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OT - measuring speaker ohmage

Started by zpyder, November 21, 2006, 04:48:04 PM

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zpyder

November 21, 2006, 04:48:04 PM
From the information I gathered from searching I found that speakers are mesaured in AC impedance, rather than DC resistance like my DMM measures.  Is this correct?  Assuming that it is, I am wondering if there is a way to measure the AC Impedance, or "ohmage" as I ineptly have put it, with my DMM...?  I've registered the DC resistance on these 4 speakers that I have here, and they all register ~17 ohms.  My guess would be that since this is close to 16, that these are 16 ohm speakers... but then doing a little internet-based background check on the 1971 cabinet that they came out of, the spec I saw was 4x 8ohm speakers....

Just want to figure out what these are so that I can use them the way I desire..

cheers!

thanks,
zpyder
www.mattrabe.com/ultraterrestrial Ultraterrestrial - Just doing our little part to make new rock go where it should have gone in the late-90's, instead of the bullshit you hear on the radio today.

R.G.

#1
November 21, 2006, 07:24:08 PM
Speaker voice coils typically measure about 75% of the speaker rated ohms when you measure with an ohmmeter.

As an example, 8 ohm speakers usually measure around 6 ohms.

It's odd that you're seeing 17 ohms. I'd have expected 12 ohms from a nominally 16 ohm speaker.

What does your meter say that a selection of typical resistors is?

Sometimes a meter is inaccurate at low ohms ranges, or if the battery is weak.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

zpyder

#2
November 21, 2006, 08:50:24 PM
hmmm... yea I just measured two resistors, one labeled 22R 10% and one 33R 10% (according to the bandings), and they measured 21.5R and 32.5R respecitvely...that's well within 10%

If the 17R reading on the voice coils represents ~75% of the impedance, I'd be looking at around a ~22-23 ohm speaker... I've never heard of 22 ohm speakers used in bass/guitar cabs...

The speakers are labeled with a date of 1971, I wonder if over time and usage a speaker voice coil's DC resistance could go up?

Are there any other tests I could do with a DMM that may be more accurate??

zpyder
www.mattrabe.com/ultraterrestrial Ultraterrestrial - Just doing our little part to make new rock go where it should have gone in the late-90's, instead of the bullshit you hear on the radio today.

brett

#3
November 21, 2006, 09:04:53 PM
Hi
Get an 8.2 (or 10 ohm) resistor.
Place it in series with the speaker and connect to an amp (preferably a very cheap one in case anything goes wrong).
Turn it up until it's reasonably loud, and measure the AC voltage drop across each of the resistor and the speaker.

If the speaker has about double the voltage of the resistor, its 16 ohms, if about equal, its 8 ohms, if half, its about 4 ohms.

cheers
Brett Robinson
Let a hundred flowers bloom, let a hundred schools of thought contend. (Mao Zedong)

zpyder

#4
November 21, 2006, 10:36:10 PM
brett-

does it matter if the resistor is before of after the speaker in series?

thanks!

zpyder
www.mattrabe.com/ultraterrestrial Ultraterrestrial - Just doing our little part to make new rock go where it should have gone in the late-90's, instead of the bullshit you hear on the radio today.
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