Measuring speaker's impedance using a DMM? - Is Erin serious, or am I missing something?

[solderdude]

That is indeed the intention of such meters. Just fun to see needles move.
The idea from Erin was showing a method that makes the 'just dancing needles' a bit more informative/useful.
In the end... they are just moving needles giving an indication of the output voltage indicated in a VU kind of way.
Depending on the used circuit such a meter could well show peak values but just as well might just give VU-style response.

 

[Cbdb2]

VU meters measure a voltage and do so with a certain delay so peaks will not register which is what one would like to see.

DC resistance of a speaker says little to nothing about the nominal impedance which is mostly measured at 1kHz.
So.. to 'calibrate' such meters just apply a known voltage (measure with a MM that has autoranging or at least 2V AC range at say 100Hz (not all Multimeters work correctly at/or above 1kHz but most do, they all will work fine at 100Hz).
Then calculate the power that belongs to that voltage using the manufacturers specs for 1kHz and 'calibrate' to that calculated power level.

Will the VU meter (with Watt scale) actually measure the drawn power from the amp ? No it won't. These meters are just 'fun' to watch and will give an indication of the outputted voltage (minus short peaks).

What if 100hz is the resonant freq. and the impedance there is 40 ohms? The DC resistance is the lowest impedance possible and majority of speakers have there lowest impedance close to that an ohm or so higher. When a speaker is only 3% efficient (fairly average) where does the other 97% of the power go? Yes no one should expect these meters to be accurate.

 

[solderdude]

What if 100hz is the resonant freq. and the impedance there is 40 ohms? The DC resistance is the lowest impedance possible and majority of speakers have there lowest impedance close to that an ohm or so higher. When a speaker is only 3% efficient (fairly average) where does the other 97% of the power go? Yes no one should expect these meters to be accurate.

The VU meter is a volt meter. A multimeter will always be accurate at 100Hz (as long as it isn't a cheapy with the lowest AC setting being 200VAC) so there we have the voltage measured correct. Amplifiers are voltage sources and not power sources. It does not matter what a speaker does at 100Hz. As it is not easy to determine the impedance at all frequencies with the available gear. So the best (when not independently measured) is the impedance at 1kHz. So that's what we have.

 

[Cbdb2]

The VU meter is a volt meter. A multimeter will always be accurate at 100Hz (as long as it isn't a cheapy with the lowest AC setting being 200VAC) so there we have the voltage measured correct. Amplifiers are voltage sources and not power sources. It does not matter what a speaker does at 100Hz. As it is not easy to determine the impedance at all frequencies with the available gear. So the best (when not independently measured) is the impedance at 1kHz. So that's what we have.

Sorry I though you were saying measure the voltage and current (power) at 100hz my bad. I see what your getting at, measure the amp output voltage and apply the nominal impedance to your power calculation.. As long as the nominal impedance is to far off the woofer (where most of the power goes) impedance you'll be close enough.

 

[Blumlein 88]

VU meters measure a voltage and do so with a certain delay so peaks will not register which is what one would like to see.

DC resistance of a speaker says little to nothing about the nominal impedance which is mostly measured at 1kHz.
So.. to 'calibrate' such meters just apply a known voltage (measure with a MM that has autoranging or at least 2V AC range at say 100Hz (not all Multimeters work correctly at/or above 1kHz but most do, they all will work fine at 100Hz).
Then calculate the power that belongs to that voltage using the manufacturers specs for 1kHz and 'calibrate' to that calculated power level.

Will the VU meter (with Watt scale) actually measure the drawn power from the amp ? No it won't. These meters are just 'fun' to watch and will give an indication of the outputted voltage (minus short peaks).

According to the IEC standards nominal impedance is 1.25 times the minimum impedance. Has nothing to do with frequency. With most box speakers this minimum will occur between 80 and 300 hz.

 

[solderdude]

According to the IEC standards nominal impedance is 1.25 times the minimum impedance. Has nothing to do with frequency.

Most speakers reach the nominal impedance around 1kHz and mostly is measured at that frequency as well.
Also that does not mean the DC resistance measured is the minimum impedance, that minimum impedance can be well below that.
Then we have current phase vs voltage phase differences as well that differ over the entire range but usually not around 1kHz.
The minimum impedance also does not have to be the DC resistance (woofer + low pass coil resistance).
Not all speakers comply to IEC standards

Measuring actual power output is a complex matter and can't be determined accurately by measuring the output voltage only as it is load impedance dependent which is frequency dependent and program material dependent.
One has to make an assumption of a 'nominal' impedance if the 'power scale' on those 'just for fun needle meters' and using the 'nominal impedance' that is supplied by the manufacturer may be slightly more accurate than measuring the DC resistance.

