Build of Loudspeaker Enclosures

Yellowstone National Park, WY USA (Photo by Yuichi)
Characteristics of a loudspeaker driver are explained by Thiele Small parameters (often called TS Parameters). TS stands for the name of A. N. Thiele and Richard Small who were both Australian engineers and they worked in 1960th and 70th. They draw out parameters which describe characteristics of the driver. Fundamental TS parameters are shown below. By using those, we can simulate behavior of the driver when it is mounted in the loudspeaker enclosure. Though TS parameters are so important, those are not shown to DIY users in some cases. This section is how to measure the TS parameters in our lab. with very simple manner.
Left are the definition of fundamental TS parameters. To simulate low frequency response of the enclosure, we need Fs, Qts and Vas for instance.
For details of TS parameters, please refer to the TS Parameter section of "Wikipedia" which is listed in the External link page.


TS Parameters explain the characteristics of the driver. However, we have to know that TS parameters has some variances between drivers, and those are not negligible small. And after one year or two years usage, the parameters are also shifted. Big contribution factor may be mechanical suspension mechanism. Therefore, we have to be sensitive about the shift.


To measure the TS parameters, web computing facility which is provided by "Weber Vintage Sound Technology" is useful. You can access this web referring "External link page". We only need very common tools for this measurement.

Tools to be provided are as listed in the left column. If we have no Audio sine-wave generator and or no Frequency counter, we can use PC based generator. Generator function can be downloaded with free of charges from some of the audio sites. In this experiment, I used PC software based sine wave generator. One of the benefit is that we can eliminate a Frequency counter because we know the frequency very accurately on the PC.


Test setup diagram is shown in the left picture. In case we use PC based Audio generator, the line output of the sound board is used with the power amp instead of the generator. And the Frequency counter can also be eliminated from the circuit diagram.

Above photo is the test setup excluding PC. We need accurate resisters and some cables.
First we have to find out Fs and Rmax by measuring impedance peak. Please see the impedance curve as below.
Following measurement shows that we can obtain maximum voltage (65mV) at 74Hz. This means that Fs is 74Hz and Rmax=65 Ohms.
Now we can fill out open windows in the left column.
Of cause the diameter and the Re can be measured directly by the scale and the Ohm meter.
Finding out Fs and Rmax

Followings are voltage measurement value by changing frequency from PC operations.


Freq ....Volt readings=Ohms

70      0.045
71      0.049
72      0.056
73      0.062
74(Fs)  0.065 (65 Ohms)
75      0.063
76      0.058
77      0.051
78      0.045
79      0.039
80      0.035

We now get
Fs=74Hz
Rmax=65 Ohms

Finding out F1 and F2

Then click [Calculate} to get Rx. Rx is calculated as 18.561Ohms. Then, F1 and F2 can be found out from Rx value in the impedance curve.

(For F1)
Freq ....Volt readings=Ohms

58     0.014
59      0.015

60      0.016
61      0.017

62(F1)  0.018   18 Ohms
63      0.020
64      0.021
65      0.026


F1=62Hz

As the same manner for F2
F2=89Hz
Here, we add some load on the diaphragm. There are two ways to add load. One is to put physical mass and the other is to apply air suspension. In this case, I put three US nickel coins shown as right hand photo. One coin equals to 5 grams. Then, measure Fs again. This new Fs is "shifted Fs" which is 51Hz.
By clicking the [Calculate] button, we can get major TS parameters in the windows as left column.

Note: (EBP = Fs multiplied by Qes)
So, EBP must be approx. 85 in this case which does not match to 59.85 in the left window. I had communication with the author and confirmed that 85 is correct.

In this measurements, we obtained Fs, Qts, Qms, Qes, Vas and Re. Now, we can simulate frequency response, impedance curve, phase curve and others on the PC. These simulations are extremely useful to design enclosures for the loudspeaker driver.

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