another impedance question

[Randall]

A friend wired up four 16 ohm speakers in a Marshall cab in a series/parallel circuit to result in 16 ohms. When he was done it resulted in 10.9 ohms. I checked it out, and other than his wiring being a little sloppy, it looked right, two speakers in series on each side paralleled together. Wanting to measure his speakers, I disconnected most of it. Each speaker measured approx 14 ohms, and each series wired pair was approx. 26 - 27 ohms.

Everything seemed right at that point, but when I wired the pos. - pos. and neg. - neg. the result is 10.5 ohms, on both our meters within a few tenths. I was expecting approx. 13 ohms, like his other 4 x 12 cab.

So, what is going on? I have never seen a cab measure 10 ohms before. Then he asked me a good question, run the amp on 8 or 16 ohms?

[The Dude]

Resistance and impedance are 2 different things. Trust the stickers on the speakers. You'll want to run the amp at 16 ohms.

[Leo_Gnardo]

Each speaker measured approx 14 ohms, and each series wired pair was approx. 26 - 27 ohms.

It's common for the DC resistance of a voice coil to measure less than the specified impedance of a speaker's voice coil. For instance ElectroVoice 8 ohm speakers often measure around 5.5 ohms with an ohmmeter. What's going on with your friend's cab does seem a little odd. Is there other noise going on in the room while you're measuring? The pressure from a noisy ambient field can cause speaker cones to move enough to generate a little voltage, and that can throw off your measurements significantly.

[Randall]

I understand that resistance is not impedance. And I do realize speakers measure less on an ohmmeter than their rating. That is not the problem, the problem is how did I get to 10~ ohms, most "16 ohm" 4 x 12's are 13 or 14. The two 26 - 27 ohm series pairs should result in ~13 ohms, no? And if it is in fact 10 ohms can he run the cab from a 16 ohm tap?

No ambient noise, and the other cab was right next to it and it measures 13 - 14 ohms.

[The Dude]

There are many factors in determining impedance. Voice coils are wound differently with different wire sizes, etc. Impedance is also frequency dependent. There could be many reasons these speakers measure the DCR they do. IMO, it's nothing to be concerned about. If they are 16 ohm speakers, they are 16 ohm speakers, regardless of their DCR. If they were 8 ohm speakers, your DCR would be less than 8 ohms.

The only thing could be is that 2 speakers are 8 ohm and 2 are 16 ohm. You could get roughly a 10 ohm DCR with that series parallel combination. Have you measured every speaker, or just one or two? Have any been reconed with possibly the wrong kit? If every speaker measures around 14 ohm DCR, then I wouldn't worry about it.

[The Dude]

My logic on that last part is this:

If you had an 8 ohm speaker (approx. 6 ohm DCR) and a 16 ohm speaker (approx 14 ohm DCR) in series, that would give you 20 ohms DCR (give or take). Put those 2 series combinations in parallel and you're going to get about 10 ohms DCR. Maybe that's what's going on?

[Old Tele man]

General "Role-of-Thumb": R(dc) ≈ 0.8*Z(ac)

[Randall]

I measured all 4 speakers, they all looked about the same. Dude your 8 ohm scenario makes logical sense on it's own, but remember I measured two 26 - 27 ohm pairs in series, so there is no way there are 8 ohm speakers in the mix. Back to my main question here, how does two speakers in series that measure 27 ohms wired in parallel with two other speakers in series that measure 27 ohms get me 10 ohms?

[The Dude]

Do you have a fresh battery in your meter? The meter measures resistance by applying a DC voltage to a resistor and measuring current. If you meter voltage is dropping with the extra load, it might measure low.

[dstrat]

I think if you want to work that out you need to know what each speaker measures separately.
and do the math.

[The Dude]

See post #1.

"Each speaker measured approx 14 ohms"

[dstrat]

I should learn to read.

Curious I measured my 1974 - 4x12 @14.2 ohms. with a fluke73.
that is an odd one.

[J M Fahey]

"Low Ohm measurements (less than 20 ohm or so) on standard multimeters are unreliable" meaning 20% or worse errors are common.
So donīt overthink it.

Besides: was the "single speaker" measured fully on its own, on the bench and disconnected from anything else?
Or, inside the cabinet, still connected?

Besides: at such low ohm values, even test probe *pressure* on contact makes a difference.

Again, donīt overthink it.

In any case, I guess this Yearīs Nobel Prize is already assigned, and even if it were empty, I doubt it would go to somebody who proves Georg Simon Ohm wrong.

[Randall]

Again, speakers were measured separately out of circuit, they all looked about the same. My meter is a new Fluke, and same or very close measurements were made on my friends meter before I got there and re-wired. I did take in account probe connections, this is not my first rodeo. I made multiple measurements with same results. The point is wired once, wired twice, still 10 ohms.

So what I'm feeling here is, I don't know why I am getting this unexpected result, and so far no one else does either it seems.

[eschertron]

So what I'm feeling here is, I don't know why I am getting this unexpected result, and so far no one else does either it seems.

All the Forum members can offer is a regurgitation of Ohm's law, and some "did you check this/that?" questions. Physics gives us a formula to predict the measured resistance. If they don't match - and you're intent on finding out why - you must review your test methods, check calibration on your instruments, repeat the experiment (or devise new ones) ad nauseum until you find out why your results don't match the prediction. Remotely, over the internet, much of the 'reality' of the situation is stripped away and we cannot repeat the experiment to see for ourselves.

So far, from what I've read in this thread, nobody seems to think this indicates a gross failure of any kind. If that's not a fair assessment, someone please step in to correct me.