Measuring Speaker T-S Parameters

To properly determine the T-S parameters of your speaker, you will need:

An Audio sine-wave generator.
A Frequency Counter (unless the dial on your audio generator is very accurate).
An AC voltmeter that will read accurately down in the millivolt range.
A 1000 ohm, 5% resistor.
A 10 ohm 1% resistor.


The test setup for measuring the speaker's impedance is shown here.
Make sure you connect everything as shown.

Set up diagram

Calibrate the system by inserting the 10 ohm resistor in place of the speaker.
Adjust the voltage output of the generator until the AC Voltmeter reads 0.01 volts.
Replace the 10 ohm resistor with the speaker to be tested.
Make sure you keep the output voltage constant throughout the tests. .01 volts represents 10 ohms, 0.015 volts represents 15 ohms, etc.

Vary the frequency control of the generator to find maximum resistance ( Rmax ), which is indicated by the highest reading on the AC Voltmeter. This is the free air resonant frequency of the speaker ( Fs ).

To determine the diameter of the speaker, take the size of the speaker (e.g. 12") times .85 and convert to cm by taking the result times 2.54
Enter the diameter of the speaker cone :   cm
Enter the maximum resistance ( Rmax )
at the resonant frequency :
  ohms
Enter the resonant frequency ( Fs ) :   Hz

  Adjust the frequency  to 1 Hz  to find the DC resistance of the voice coil. ( Re ).

Enter the resistance at 1 Hz ( Re ) :   ohms

Impedance to Frequency Plot

  resistance ( R x ) = ohms.

Enter the lower frequency ( F 1 ) at R x : Hz
Enter the higher frequency ( F 2 ) at R x : Hz

With the speaker facing upward, place small weights or  U. S. nickels evenly around the dust cap in the center. 6 nickels equal about 30 grams. With the added mass on the speaker, find the shifted resonant frequency using the same method you used before to determine the free air resonant frequency.

Enter the Mass : grams
Enter the shifted resonant frequency : Hz
The T-S Parameters For This Speaker

Symbol Value Definition
Qms =    The mechanical Q.
Qes =    The Electrical Q.
Qts =    The total Q of the speaker.
Cms =    m / N The compliance of the speaker.
Vas =    liters The equivalent volume of air having the same compliance as the suspension system of the speaker.
EBP  =    The Efficiency Bandwidth Product.
If the EBP < 50, the speaker should be used in a closed back cabinet.
If the EBP > 90, the speaker should be used in an open back or ported cabinet.
If the EBP is between 51 and 89, the speaker may be used in either cabinet.
This is a rule of thumb only. Your results may vary.

To find the voice coil inductance of the speaker, set the frequency of the generator to 1000Hz and measure the resistance using the same method as before.

Enter the resistance at 1000 Hz  : 
Inductance of voice coil  =  H


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