When researching soundproofing solutions, it's common to hear terms like mass law, mass-loaded vinyl (MLV), and massive drywall. From these terms, it's easy to conclude that mass is the only thing that matters to soundproof a room.
This is only partly true.
Essentially, mass is not the only factor to consider with soundproof construction. It's what you do with the mass that counts: there are many examples where mass is increased but performance does not increase.
It's how you assemble the available mass that defines your success in your sound isolation and soundproofing efforts. Check out this article on the Triple Leaf Effect, which describes the issues involved in such an assembly.
This website also has a very good discussion on this topic. House Repair Talk Forum