One guy that used the 90lb role (very heavy and hard to find) just put it under the drywall on the home theater side, as well as above the ceiling drywall.
One guy that used the 30lb roles like I have from Home Depot put it under the drywall on both sides of the wall and ceiling.
Another guy took a lot of time and weaved the first layer around each stud to the halfway point of each cavity all across the wall to form a barrier at the half way point, then a second layer above that under the drywall. I tried this and it was not easy to work with.
For me I am going to try just putting a layer behind the drywall on each side of the wall. Since the material is 30" wide I will have to cut sections and overlap as I go across. So far I have been stapling and havent' decided if I'm going to use sealant/caulk yet. I think the drywall should hold it pretty flat. Who knows it might be a waste of time.
