Lead Soundproofing for Noise Reduction

http://www.nuclead.com/soundproofing.html

excerpt:
Lead, on the other hand, is soft and malleable. This makes it impossible to radiate sound and very difficult to be set into vibration, and has the flexibility needed to make a good sound barrier. Since lead is a metal it has a uniform density compared with wood and other materials and finally it is easy to attach lead sheets to other materials to produce structurally sound walls.