I can't argue with the manufacturer specs although the Vd is directly related to the surface area of the cone so a bigger cone usually means more air movement. Very cool information. I was blown away by the difference in the two. Thanks for the specs. We bass players need to move a lot of air! (Pun intended)
http://www.members.shaw.ca/loudspeakerbuilder.ca/thiele-small.html
Vd - No, it not that kind of VD. This Thiele-Small parameter is the Peak Diaphragm Displacement Volume which the total volume of air the cone will move. It is calculated by doubling Xmax then multiplying the result by Sd (Surface area of the cone). The higher the Vd figure is the better it is for use as a sub-bass driver.
Its the product of Xlim and surface area. Its not just "Sub bass", its a pretty good indicator of actual content below 150hz or so.