# Help! speaker ohms when none are marked



## whammybar (May 7, 2008)

Is there a test to find out what the ohm rating of a speaker is when there is nothing stamped on the speaker? The company has long gone out of business so no help there, Google was not my friend, at least in this case. Also, is there to do the same with the watts rating? Same speakers, same problem.


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## Fader (Mar 10, 2009)

An 8 ohm speaker will be around 5 ohms when measured with a multimeter.
A 16 ohm will be around 12.

The wattage will be scalable to the wire gauge of the coil.


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## nonreverb (Sep 19, 2006)

The impedance can actually be anywhere from 5 to 6.5 ohms for an 8 ohm speaker and around 10 to 14 for a 16 ohm. A 4 ohm will read around 2.5 to 3.5 on a good quality multimeter.


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## Fader (Mar 10, 2009)

nonreverb said:


> The impedance can actually be anywhere from 5 to 6.5 ohms for an 8 ohm speaker and around 10 to 14 for a 16 ohm. A 4 ohm will read around 2.5 to 3.5 on a good quality multimeter.


I did some calculations and as it turns out, 5 to 6.5 ohms happens to falls into the "around 5 ohms" value. Surprisingly my 12 ohm projections are good as well.

Thanks for clearing up that confusion Nonreverb.


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## Printer2 (Apr 8, 2012)

And I just measured a speaker that is 7.2 ohms. A 10 ohm speaker maybe?


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## nonreverb (Sep 19, 2006)

Flaming not required buddy 



Fader said:


> I did some calculations and as it turns out, 5 to 6.5 ohms happens to falls into the "around 5 ohms" value. Surprisingly my 12 ohm projections are good as well.
> 
> Thanks for clearing up that confusion Nonreverb.


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## nonreverb (Sep 19, 2006)

As long as it's below 8 and above 5 you're pretty well assured it's 8 OHM. It can get tricky if the probes or speaker contacts have any oxidation or are dirty. I've had stuations with unknown speakers where I've had to take a couple of readings just to be sure as the readings varied.
As for the power handling, that's a little more challenging. Probably, the best way visually is by looking at first the magnet size and then the voice coil diameter. The general rule is the bigger the magnet/voice coil diameter the more power it can handle. That doesn't tell you exactly but if they have hefty magnets and voice coils an inch and a half in diameter, you can take an educated guess that they can probably handle around 50 Watts.
Where you can run into trouble though is when they're "vintage" speakers. If they were Jensen's from the '60's for instance with original cones, they have about half the power handling capacity that they did when new.



Printer2 said:


> And I just measured a speaker that is 7.2 ohms. A 10 ohm speaker maybe?


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## Fader (Mar 10, 2009)

nonreverb said:


> Flaming not required buddy


Duplicating my post not required either, Pal.


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## nonreverb (Sep 19, 2006)

Let's look at what you said in your post...

"An 8 ohm speaker will be around 5 ohms when measured with a multimeter."

As Printer2 quite rightly pointed out, he got a reading of 7.2...kinda off from around 5 isn't it? For all you know this guy might be a newby and may be taking what you say literally. I myself should have been more accurate as well as I know those readings can be close to 8 sometimes.

"The wattage will be scalable to the wire gauge of the coil."

As true as that is, how is that supposed to help someone estimate the wattage of a speaker he's holding in his hand? Destroying the speaker to measure the wire gauge is probably not the preferred option. 
It's important to know who your audience is when posting here. Some people will take your direction/information literally so accuracy is paramount as well as wording it in a way they can understand. By the way, I don't doubt for a second that you know what you're talking about if that's what has you on the defensive. I was merely trying to be a little more accurate.


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## Wild Bill (May 3, 2006)

I have a slightly different way of thinking about this problem. First off, the dc resistance of the wire used in the voice coil is always lower than the AC impedance value. So you are virtually always safe rounding up to the nearest standard value. Virtually all speakers come in 3.2, 4, 8 or 16 ohm loads. I think I have seen only one speaker in my life that was different and that was a 32 ohm unit used in a special application.

So if it's less than 8 but more than 4 call it an 8 ohm speaker. 3.2 ohms are rare in guitar amp circles. They were common in car audio and are close enough to 4 ohms to be used as the same.

