the boxing with sand to
deaden the sound, but that would cause too much grief (can't quite imagine
filling up all my boxing - which has to be quite large - with sand; plus
there's a rodding eye at the base for which access is needed). So I
propose to fill the void with Rockwool.

Question is - should I insert Rockwool in it's 'natural' state, ie, unpack
the stuff and let it expand to its maximum, and not allow any compression
when I put it round the pipe (which I know would be best if I was dealing
with thermal insulation here), or should I cram as much Rockwool into the
void as I possible can? Would that be better/worse/similar to option 1?
The stuff is pretty dear, so I don't want to overstuff the boxing if it's
not going to be beneficial.

Thanks
David