Speaker Impedance Testing

We looked at this same diagram to find the FS of a speaker in another article.  A speakers impedance will be different at different frequencies, it is useful for us to know what these impedances are. To measure this use the hookup like in the picture but we have one extra step and that's calibrating the set up.

To calibrate the set up is simple, use a 4 (or 8 ohm) resistor in place of the speaker for the calibration and set the volume of the oscillator so that the voltmeter reads 4 (or 8, or whatever calibration resistor you use).. 

Now that it is calibrated, put your speaker you wanna test back instead of your resistor, it should read the impedance of the speaker. As you sweep the oscillator through the audio range, you will see the impedance change with frequency. 

You can write these measurements down on a sheet of graph paper and have a permanent record if you wish.

Or you can use a more sophisticated hardware software package like LMS (Loudspeaker Measurement System) and get a graph like these impedance graphs below. Note the peaks in the bass, this is the FS of the speakers (three different ones in this graph) under test...

Note below, the loudspeaker with the red trace had a low impedance of below 5 ohms at about 70Hz and at 25Hz the impedance was close to 50 ohms!  The green trace is a different loudspeaker with the low impedance being about  7 ohms at 350Hz. and at the resonant point of 60Hz has an impedance of 40 ohms.!  

Good luck and have fun!