How is lead soundproofing installed in walls being renovated?
Well, what we ended up doing is cutting the sheets into pieces that fit into the bays between the joists. So, the cross section looks like this:



An acoustic caulk was used between the sound proofing layer and the joist flanges.

The result was very effective. There is no sound transmission between the two rooms through the floor. It also made the floor much more stable. I can jump up and down on the floor, no vibration at all.