It's possible to get most of the ThieleSmall parameters from a loudspeaker by just accurately measuring the impedance versus frequency. If this is done twice, one with the driver in open air and one with an added (known) mass it should be fairly easy to calculate the fs, Vas and Qt.
What you need is a sine wave generator with reasonably low distortion (<1%), as flat frequency response as possible, and good, stable frequency. You'll need an AC voltmeter that is also flat over the indented range and has the needed sensitivity. A frequency counter is also useful, since the frequency calibration of most oscillators is pretty awful. You'll need an accurate means of measuring DC resistance as well. Add to that a precision 810 ohm resistor for calibration purposes, and a 1 kOhm resistor to turn your frequency generator into a virtual current source.
Here's how to proceed:
o 1kOhm +<+    Sine gen. 8 ohms AC voltmeter    o+<+This circuit is called an impedometer, where the voltage across the load is proportional to the impedance of the load. The 1 kohm resistor turns the oscillator into a pretty good approximation of constantcurrent source.
Fs sqrt(Rc) Qms =  f2  f1 [in the example, it will be: 32 sqrt(6.46) 32 * 2.54 81.3 Qms =  =  =  = 3.58 45.3  22.6 22.7 22.7
Qms Qes =  (Rc  1) [in this example, it will be: 3.58 3.58 Qes =  =  = 0.66 6.46  1 5.46
Qes * Qms Qts =  Qes + Qms [here, it would be: 0.66 * 3.58 2.36 Qts =  =  = 0.56 0.66 + 3.58 4.24
Fc Qec Vas = Vb [   1 ] Fs Qes [In our example: 80 * 0.95 76 Vas = 20 [   1 ] = 20 [   1 ] = 20 * 2.62 = 52L 32 * 0.66 21 Our Vas is 52 liters].
The information in this article is mainly based on news articles from Dick Pierce posted to rec.audio.pro newsgroup at 1998.
Last modified: May 28, 2002
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