Okay, time for more post. Here’s part II of how to soundproof a wall.
By this point, I’ve already sealed the gaps in the other side of the wall with goop, and padded up the boxes and other protrusions. The blue-jeans cotton insulation has gone in, and it’s onto finishing the mass-loaded vinyl.
Once you’ve got the first course up, you’ve got to put up the second course above it. This is harder than it looks, since it weighs about one pound per square foot, comes in four-foot wide rolls, and you have to hold it up eight feet off the ground. I suggest you get help from your resident fourteen-year-old.
You want to make sure it covers everything and is continuous, since it acts like a barrier to block the higher frequency sound as well as medium frequency. When it comes to openings like doorways, though, you can either have it stop or wrap around the wood. You’re making the wall assembly heavier (more massive) and less able to vibrate and transmit sound, so any bit should help.