Of course, when a manufacturer only specifies the minimum impedance then one could well put in 1.25x the value for the nominal impedance in the power calculation in order to calibrate the 'meter' using a continuous tone.
Or... use the, for an amateur speaker builder affordable, Dayton Audio DATS V3 and eyeball the nominal impedance from the plot or use the 1.25x minimum impedance found on the plot for that speaker.
I do wonder how accurate that 1.25x minimum spec is though.

 

[Blumlein 88]

Most speakers reach the nominal impedance around 1kHz and mostly is measured at that frequency as well.
Also that does not mean the DC resistance measured is the minimum impedance, that minimum impedance can be well below that.
Then we have current phase vs voltage phase differences as well that differ over the entire range but usually not around 1kHz.
The minimum impedance also does not have to be the DC resistance (woofer + low pass coil resistance).
Not all speakers comply to IEC standards

Measuring actual power output is a complex matter and can't be determined accurately by measuring the output voltage only as it is load impedance dependent which is frequency dependent and program material dependent.
One has to make an assumption of a 'nominal' impedance if the 'power scale' on those 'just for fun needle meters' and using the 'nominal impedance' that is supplied by the manufacturer may be slightly more accurate than measuring the DC resistance.

Of course, when a manufacturer only specifies the minimum impedance then one could well put in 1.25x the value for the nominal impedance in the power calculation in order to calibrate the 'meter' using a continuous tone.

Bottom line is still Erin's suggestion is less accurate versus about any other method. Most methods that don't involve measuring the speaker are likely to be inaccurate by significant degrees. Which is one reason I've never cared for such meters and other than nostalgia don't see the attraction. They've always been relatively useless.

In essence all of them are volt meters with different scaling from volts with different (and usually useless) assumptions. Moderately more useful would be for the read out in volts, and a peak hold function in volts, and db.

I suppose a real attempt would combine a hall effect clamp meter on the speaker wire, with a voltmeter and even then you'll get VA instead of watts.

 

[solderdude]

Yep, exactly my point.

That said... using the method of calibration but instead of using the DC resistance measurement using the 'nominal' impedance (or 1.25x the minimum impedance ?) to calculate what power the 'meter' should be indicating at the (with a MM measured AC voltage at 100Hz) voltage is probably going to yield slightly more accurate results.
One should not get the idea that the indicated power by those needles is accurate in any way though.

They are eye candy for people that like to see swinging needles but could be useful when comparing amps and trying to match levels.
The multimeter will be more accurate for that purpose anyway though and is cheaper but ... does not have swinging needles.

For single driver (full range) speakers the DC resistance is indicative for the the nominal impedance and DC resistance is nearly always the same as the impedance at 1kHz.

 

[TLEDDY]

Will the VU meter (with Watt scale) actually measure the drawn power from the amp ? No it won't. These meters are just 'fun' to watch and will give an indication of the outputted voltage (minus short peaks).

^^^^^^^^^^^^^

This!!

Tillman

 

[JimA84]

Sorry, but I assumed that Erin has some knowledge of basic electricity. Have a look at this:

He's advising people to use a DMM to measure the speaker's impedance? DMMs Ohm measurements are a simple DC measurement, not actual impedance.

Even with an LCR meter, you'll probably get the speaker's impedance wrong because a speaker + passive crossover is a complex circuit which have inductance, capacitance, and stray resistance.

It also has an induced back EMF, which together with the drawn current forms the main portion of the Ohmic (real) part of the impedance. The back EMF depends on the output power of the speaker and of course, the frequency.

When you measure the DC resistance of a commercial loudspeaker, all you get is the static equivalent resistance of the crossover + voice coils of the woofer and midrange drivers (not the tweeter because of the DC blocking capacitor).

I've just measured the resistance of my Forac Aria 906 speakers using a DMM and got a value of 3.6Ohms which isn't even close to the nominal 8Ohms.

I always assumed that to measure nominal impedance you need to drive the speaker near rated power using an amplifier and a tone generator, measure voltage and current per frequency, and then calculate the impedance and stage an average. I'd expect the Z(f) graph to change if you change the power because the higher the motor's excursion, the higher the back EMF would be.

I'd love to learn how exactly impedance graphs are generated.

Actually you can buy a cheap handheld impedance meter to measure impedance.

The problem with HIFi is that this typically just measures at one frequency.

If course it's dubious how important this is unless you want avoid blowing a fuse.

In terms of HiFi you should be more concerned with what comes out of the speaker.

So long as you're not blowing fuses or designing analog crossovers, speaker impedance is far down on the list.

If it works, it works.

Obsessing on the theoretical dynamics is time wasted from listening to music.