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.!
luck and have fun!