We have measured all the T/S parameters, by the impedance curve, with the exception of the Vas; to get this value we need to measure the driver impedance using a lossless closed box, whose volume should be equal to or less of the driver Vas, or with the added mass method, in which some nonmagnetic weights will be placed on the driver cone to lower its Fs. Measuring the Vas is very difficult, since depends on temperature and air density, thus it's variable in time. However is not necessary to have an high precision in this value: Joseph D'Appolito states that a 25% variation of the Vas results in less then ±1dB  variation in the pass band response of a B4 or QB3 vented box alignment; in a closed box alignment, α>3 e Qtc<1.1, that variation of the Vas brings an even less evident pass band response variation. The closed box method, suffers errors due to leakage of the box, driver-box, and even of the driver surround and dust cap; this method measures directly the Vas. The added mass method suffers errors due to suspension nonlinearities, thus this measurement is done at the lowest possible voltage level; an erroneous calculation of the driver Sd affects the result too, since it is present in the Vas formula squared; it also important not to use too heavy weights, never higher then the Mms , not to enter in the suspension nonlinearities as well as not to change the Cms. Just attaching a mass to the cone causes a shift in the driver compliance, since the cone position is shifted: it's safe to apply a signal, equal to the driver resonance, to the driver with the added mass for a minute or so, to reset the suspension shift, and then perform the measurement. The added mass method calculates  the Vas through the Mms (cone mass) and the Cms (compliance). D'Appolito reports a 4% variation of the same driver Vas measured with the two methods, to confirm the high tolerances of this value.

Here I will show you the second method, the added mass one. As for weight we can use the Blue Tack or some coins (use some bi-adhesive to stick them to the cone)  but is very important to weight them with a precision balance like the one you find in jewelry. But of how much weight do we need? As reference use the Cone Mass, the Mms; if using a weight bigger then the Mms, don't put it in just one place of the cone, otherwise a misalignment of the cone will occur. If using blue tack, just model it to make a rubber ring, while if using coins put them symmetrically. The driver has to be measured horizontally, cause of the weights.

 ONE WEIGHT IN THE CONE CENTER

4 SYMMETRIC WEIGHTS

 

Once measured the impedance with the added mass, you will notice that the response frequency will be lower than the free air one: this difference has to be at least of 25%; if not you have to increase the weight. This is the limit of the added mass method: with big woofers with low Fs, looks difficult to lower it more then 25 % without entering in the cone nonlinear area. In these cases, the closed box method is a must.

With the weight in place and the cable connections as before, double click on  and the DRIVER PARAMETERS window opens; click in it with the right mouse button and select PROPERTIES and then DATA tab; check the added mass impedance box and write the weight value in the Added mass box; if you are using the closed box method, just mark it and write its volume:

 

Close the window clicking on . Then open menu MEASURE/ IMPEDANCE WITH ADDED MASS; again the driver will whisper and then a new file will be added in the folder: .  Open it and check that the  Fs is at least 25% lower then the free air Fs. The Fs value is easily seen if you look at the point where the phase (yellow line) pass for the zero, that is also the impedance (blue line) peak. Just pass the mouse cursor in that area and you will be able to read the Fs value at the bottom of SW window (be sure to have selected the STATUS BAR in the VIEW MENU). If the Fs difference is less then 25%, you have to increase the weight and repeat the added mass measure.

 

Now let SW calculate the Vas: menu DRIVER/ ESTIMATE PARAMETER..  If the measured curve is clean, as the one I measured, we can leave the first option; otherwise use an appropriate frequency range: we are repeating what we did in the previous section, but this time will not appear the Vas error window. Click on , and the Focal data window will show the Vas too, now:

 

We have almost finished, just repeat the procedure for the Focal B and then we can verify.