Can I inject my walls or ceiling cavity to soundproof from my neighbour?
Adding mass can help soundproof a room.
Mass can mean a layer of drywall or a layer of Quietrock etc. You’ll use a series of techniques to add QuietGlue damping compound and drywall or quietrock to the walls and ceilings. You can soundproof an existing room. All you have to do is apply the Glue to your existing surface (as it is), screw the new drywall over top, fill in the seams with acoustical caulk, spackle, and repaint.
Hope this helps