I'm also thinking that it's not a bad idea to take away the physical sound transmission path between drywall planes through the use of a resilient caulk. I have also always thought that the backer rod prevents the acoustic caulk from migrating deeper into that joint over time and losing its connection with the drywall. Stevewhy would you not place the layers flush and then caulk/seal along the joins?
How to drywall and caulk? Step Sequence....
Originally posted at johnlsayers.com, topic 2765.