I’m pretty sure this has been discussed before, but I don’t remember the answer. Can you measure the impedance of a speaker with a standard multimeter? If not, then how can you do it?

200ohm setting on VOM.

That measures DC resistance, not impedance.

You can measure the voltage drop across a resistor at a fixed frequency, but it's not possible to easily measure actual impedance. You can measure the DC resistance and get a ballpark idea of the total, though- an 8 Ohm speaker will measure roughly 6 Ohms when a multi-meter is used.

Parts Express had a piece called WT-3 (now upgraded to the DATS v3) and it will gather the Thiel-Small parameters, including an impedance plot. If you have a wholesale account, it knocks a bit off of the price. I have the old WT3 and it has been very useful when working with drivers that were made before the parameters were widely used, or when the data is hard to find.

[Link: parts-express.com]

If an 8ohm speaker measures 6, what would a 4ohm speaker measure? And more for the issue, what would two 4ohm speakers probably measure?

Depends on the voice coil, but the 6 Ohm measurement is fairly typical- could be 6.2, 6.4, etc. 4 Ohm drivers often measure around 3.2 Ohms- if you're asking about two 4 Ohm speakers wired parallel, most amps won't like that because the minimum impedance will likely activate the protection circuit.

What's the application- in-ceiling speakers?

It may not be important to your needs but speakers will measure a variety of different impedances at different frequencies, hence the comment about the impedance plot. And often that same driver will measure differently in open air vs in a loaded cabinet. So the term "nominal" is a ball park figure to suggest where the appx impedance range of said driver is.

For parallel-series calcs, and its effect and load on an amplifier, sometimes the D.C. resistance is usually good enough, unless you're on the threshold of an amplifier's tolerance, then maybe it's not good enough

It’s in ceiling Def Ted speakers on a C4 amp. The amp is rated for 2ohm, and they say it’s safe down to 1.6ohm. But the guy drives them at almost full power with streaming music. So it’s probably dipping below 1ohm in the crescendos. I think it’s just a bad combination of everything and not a bad speaker.

As mentioned, DC resistance is about 80% of impedance.

If I ever thought there was a problem in a speaker I was testing for simple resistance, comparing the other identical one would usually confirm whether I had a problem or not.

TOA ZM-104 (plenty on eBay) is a pretty standard unit in the industry.

These are also pretty indispensable in 70v troubleshooting, where a DVM is useless.


The customer plays them so loud he is deaf Ted?

If they are Def Tech, you could wire two in series to keep the amp from shutting down.

What is your purpose in gathering this information? A DVM can get you an answer that is useful for some things, such as the DC resistance typically (not exactly) being 3 ohms for a 4 ohm speaker while it's 6 ohms for an 8 ohm speaker.

But do you need to know the actual impedance at some particular frequency? That is WAY different from distinguishing between a nominal 8 ohm speaker and a nominal 4 ohm speaker. (These are called "nominal," which means "it's not exactly that, but that's what we call it, because we either don't need better information or we don't know that we need better information.")


When I worked at Marantz around 1980, my boss had come up with a pretty good method for this. We ran plots of impedance versus frequency with the intent of knowing whether the crossover/speaker combinations presented well-controlled signals to the individual drivers. For instance, we had one midrange speaker for which the response curve in its useable spectrum was not a (more or less) level line, but instead rose across its spectrum. By manipulating the impedance of the crossover he created a crossover with a complementary non-flat impedance curve, which resulted in a flatter audio response. He wanted a skewed voltage curve to present to the driver in order to counteract the driver's audio response curve.

His method was this:
Set the output of a power amp at ten volts.
Put a 1000 ohm resistor in series with the speaker.
Measure the voltage across the speaker.

This voltage divider had two portions: a small voltage across the speaker and a large voltage across the resistor. The voltage across an 8 ohm load was almost exactly 8* mV. If the load were 10 Ohms, the voltage would be almost exactly 10* mV. This voltage was fed into the Bruel & Kjaer plotter and scaled to give us a chart showing impedance versus frequency.

This was quite accurate, especially since we were aware it was almost exactly right, at impedances under, say, 30 ohms. The higher the impedance, the less exact we needed it to be, which was nice, since it got less accurate at higher load impedances.

In addition to its being accurate, it did not lend itself to field use. You'd have to drag around an amp that could output ten volts and you'd have to spend the money on a meter that would accurately measure AC down to less than ten mV. Look at the various meters you've got -- accurate measurement of very low voltages costs money!

Was there other information you needed about this?


*or 80 mV. Somebody check my arithmetic, please.

Typically around 3 at rest. 2 - 4 ohm speakers in parallel will measure approx 1.5 while in series approx 6. In my car audio days I had a ton of test equipment and I'd take a 4ohm speaker and run it through an impedance test and would see values from 2 to 60.

Grab a multimeter and check the resistance of a whole bunch of different speakers. That will give you a feeling of whats going on and go from there.
The measurements will vary but, will give you an idea of what to expect in the real world.
If you were wiring a series/parallel setup, use the manufacturers spec and calculate from there. When finished wiring them up, read the resistance across the final load to see if you are in an accepatble range. If your close to the expected resistance, then you should be alright given your amplifier can handle that load.
Impedance is dynamic in loudspeakers. Can, and does, vary across the frequency spectrum.
You can understand alot by using a multimeter for everything you do electrically.
I too came from the car audio industry. Attended Rockford Fosgate's 3 day course they offered back in the late '80's in Tempe AZ. Still have the 3 ring binder somewhere in my shop.
It seems that car audio guys,many times, have a better understanding of system design from experience. (Breaking shit)

Inserting a large value resistor in series with a signal generator (1000 Ohms in Ernie's example) you are essentially converting the the generator to a constant current source because the magnitude of the speaker's impedance is much lower than the resister's value. This scheme is not absolutely accurate, but it will be within a few percent and this is good enough for practical purposes. You can use a 10K resistor, but the voltage readings will be much lower. It is trivial to convert the meter reading into the magnitude of the impedance.

If you want valid results for a wide frequency range, pay attention the frequency response of the AC voltmeter. Many are designed for reasonable accuracy at 50, 60, or 400Hz and accuracy at other frequencies is not specified. You can quickly check your setup by assuming that the power amplifier in Ernie's example is flat, then measure the output voltage of the power amplifier at various frequencies. If the voltage indication varies significantly, suspect the generator or the voltmeter. If the generator is reasonable quality, then the voltmeter is not appropriate for the task.

You can use this same scheme to check a network of in-wall volume controls. This is a very handy technique to quickly check the complete system. You can determine if the system configuration is reasonable by inserting the resistor and making a quick impedance sweep. Mercifully, there will not be grossly loud strange noises to frighten inhabitants of the venue.

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I've seen a few cheap VOM's or dedicated devices that claim to be able to measure speaker impedance. Unfortunately, the designer has no grasp of the purpose of his device because it measures the magnitude of the impedance at a single frequency, usually 1kHz. This is not a very interesting frequency for our purposes.

I used the Gold Line ZM-1P for years, and it came in handy many times.

Seriously overkill, but none of the cheap Chinese meters were around back in the day.

Here’s what I use:

Tenma 72-6948 Audio Impedance Meter Tenma [Link: amazon.com]