Determining Thiele & Small of PL18WO-09

One highly useful feature of LMS is the ability to calculate drivers' Thiele & Small Parameters. This is invaluable to a designer. If the manufacturers' specs are accepted blindly, it poses a very high risk in development. In the event that the components fail to conform to the manufacturers' specs, the entire bass alignment will be seriously compromised.

To determine the Thiele & Small Parameters, two measurements must be taken, one in Free Air and the other, either with Delta Mass or Delta Compliance method. In the absence of a test box, I have opted to use the Delta Mass Method.

Free Air Measurement

This measurement essentially tells us the Electrical Parameters of the driver under test. LMS is set for Precision Impedance Sweep with 300 data points. This is a fairly high resolution sweep.

Figure 1 shows the result of the sweep from 10Hz to 40kHz. From this measurement, Revc, Zmax, Fo, Qms, Qes and Qts can be generated. From the graph itself, it's evident that the Free Air Resonance (Fo) is about 48Hz and DC Resistance of Voice Coil (Revc) is 6 ohms.

(Figure 1) Free Air Impedance Sweep of Vifa PL18WO09

Delta Mass

For Delta Mass measurement, a known mass is used to lower the Fo of the PL18. In this instance, 30g mass is added to the cone and another sweep is done. From the result shown in Figure 2, we can clearly see that the resonant peak has shifted downwards from 48Hz to about 27Hz. This difference is deemed sufficient for LMS to generate the Electro-Mechanical parameters with a respectable degree of accuracy.

(Figure 2) Delta Mass Impedance Sweep of Vifa PL18WO09

From these two curves, the following Thiele & Small Parameters were generated by LMS (Figure 3). Before we make any comparisons, I should add that these LMS figures are accurate to within +/- 10%.

Going down the list, we can see that my PL18 has a Free Air Resonance of 48.4Hz whereas the published specs is at 38Hz. Qes is higher at 0.50 compared to 0.40. Likewise, Qts is higher too at 0.42 compared to 0.34. Lastly, my Vas is smaller, coming in at 20.15 Liters whereas Vifa specs lists 25 Liters.

This does not imply that Vifa's specs are inaccurate. After all, my PL18 is not brand new. It has seen many years of use and it's likely that the compliance has changed slightly over time. In any event, the differences are not that dramatic to throw the alignment completely off.

Revc = 5.8 Ohms
Zmax = 34.2 Ohms
Fo = 48.4 Hz
Qms = 2.46
Qes = 0.50
Qts = 0.42

Vas = 20.15 Liters
Cms = 801.99 uM/N
Mms = 13.46 Grams
Sd = 0.0133 sq. Mtr
BL = 6.87 T.M
nO = 0.44 %
SPLo = 88.45 dB
(Figure 3) T/S of PL18WO09
by LMS

With these fresh data, we can now confidently proceed with some Box Modeling for the bass response.

(Figure 4) From Vifa PL18WO09 PDF

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