Vas: equivalent compliance volume

Vas is a way of expressing suspension stiffness in units a box builder can use directly:

V_{as} = \rho_0 c^2 S_d^2 C_{ms}

It is the volume of air that would be exactly as springy as the driver's own suspension. A soft suspension on a large cone has a big Vas; a stiff suspension on a small cone has a small one.

Why it is quoted this way

Because the comparison you actually need is between the suspension and the air trapped behind the cone. Sealing a driver in a box adds the air's springiness to the suspension's, and the ratio of the two is what sets the result:

\alpha = \frac{V_{as}}{V_b}, \qquad f_c = f_s\sqrt{1+\alpha}, \qquad Q_{tc} = Q_{ts}\sqrt{1+\alpha}

A box equal to Vas doubles the total stiffness, raising resonance and Q by \sqrt{2}. A box a tenth of Vas is a much stiffer spring than the suspension and dominates completely.

This is why Vas, not cone size, determines how big the enclosure has to be. A 10-inch driver with a stiff suspension may need a smaller box than a 6-inch one with a floppy one.

Typical values and a warning

A 6-inch midbass is often 20 to 60 L; a 12-inch subwoofer driver 80 to 250 L.

Vas is the least reliable published T/S parameter. It is measured indirectly, by added mass or by a known test volume, and it depends on S_d squared, so a small error in the assumed cone area becomes a large error in Vas. Disagreements of 20% between a datasheet and a careful measurement are common and are usually the datasheet's fault. If your simulated response does not match a build, Vas is the first number to suspect.