I've actually been looking for some sort of study or numbers on the resonant properties of concrete (Google let me down, for the most part). Anyone here got any leads?
From my amateur testing, I put a butt-kicker on one end of a concrete block (nylon webbing holding it on) put my hand onthe other side, and ran 20-200 HZ sweeps to see if I could feel/hear anything. Could hear some rattling (nature of the butt-kicker), feel nothing. Acknowledging the uneven response of those butt-kicker devices, this simple experiment seemed to indicate to me that concrete blocks would make good speakers stands (surely better than metal or wood), and should also make a decent isolation stand for a sub woofer. Yeah, my hand and ears aren't a very accurate test device, but for the purpose of determining material for speaker stand, they sure seemed adequate and practical.