A.Fernando Pagés Ruiz, a contractor in Lincoln, Neb.,and the author of Building an Affordable House,responds: According to Appendix K of the IRC (which, of course, might not be locally adopted), dwelling-unit separation walls must have a minimum sound-transmission class rating of 45 (at which loud speech can be barely heard through the assembly). In your case, the double-wall construction you describe should meet this requirement, but I suspect that construction defects are still allowing considerable noise to seep through. So the first thing you need to do is find the sound leaks, which are reducing the wall's effectiveness.
Start by getting down on your hands and knees and listening. You may discover that most of the sound is filtering in at baseboard level. Drywall hangers often leave a gap between the bottom sheet of drywall and the floor, which allows air and noise infiltration. After removing the baseboards, fill any large gaps with drywall and tape the seams,...
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