Advanced Acoustics 4" Class '0' Studio Foam Panel 2ft by 4ft Acoustic Treatment

It's incredibly dense and therefore, very heavy at around 9kg per panel. Despite the high price, I think it's a worthy investment because it will absorb lower frequencies much more efficiently than less dense acoustic foam. Much of the light-weight foam you see on sale does nothing for lower-end frequencies, whereas this stuff will suck them up. The enhanced fire retardancy is also a very good thing of course.