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Old 18th April 2008, 12:11 AM   #1
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Default Testing a speaker's impedence

I searched for a post of this question and couldn't find it. I'm sending a frequency generator to an amplifier, and the output of the amplifier to the speaker. How do I test the speaker's resistance using a multimeter? I've included a picture of the digital multimeter that I am using, which I assume is fairly standard. I'm looking for the frequency with the least resistance so that I know the tuning of my ported cabinet. Thanks ya'll.

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Old 18th April 2008, 01:45 AM   #2
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Hi,
First, you need a known resistor in series with the speaker load,
then you need to accurately measure the voltage going to the speaker from the amp. You then you need to measure the voltage drop across the speaker terminals.

Rod Elliot's ESP site has an article describing the procedure, with a diagram and a handy spreadsheet to put the values in. It's here
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Old 18th April 2008, 02:28 AM   #3
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Thanks John. Are you sure the resistor is needed for this test though? This is just to see the resistance fluctuation between 30 and 100hz. http://www.diysubwoofers.org/faq.htm section 1.03 gives a brief explanation saying that the resistor is more accurate but not necessary. Also, I don't think my multimeter does millivolts.
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Old 18th April 2008, 02:36 AM   #4
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Gee, I didn't even look at your MM, but yeah, no mVAC scale. Looks like you are SOL, as the only way to determine the impedance is to measure the voltage drop , which will be AC and tiny.

The fixed resistor is needed to calculate the current draw. Without it, you need another MM to measure mA draw. A resistor is cheaper.

EDIT: Sorry, just looked at the Brian Steele link you posted. If all you need is the Fs, then that will work ok. mA AC measurements on these cheaper DMM are not very accurate though.
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Old 18th April 2008, 12:21 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

The multimeter shown does not have a AC current setting.

It should be possible by observation of the port output maximum
(fingers slightly parted and in the way) and the coincidental cone
movement minimum to directly ascertain the port tuning frequency.

/sreten.
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Old 18th April 2008, 12:35 PM   #6
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Thanks again John. I found a good site with diagrams as well. It's here:
http://www.bcae1.com/spboxad3.htm
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Old 18th April 2008, 01:20 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
for an approximate answer measure the DC resistance of the speaker at the input terminals. Add about 25% to that DC figure to get impedance.

If you require a more accurate impedance value you need to send audio band pink noise to the speaker and need special instrumentation to measure the full audio bandwidth of the signal across the speaker and the resistor.

But probably easier to open the speaker and read off the impedance printed on the driver labels.
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Old 18th April 2008, 02:52 PM   #8
sreten is offline sreten  United Kingdom
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Hi, I think you have completely missed the question, /sreten.
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Old 18th April 2008, 03:11 PM   #9
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With a small amount of thought capital and some inexpensive cables to your computer soundcard, you could use the free software called "speaker workshop" to measure and plot the impedance.

The benefit of this method is that you will already be on the learning curve should you decide that you want to make acoustic measurements later on.

The software is available here:
http://www.speakerworkshop.com/

A comprehensive tutorial on how to use this software is here:
http://www.claudionegro.com/

A tutorial of the cables you will need to setup is here:
http://www.claudionegro.com/swsetup/cables/cables.html

Regards,
David
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Old 18th April 2008, 03:19 PM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by sreten
Hi, I think you have completely missed the question, /sreten.
I can see you're right. I must learn to read
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