ornithology
Member
I hear bags of polyfill in plastic bags are also effective in killing vibrations in the air. A friend of mine put some of that in the headliner, put some in the rear cargo area (DO NOT PUT IN DOORS, water goes there), and behind speakers and it effectively dampened the cabin noise and increased the boom factor of his subs. Another thing is when using polyfill, be sure to put it in plastic bags and do not put them where water/vapor is. It will cause rust! If used properly it is effective and cheap!