Impedance??? Am I doing this correctly?

[moodyda]

Maybe I am not doing this right. I have a Radio Shack multimeter (the little yellow fold-up one). I set the meter to impedance (the upside down horseshoe) and I get some strange results. The speakers list their impedance as 6 ohm and I get a 3.1 reading on my display. I also tried some 8 ohm speakers and they were reporting 3.9 to 4.1. Am I doing something incorrectly? The meter has fresh batteries and works properly on all other sources. The reading is taken on the terminals of the speaker cabinet with and w/out the wires connected.

 

[j_garcia]

The impedance is NOMINAL across the range to give you an idea of the type of load it will present to an amp, it is not a measure of the impedance of the speaker with no load. Impedance varies with frequency, so the impedance of the speaker will change depending on what is being played through them at the time.

 

[moodyda]

So how do I measure it. Or do I just go with the rating stamped on the speaker?

 

[j_garcia]

I don't think you'd actually be able to measure with only a DVM, so yes you should use the manufacturer's rating to base your amplification needs on. Not too many manufacturers provide the measurements of impedance across the range that the use to determine the nominal rating, but some will at least give you max and min of what the speaker will exhibit.

 

[zhimbo]

As an example, here's an image from a recent speaker review:

You can see that, in this case, over the frequency range, the measured impedance varies from less than 5 ohms to over 10 ohms.

The review can be found here.

 

[TLS Guy]

Maybe I am not doing this right. I have a Radio Shack multimeter (the little yellow fold-up one). I set the meter to impedance (the upside down horseshoe) and I get some strange results. The speakers list their impedance as 6 ohm and I get a 3.1 reading on my display. I also tried some 8 ohm speakers and they were reporting 3.9 to 4.1. Am I doing something incorrectly? The meter has fresh batteries and works properly on all other sources. The reading is taken on the terminals of the speaker cabinet with and w/out the wires connected.

Impedance is not resistance. You can only measure the DC resistance of the voice coil with a multimeter.

A loudspeaker voice coil is an inductor, which resists current which results in a change of magnetic flux. As has been explained this is dynamic with frequency.

To measure impedance the easiest device to use is the woofer tester from Parts Express.

 

[j_garcia]

That WT3 is supposed to do a great job too. Just a little expensive unless you are designing speakers for a living.

 

[highfigh]

That WT3 is supposed to do a great job too. Just a little expensive unless you are designing speakers for a living.

It's good for measuring inductance, too.

 

[highfigh]

That WT3 is supposed to do a great job too. Just a little expensive unless you are designing speakers for a living.

It's good for measuring inductance, too. Not at the saturation point, but it does have the ability to measure it.