Joe - Celetex is a brand name.  I think it's called particle board? Every time we called our favorite home supply store, it had a different name and no one knew what we were talking about.  We usually ended up with 'sound deadening board' as a universal descriptor.  The nice part is that is was dirt cheap, so not expensive, unlike alternatives - the students like it because it was light weight - not so much the Masonite!