Originally Posted by xander_crews
all it's for is to show you the actual area of the cone, not the entire volume or air that will be displaced
vd is xmax/cone-area, max
displacement...I think you thought he meant that instead of sd. EDIT: nvm now i see what you meant.
He may mean that some subs may have slightly different cone area, it may depend on how the brand rates theirs.
Quick little info that is usefull:
The whole wide surround vs half roll myth, some people believe that a wider surround reduces cone area since the cone is sometimes smaller by very little amount. Others believe that 1/2 the surround counts, and others, 1/3 counts.
I believe that part of the surround counts, because a 1/3 - 1/2 is usually moving much with the cone while the rest closer to the basket mount is stationary and shouldn't count.