This is a trickier question than might first appear. Many claims are made for wool as a bulk absorber of audio energy (read any discussion of transmission line speakers if you don't believe me) , but in this case, porosity counts more, though absorption certainly isn't to be sniffed at. We're not using the felt the way speaker people do. We're using it as an acoustic resistance, so whether the fibers rub against one another and turn some of the audio energy into heat energy doesn't matter so much to us, though we'll take it if we can get it. So for those of you about to go shopping, try the wool felt, but if you can't get it, don't feel bad.
Edited by wualta - 7/18/10 at 3:01pm