I found further directions to measure VAS (two sections in the manual) I'll play with this some more soon as I can. I'll reinstall DATS software and make sure everything is setup properly and remeasure everything.

Measure V(as) by the Selected Method
Before any measurements are made this command is disabled (grayed out). After making a free air measurement this command becomes enabled and can be used to measure the V(as) of a driver by any of four methods. The V(as) test method is selected at the Parameters Bar at the right side of the main window. You can also Measure V(as) by clicking the same named button at the Impedance Bar at the left side of the main window. The four methods are: Test Box, Added Mass Specified SPL and Specified M(md).

  • Test Box: This method is probably the most widely used and trusted. For this method you will need a test speaker enclosure of suitable size for the driver being tested
  • Added Mass: Although the added mass method can employ added mass in the simple form of coins added to the cone it is best to make sure the added mass actually adheres to the cone so that it moves with the cone at all times. If the mass "decouples" as the cone moves then accuracy will be compromised. Use an added mass that will shift the F(s) lower by about 25-50%. An added mass equal to the driver's Mms is just about right. DATS will warn you if the shift in the measured F(s) is not within the above recommended range.
Items commonly used for added mass include: coins, modeling clay, carpenters putty, rope caulk and tape rolls. In general you should measure the weight of the added mass accurate to 1/10th of a gram. In a pinch, US coins can provide an easy source of standard weights. One US nickel ($.05) weighs 5.0 grams.

  • Specified SPL: This is a simple V(as) method as it requires no test box or added masses. It is also probably the least accurate as it depends strongly on the accuracy of the SPL you specify.
  • Specified M(md): This is another simple V(as) method. It requires that you know the actual moving mass of the driver. This method is especially suitable for use by driver manufacturers where the driver's components are readily available for weighing. It is also a good method for comparing similar drivers.
Each of the four methods of measuring V(as) requires that you enter the Piston Diameter of the driver. The piston diameter is the effective diameter of the acoustic piston formed by the speaker. The piston diameter is measured as the diameter of the speaker cone and normally includes about half of the surround on each side of the cone. For example, a nominal 8 inch woofer would probably have an actual piston diameter in the range from about 6.25 to 6.5 inches.