How to Measure the Impedance of a Speaker (INS032E)

The resistance measured with the multimeter is not the impedance of a loudspeaker, but the ohmic resistance of the mobile coil, a value much smaller than the impedance. To measure the impedance it is necessary to make use of a special procedure, for which we will need the following material:

a) Sine-wave audio generator (or function generator)

b) Audio Voltmeter or Multimeter

c) Small power amplifier

d) 50 ohm wire potentiometer







The connection of the elements is shown in the figure. This procedure can be used to determine the impedance of speakers from 2 to 100 ohm.

Initially, we apply the signal to the speaker in order to get close to saturation.

The signal must be 400 Hz with a sinusoidal waveform. Then, we turn on the potentiometer as shown in the figure, interrupting the connection to the speaker at point X. We record the measured voltage on the multimeter.

After the potentiometer is turned on, as shown in the figure, we set this potentiometer so that the signal strength measured in the potentiometer drops to half. Simply turn off the circuit and remove the potentiometer without changing its resistance.

Measuring with the multimeter, the resistance set in the potentiometer will have the same value as the impedance of the tested speaker.