Just noticed this on the parts express site:
http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=264-832An interesting option for compact subs, desk systems etc. Here are some response curves, dark blue is vented in 11 liters, light blue one is 24 liters using the Dayton 10" passive radiator which they sell for less than twenty bucks - !
http://www.audiocircle.com/circles/galleryimage.php/album31/Tangband_W6_1139SC_Compare_Response.sized.gif">