Take a look at these.
http://www.readyacoustics.com/index.php?main_page=index&cPath=26_3_21
http://www.soundproofingamerica.com/Echoless.pdf
http://www.readyacoustics.com/index.php?main_page=index&cPath=26_3_21
http://www.soundproofingamerica.com/Echoless.pdf