Sound travels through air vibration, so you need to make the ceiling voids air tight first. This can be done by ripping down the existing ceiling and battening the joists along their length. You then cut plaster board to fit each space, screw and then mastic the edges & joints twice to make sure its sealed. Then over board the joists with foam backed plaster board and skim with new plaster. (It also ups the fire rating between the flats too.) I did a top floor flat for a DJ in Notting hill, he had regular late parties after he'd played a club. This really cut the sound down to a minimum. Costly though. W**F Also John Lewis rubber heavy underlay is the best sound proofer from the upstairs floor side, thin plyboard & fix down the floor first , lay the underlay then hard tuck the new carpet edges.