TubeNet

I had a tenant that was a graduate student in composition that did a lot of recording in his room. He used this product on all the walls and the difference was very notable. Probably one of the more cost effective means of dramatically reducing noise transfer and reverb in a room.

That said, they basically looked and felt like thick heavy blankets. It made for a remarkably ugly room and I always wondered if a similar level of 'sound proofing' could have been achieved with a pretty quilt from Kmart.