Thiele and Small Parameter For Developing Loudspeakers

A driver should be placed in a matching cabinet of adequate size. To determine the correct size the Thiele and Small parameters are being used (especially the Equivalent Volume Of Compliance Vas, the Total Q Factor Qts, the Driver Free Air Resonance Fs).

These parameters can be looked up in data sheets supplied by us for all drivers or they can be measured. Usually two impedance spectrums (impedance dependant on frequency) are measured, for example:

I) Measuring the
a) free air impedance spectrum and the
b) impedance spectrum in a closed cabinet or

II) Measuring the
a) free air impedance spectrum and the
b) impedance spectrum with an additional weight
    (e.g. Plasticine) attached to the diaphragm.

Measurement Ia) and Ib) with Clio are shown on the right. Using Clio is particularly easy since all switches are hidden in the Switchbox and only one cable is necessary (e.g. for impedance measuring).

After both impedance spectrums have been recorded, the results of simple devices to be analysed are shown in graphical format. Using Clio, the impedance spectrums - according to spectrum I) or II), as you prefer - are then analysed and all relevant data including voice coil capacitance etc. are shown.

Measuring Thiele and Small-Parameter with Clio

First, the impedance of the loudspeaker needs to be measured with Clio, like described above. To do this, an input channel has to be chosen (we are using channel A here) and we have to decide what has to be measured (here: Ohm). With the boxes 1-5 several impedance spectrum that have been measured already, may be faded in or out. The red curve is the driver free air resonance, while the impedance for a 11 litre cabinet is white:
 
 
After the impedance spectrum have been recorded and stored, the Thiele and Small parameters may easily be calculated. To do this Clio will ask you for some missing data (like diameter of the diaphragm) and then gives you the result:
 
 
Important are : Qts, Vas and Fs (sometimes also called f0). The remaining parameters may also be helpful, when you are using software that requires additional data.

For comparison we have also used the method with additional weight. The findings are very similar and showed exact the same results in later loudspeaker calculations.