I'd like to hear from others about the methods used when measuring a driver's Fs and for calculation of Vas, specifically the mounting and orientation of the driver being tested. In the past, I've hung the driver from the ceiling for free air measurement, and a test box for Vas. Although my results are often fairly near published data, I've never been confident that I'm getting useful data. Below is some text from Speaker Workshop's help file on the subject, which doesn't seem entirely logical either;

"Impedance in Free Air
To measure driver impedance in free air, the driver should be suspended in air far from any acoustically interfering surfaces. Hanging it from the ceiling will NOT work - the driver must be kept physically stable or the shaking from the sound will affect the impedance measurements. The recommended approach is to take two heavy tables and two clamps then put the tables near each other and clamp the driver between them. "

This seems to imply the DUT is situated with the axis vertical, which brings gravity's effect into the equation (only there's no allowance in the real equation). Also, isn't being clamped between 2 heavy tables a little contrary to the "free air" stipulation?

"Impedance in Sealed Box- The Delta Compliance method
When measuring the impedance in a sealed box (for estimating T/S parameters) the critical value is the volume of the box. This measurement is only used for estimation of Vas."

Straightforward except no mention of axis orientation. Also, for best results, the test box needs to be fairly near the DUT's Vas in volume, which could mean having a variety of test boxes on hand.

"Add extra mass- The Delta Mass method
Add additional mass to the cone in such a way that the additional mass is known exactly and the mass won’t flex when the cone moves."

Again, implied vertical axis but no factor for gravity in the equation.

How do you guys do it, and how close to published data do you get?