RUBBERFELT Here's a 13mm thick, heavy Rubber / Felt composite that has pretty good sound insulation properties.
Three bonded layers of dense felt with a bonded layer of granulated rubber.
Easily cut with a sharp knife or heavy scissors.
Sold in multiples of 1mtr length from a 1.37mtr wide roll - 1.3 Square Metres.
Maximum length in one piece 7mtrs (10 square metres.) Weight 2.2 Kg per square metre.