You will need an
audio sine-wave generator, frequency counter, ac voltmeter, 1000 ohm resister, a 10 ohm 1
% precision resistor, and the driver to test.
The test setup for
measuring driver impedance is shown here. Connect everything as shown.
First, calibrate by
inserting the 10 ohm resistor in place of the driver.
Adjust the voltage
output of the audio frequency sine-wave generator until the ac voltmeter reads 0.01 volts.
Replace the 10 ohm
resistor with the driver to be tested.
Keep the output
voltage constant. 0.01 volts represents 10 ohms, 0.015 volts represents 15
Vary the frequency
control of the audio frequency generator to find maximum resistance ( Rmax ). At
that point, frequency is resonant frequency ( Fs ).
frequency to 1 Hz to find ( Re ).
resistance ( R x ) =
Adjust the sine-wave
generator until the resistance is equal to Rx. This will occur twice as shown in the
graph above. F1 is the lower frequency when the resistance equals Rx. F2 is
the higher frequency when the resistance equals Rx.
With the speaker facing
upward place small weights or U. S. nickels evenly around the dust cap in the
center. 6 U. S. nickels equals 30 grams. With the mass on the speaker
find the shifted resonant frequency.
To find the voice coil
inductance of the driver, set the frequency of the sine-wave generator at 1000 Hz and
measure the resistance.
Audio Speaker Design Calculators