33
Finding the Thiele / Small (T/S) parameters of
a driver (Advanced Topic)
You don't need more equipment and a computer to do this.
With the IM-SG, a small digital postage scale, and some
play dough; it is possible to derive all of the T/S parameters
that are needed to design a perfect custom subwoofer
enclosure specific for your driver. This is important as
manufacturer's published T/S specs can be off by 20% or
more due to manufacturing tolerances.
If you are afraid of a calculator you may want to avoid this
section and use the software available on our website.
Make sure the driver is ready to be measured (See General
Notes for Measuring Raw Speaker Drivers page 8).
1. Using a digital voltmeter, zero the probes, and then
measure the DC resistance of the driver. Record this
value. We will call this R
E
. On our 12 inch woofer
example we measured R
E
= 3.4 ohms.
2. Calibrate the IM-SG to compensate for the harness
(Page 7)
3. Connect the IM-SG test leads to the speaker, if it has
multiple voice coils they all need to be connected.
Connecting the coils in parallel will work fine.
4. Turn on the IM-SG by holding the
On
button for 1
second.
5. Adjust the frequency on the IM-SG to the low end of the
f
S
range in the chart on the next page. Then slowly raise
the frequency until the peak in impedance is found. If
you go too far and pass the peak, just back down the
frequency until the peak in impedance is found. Figure 8
shows the impedance curve for a 12 inch woofer we
tested. We measured the f
S
to be 27.1Hz.
Summary of Contents for SMD IM-SG
Page 1: ...IM SG Impedance Meter Signal Generator Owner s Manual ...
Page 20: ...20 Figure 2 Impedance curve of a typical 4 ohm 1 tweeter Notice the fS at 1 59 kHz ...
Page 22: ...22 Figure 3 fC of a single 4 ohm 15 inch subwoofer in a 2 cubic feet sealed box ...
Page 44: ...44 Notes ...
Page 45: ...45 Notes ...
