Sound Boards?

Tedd Benson refers to the use of "sound boards" for interior wall construction in one of his books. ANybody know what this stuff is? Thanks.

I've done some research on you're question and have two answers. First, probably not correct, is that "sound boards" are boards that are not rotten. Secondly, "sound boards" are know as "Homesote". A fibreboard made up of recycled paper products, mostly cardboard and newspaper.