Vab is defined as the volume of air having the same acoustic compliance as the enclosure.
Vb is defined as the Internal Volume of air in an enclosure.
Why is Vab different from Vb, and how is it calculated? Does it have something to do with the stuffing?
Nah, he is correct with Vab.Its Vas not Vab
soho54 is correct. Section 7 of Small's paper "Closed-Box Loudspeaker Systems Part I: Analysis" has a complete description of the theory of the effects of stuffing on Vab, as well as a (not so easy) method of measuring the effects of stuffing. Thanks.