The previous post is exactly correct but here is a little more detail. it is densly compressed fiberglass. It is far more effective than sonex,aurelex or any of those types of foam treatemenst because it is more efficient and works over a wider range of the spectrum. It cost about 5-6 bucks for a 2foot by 4 foot sheet, but you will want to cover it with some kind of pourous fabric because its ugly and it sheds.
It is tough to find because it is only sold to commercial contractors (insulation contractors) so you need to get a contractor to find you some or find a distributor that sells to contractors.