Any loudspeaker experts out there? I know Vas is difficult to measure, and the "added weight" method is easier than building a test box, but what am I missing here?

Here's the page I'm referring to:

http://www.mh-audio.nl/Measuring_Loudspeakers.asp

and here's the section of it that has me baffled (excuse the pun):

Calculate Vas with Mass:

With the speaker facing upward, place small weights evenly around the dust cap in the center. With the added mass on the speaker, find the shifted resonant frequency using the same method you used before to determine the free air resonant frequency.

The formule is: Vas = 1.42 * 105 x Sd2 * Cms

Sd = Cone area

In our example, Mass = 30 grams, shifted resonant frequency = 13 Hz, VAS is 58.5 Liter.

The formula for Vas uses only the cone area and Cms. Where has Cms come from?