Is there something special about measuring speaker ohms?

[heresrobert]

How do you determine speaker ohms? I have some speakers that are not marked as to what their ohms rating is. On one I'm getting a meter reading of 22 ohms. Can that be right? I then measured a speaker rated at 4 ohms and it metered at 12 ohms. Could the DMM be loading the speaker coil and giving an inaccurate reading?

 

Thanks for any help.

Robert

[PRR]

I wonder if your meter has trouble reading low ohms.

Try reading some real resistors in low-ohms values.

Check/replace the battery.

It's fairly normal for meters to have trouble below a couple ohms. However they should read 6 or 8 or 13 ohms good enough.

[heresrobert]

Yes. I'm testing components... bare speakers. I have a TPI model 135.

I bought a new battery and I'm getting better results. The smallest resistors I have are Doug's 5% 2 watt 1 ohm. I'm still looking for the 1% resistors I have somewhere. With the 5% 1 ohm, I get 1.3 and 1.4 ohms on the meter. Not what I was hoping for. The marked speakers all measure a little higher than their rating. I think this is new that these resistors are measuring this far off of 1 ohm. I'll dig out the 1% stock and see how that goes. Thanks for the help. I'll report back after more testing.

Robert

[Willabe]

Quote from HBP in; Re: Impedance of speakers« Reply #7

http://www.el34world.com/Forum/index.php?topic=14070.0


Not true.

Impedance = Resistance + Reactance. Resistance of the wire, which is then wound in a coil having mostly inductive reactance.

Except it's not plain addition, but vector algebra; Resistance2 + Reactance 2 = Impedance2

Gerald Weber suggested there was a fixed ratio of d.c. resistance for a given speaker impedance, but that's just wrong. There is a typical range, but it really depends on how the speaker manufacturer designed and constructed their speaker.

Maybe this will add to helping to answer your question?


                              Brad      

[jeff]

What do you read when you touch the two leads together? Not testing anything, just red probe to black probe.

[HotBluePlates]

With the 5% 1 ohm, I get 1.3 and 1.4 ohms on the meter. Not what I was hoping for.

A 5% 1 ohm resistor show read between 0.95 and 1.04 ohms, not 1.3 or 1.4 ohms.

...

Is Heresrobert DMM still giving him problems?  probably.

I have a Fluke 87III DMM. If I measure those 1Ω resistors, I'll get 1.3 or 1.4Ω, too.

At that point, I'd engage the offset capability of the meter, and touch the meter probes together. That would set the probably 0.3-0.4Ω resistance of the meter leads as the new "Zero".

So don't fret, and you don't need 1% resistors for the test. 99.995% chance your measurement of resistance will be that 0.3-0.4Ω high due to meter lead resistance.

... I'm getting a meter reading of 22 ohms. Can that be right? I then measured a speaker rated at 4 ohms and it metered at 12 ohms. ...

You confirmed your meter is in the right ballpark, which is what PRR wanted you to check. If you still get resistance readings higher than the stated impedance of the speaker in question, there may be damage to the voice coil, or more likely a thick layer of something that's a poor conductor where you're probing. It's also possible to have a cold solder joint where the tinsel wires are brought out to the speaker terminals. Be sure to use good, firm pressure with the points for the meter leads when making a resistance measurement. That will help you press through any surface oxidation which will skew the readings.

I am not questioning Hot Plate credentials ...  I believe the quoted answer is very simplistic, because impedance is a function of frequency.

...

Typically the DC reading of a speaker component is about 70 to 75% of the speaker impedance rating.  Therefore, if the speaker is labeled 8 ohms nominal, the DC reading would about 6.3 ohms.

Dr Gonzo, you and I will have to agree to disagree.

On the one hand, you say d.c. resistance is typically a fixed percentage of total impedance, then say that impedances changes with frequency. For both statements to be true, resistance would also have to increase with increasing frequency. Since this is known to not be a valid property of resistance, it follows that d.c. resistance cannot be a fixed percentage of speaker impedance.

But this is what I've been arguing in my simplistic posts! PRR pointed out an issue I overlooked, being "air impedance" the speaker works against. I won't confuse the issue further here, but say that total speaker impedance = resistance + reactance + air impedance. That's A=b+c+d. What's the ratio of the a, b, c?

Simply put, there's no fixed ratio that is always valid because it depends on the particulars of the speaker design. The only thing you can honestly know is resistance is less than total impedance.

As an example, I have a JBL D130 that is a 16Ω speaker; It has a d.c. resistance of 6.2-6.4Ω, or less than 40% of total impedance. From PRR's information, much of the remainder is made up of air impedance due to the curvilinear cone.

[heresrobert]

Hello folks,

I just got home from a weekend trip. I apologize for delayed responses.

Jeff,  Firmly touching the probes in continuity mode, I get 0.3 ohms. On the 400 ohm setting, I get 1-8 - 2.0 ohms. I've repeated this a bunch of times with the same results. I've had this meter for a long time. I'm thinking a new set of leads is in order.

I checked some speakers that I have. The 4 ohm speakers are measuring about 5.4. An 8 ohm is measuring 8.8. The speakers originally measuring wild numbers are somewhere 5.2 - 5.5 ohms. There's no guarantee that any speaker is supposed to measure what is stamped on them. Nothing horribly wrong with the DMM, but I do want to resolve getting an accurate reading. I found the user's manual for my DMM. What it doesn't talk about is zeroing out the darn thing. I'll go after the company for some answers.

Thanks for everyone's help. I really appreciate it.

Robert

[HotBluePlates]

Firmly touching the probes in continuity mode, I get 0.3 ohms.

Very typical, and not a problem. I get the same with a Fluke.

On the 400 ohm setting, I get 1-8 - 2.0 ohms.

May also be typical. 2Ω is 1/2% of full-scale, and probably exceeds your meter's capabilities/accuracy.

I'm thinking a new set of leads is in order.

I don't think anything is necessarily wrong with your leads, or battery, based on the results of the tests you've performed.

I checked some speakers that I have. The 4 ohm speakers are measuring about 5.4. An 8 ohm is measuring 8.8. The speakers originally measuring wild numbers are somewhere 5.2 - 5.5 ohms. There's no guarantee that any speaker is supposed to measure what is stamped on them. ...

Borrow a meter better than what you have now. Or make your measurements in continuity mode.

Something is confusing your meter. Trying a different meter may be better than trying to find the right crutch for your present meter.

[heresrobert]

HBP wrote:

Borrow a meter better than what you have now.

Thank you. That's a good idea and I do have access to better meters.

Robert