Just a though, it works for me...
I have one outside wall with a window in my studio and I was able to considerably dampen the sound to the outdoors by putting up matresses on the wall. I have 1 king and a super single edge to edge and it covers the entire width of the wall. I live in an old part of town where the houses are all close together and I don't bother the neighbors, they say they can barely hear it when they sit on the front porch. I also have insulated plaster walls, with another layer of drywall, everywhere else and a solid core door with weatherstripping. We get pretty loud in here and many times late at night, with no problems. The last room I used was much larger and had three exterior walls, the cops showed up all the time so I had to move to the smaller, and now quieter room. Good luck.