Where we see a bit of confusion is in examples like the one given above, where the dc resistance measured over 7 ohms but less than 8. This is not that uncommon. We should remember that speakers of higher wattage need thicker wire in the voice coils to handle more power. Thicker wire has lower resistance. So a 20 watt speaker might measure 7 ohms while a 100 ohm speaker might measure 5 or 6 ohms

The ranges still are a good rule of thumb but higher wattage rated speakers will tend to the lower end of the range.

Wild Bill - Busen Amps


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## nonreverb (Sep 19, 2006)

Excellent explanation Bill.



Wild Bill said:


> I have a slightly different way of thinking about this problem. First off, the dc resistance of the wire used in the voice coil is always lower than the AC impedance value. So you are virtually always safe rounding up to the nearest standard value. Virtually all speakers come in 3.2, 4, 8 or 16 ohm loads. I think I have seen only one speaker in my life that was different and that was a 32 ohm unit used in a special application.
> 
> So if it's less than 8 but more than 4 call it an 8 ohm speaker. 3.2 ohms are rare in guitar amp circles. They were common in car audio and are close enough to 4 ohms to be used as the same.
> 
> ...


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## whammybar (May 7, 2008)

Um.... to call me a 'newby' would be a compliment. I know bubkas. Thanks you for all the advice. So when I use a multimeter to measure, do I place the leads of the multimeter on the lead wires coming off the speaker contacts (I mean the place where I would normally hook the wires to the speakers?) I'm just not sure where to take the reading from. And yes... I am really incompetent when it comes to all this so I very, very much appreciate all your patience.


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## loudtubeamps (Feb 2, 2012)

How to Troubleshoot Speakers


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## greco (Jul 15, 2007)

Just remember that you have to take the speaker wires off the speaker (unsolder the wires [if soldered] or GENTLY pull off the spade terminals attaching the wires to the speaker) before taking the DC resistance measurements.

Alternately, if the speaker wires go to a 1/4 inch plug, you could take a reading of the DC resistance across the tip and sleeve of the plug. 

My apologies if you already know this.

Cheers

Dave


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## Printer2 (Apr 8, 2012)

Wild Bill said:


> We should remember that speakers of higher wattage need thicker wire in the voice coils to handle more power. Thicker wire has lower resistance. So a 20 watt speaker might measure 7 ohms while a 100 ohm speaker might measure 5 or 6 ohms


Not sure if this is particularly accurate. Lets look at two speakers, one 10W and one 100W. They both have an impedance of 8 ohms. Which one has the larger diameter wire? Kind of depends on the length of the wire, right? How many turns does each coil have? If the higher power speaker had one layer of wire along a 1/2 inch of voice coil form (Which gives a lot of radiating area, and the other has multiple windings stacked on each other on 1/4 inch of the voice coil form you can be fooled. Also not quite sure on the 100 ohm speaker thing. Typo?


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## greco (Jul 15, 2007)

*Thiele-Small Parameters: *

Voice Coil DC ResistanceR[SUB]E[/SUB]6.70 Ω

I was looking up the specs on a Jensen speaker and noticed that they show the DCR value.
The speaker's impedance is 8 ohms.

Nice of them to provide this info.

Cheers

Dave


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## whammybar (May 7, 2008)

Thanks to everyone. At least I can understand enough to give it a try now. Sorry for being such a noob. It's pretty much my first time 'under the hood'.



greco said:


> Just remember that you have to take the speaker wires off the speaker (unsolder the wires [if soldered] or GENTLY pull off the spade terminals attaching the wires to the speaker) before taking the DC resistance measurements.
> 
> Alternately, if the speaker wires go to a 1/4 inch plug, you could take a reading of the DC resistance across the tip and sleeve of the plug.
> 
> ...


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## nonreverb (Sep 19, 2006)

No appologies required! There are great folks here to help you out. Best of luck 



whammybar said:


> Thanks to everyone. At least I can understand enough to give it a try now. Sorry for being such a noob. It's pretty much my first time 'under the hood'.


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## greco (Jul 15, 2007)

whammybar said:


> Sorry for being such a noob.


You never need to say you are sorry for being a noob. 

We are all noobs at everything at some point in time.

When I was telling you in my post to be gentle pulling off (and pushing back on...BTW) the spade connectors on the speaker terminals it is because I broke two of before it finally sunk in as to how fragile these fibreboard terminal holders are. However, as a result, I taught myself how to construct and attach new terminal holders and how to solder tinsel wires. GC member _Nonreverb_ had "walked me through" all the aspects of soldering the tinsel wires previous to that (in a thread in this forum), so I was ready for that part.

Cheers

Dave


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