Im no genius but, wouldnt the air velocity be changed according to the note? If a subwoofer played a low sound like 32 hz it would move more air than if it played 100 hz. Moving more air would mean a greater 'air velocity'. So isn't this the same as a box's fs? I don't know the formula off the top of my head but i'm sure its all over the internet.
Is this a question or just you trying to riddle the forum?