If you look into the lead type you will find it comes in two sided or single side and has a metal film. some of it is so heavy you myst support it with a small wood or aluminum frame to keep it in place. it may even require a 1/4" piece of ply
(back board)with a few stainless screws through it and the board is attached to the engine room wall and the insulation hung on it. This makes it real quiet but allot of work.I have found that at higher engine RPM the problem is viverations and not so much the engine noise.Having a few locker lisd buzzing in harmony will drive you nuts.