Measuring impedance with multimeter

I am measuring a new full range speaker impedance that is advertised as 12 ohms and I am getting  a consistent reading of 4.2. 

I checked the multimeter on another bookshelf speaker advertised as 6 ohms nominal and I get exactly that.

I am using a multimeter at the speaker leads not connected to amp.

Why is this reading so low?
The multimeter measures the speaker’s impedance at zero Hz (i.e., DC). That reading will usually be significantly lower than the impedance at audible frequencies.

Not to worry!

-- Al
Thanks for the reply. A internet search indicates the measurement should be in the ballpark ( ie 15 to 20%off). 

Would different or more complex speaker designs give widely variable levels of accuracy?

As I stated, the little elac debut reads exactly  6 ohms .... as advertised;-)
There are drivers that range from 4 ohm to 25 ohm depending on the frequency. Multimwters are pretty much useless for measuring impedance. 
Even if the frequency- impedance graph indicates the lowest impedance as 12 to 14 ohms?
While the Elac Debut B6 has a specified nominal impedance of 6 ohms, it can be seen in Stereophile’s measurements that its impedance is above 10 ohms at all frequencies above 500 Hz, and is above 20 ohms at almost all frequencies above 1 kHz! 6 ohms in this case is its minimum value at any frequency, which it reaches at 10 Hz (the lowest frequency they measure, which of course is not too far from the 0 Hz frequency at which your multimeter measurement was taken), and also around 200 Hz.

IMO most manufacturers would have specified the nominal impedance of that speaker as being much higher than 6 ohms. Certainly at least 8 ohms, and perhaps 10 or 12 ohms or even more.

Many variables affect the impedance of a speaker, and how that impedance may differ from what it is at DC, including the drivers, the crossover networks, the enclosure, and how the designer chooses to specify the impedance. I would ignore the 15 to 20% rules of thumb you mentioned reading about.

-- Al

To be technically accurate, you're not measuring impedance with a multimeter. You're measuring resistance. Impedance applies to the resistance of an alternating signal. Also consider the thing you're measuring. A driver is an inductor placed inside a powerful magnetic field. Obviously it's got inductance that's going to resist higher frequencies. You're also moving wires back and forth inside a magnetic field. That generates electricity in itself that can drive impedance way up while rotating the phase angle around in weird ways depending on the mechanical properties of the driver and how it's enclosure loads it. 
Add into the mix the reactive nature of a passive crossover and there's a LOT of factors that contribute to the impedance of a functioning speaker. 
Multimeters are cheap. Audio Precision analyzers aren't. That's why. 
Speaker impedance is expressed as an average in ohms; impedance varies with frequency of input signal.
Thanks for all of the explanations and links.

For non EE types, what sort of equipment would be needed then to get a reasonably accurate measurement?


Thanks in advance
If you connect your speaker to a signal generator and run a frequency response graph for the entire audio spectrum (generally 20-20,000cps) you can see on the graph how the impedance varies quite a bit (from maybe a low point of 2ohm to a high of over 20ohm) over the frequency range. The mfgr of your speaker states it is 12ohm but that is an average of the low and high impedance shown on the graph. You should connect your speakers to the 8ohm outputs on your amplifier.
Bsmg, as I stated in the prior post, the graph provided by the maker of the speaker never dips below 12 ohms over the entire frequency spectrum. And I never mentioned anything about what taps I have.

So is there no instrument that can be acquired without breaking the bank for nonEE to broadly measure impedance, or in this hobby where snake oils are rampant does everyone blindly except vendor claims?

Speakers are expensive and impedance matters ;-)
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I have never heard of a 12ohm speaker. Guess you need to get in touch with the mfgr. Good luck.
For non EE types, what sort of equipment would be needed then to get a reasonably accurate measurement?

So is there no instrument that can be acquired without breaking the bank for nonEE to broadly measure impedance, or in this hobby where snake oils are rampant does everyone blindly except vendor claims?
You need a sine wave generator and probably a 100 ohm potentiometer in addition to your meter.

The sine wave generator drives the speaker, which is wired in series with the potentiometer; the latter only uses the center lead and one other lead.

Set the meter to AC volts.

You adjust the pot so that your meter reads the same voltage across the speaker terminals as the across the pot. Then remove the pot from the circuit and measure the DC resistance- that is the impedance of the speaker at that frequency.

So you need to do this a bunch of times but with a bit of graph paper you can draw up the impedance curve.
You can measure speaker impedance using Room EQ Wizard and a home made harness.

I personally recommend DATS V2 as a complete, out of the box solution:
Around $99 at Parts Express
As said above, the 'resistance' that you measure with your ohm meter is in DC, i.e. direct current. Impedance is an entirely different measurement that uses an alternating current (The music signal happens to be just that, AC). It might like understanding the horsepower of a car. The stated horsepower isn't exactly what you will be experiencing in driving, at least considering differing speeds, roads, driving habits, etc. It gives a baseline for you to consider. 
If its the m3 and its $300 in Eminence drivers that DC of 4.2 is about right. 4.2DC is about 8 ohm average. If you want a 16 ohm loudspeaker front horns and a few Fullranges are about the only choices I can think of off hand.
This is an area of interest for me too.  A close friend's church had lots of PA gear stolen.  I'll be doing the installation and don't want to be attaching a new power amp before testing the long speaker cables.  Strangely another friend's church needs work on their 70v system with 8 speakers in the ceiling.   I've been running sound for churches for decades, not doing installations.  So I need to purchase something reliable. 

There are relatively inexpensive devices available that will test AC impedance for audio applications--not DC resistance.

The more expensive ones will test impedance at various frequencies.  Some will also calculate "watts" for those testing a larger number of ceiling speakers running on a commercial 70v system, etc.

There is a software based app that has lots of bells and whistles.  The package is very pricey due to the required $500 audio interface plus the app

The inexpensive  Chinese impedance meters only test at 1khz.

The choice of 1khz seems strange to me.  My understanding is that the standard frequency for testing woofer impedance is 400hz. 

You can see why 400hz is more suitable for testing speakers since impedance varies greatly according to frequency! 

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Dayton Audio DATS is the only answer to this question.
Erik, thank you very much for the information on the DATS system.  It seems to be excellent for my situation and a good option for Recluse's situation.  I will likely purchase it. 

Do I understand correctly that it will not calculate the watts in a 70v system as some of the high-end hardware units do?

However I don't see any reason why DATS is "the only answer" to Recluse's question.   Some people may prefer the convenience of a very portable, small, dedicated piece of hardware.   A sound tech on a ladder troubleshooting a problem with ceiling speakers would appreciate the convenience of a small battery powered hardware tester rather than a laptop computer.  
Of course it is not "the only answer" but I wanted to avoid endless hackery and half baked solutions propping up here.
DATS is pretty comprehensive, allowing the measurement of drivers, entire crossovers and every component thereof. 
It is inexpensive, USB driven and pretty accurate. Certainly accurate enough for Magico to use (but not of course alone).
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