# Measuring Speaker Impedance

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Can anybody tell me how to measure the impedance of unmarked speakers?

Pete

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You will get a rough idea by using an ohmeter across the terminals set on Rx1. It won't be exactly 4, 8 or 16 but it should be close enough to make a good guess. Some measure a little high. some a little low. It is a dynamic value, not a static one so you might even find two identical speakers that measure different.

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Won't that just give me the d.c. resistance of the coil?

Pete

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Yep, that is why it is an approximate reading. I don't use digital meters for a lot of reasons but one is that an analog meter puts a light load on everything it tests. The reading you get with an analog is closer to the impedance than the resistance reading you will get with a digital. Watch the arguement that follows this one.

Edited by thedoctor
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Won't that just give me the d.c. resistance of the coil?

Pete

Yes, it will. And the resistance will be lower than the nominal speaker impedance of the speaker. However, it's close enough to let you decide what it is, because there are only a few values.

Most 4 ohms speakers measure 3.2 to 3.6 ohms DC.

Most 8 ohms speakers measure around 6 ohms DC.

Most 16 ohms speakers measure around 11-14 ohms DC.

The DC resistance will always be less than the speaker nominal impedance, because part of the speaker impedance goes to putting power out as sound. The DC resistance is the part that wastes electricity as heat inside the speaker. The two parts added together are the impedance that the speaker is rated at.

The actual impedance, not the nominal, rated one, varies by at least 3:1 as frequency varies, sometimes more than that.

What he said!

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Wow, is it really that simple??!!

I never tried, figuring I'd get a wildly inaccurate and useless reading.

I thought I'd have to set up a signal generator and a 'scope and a frequency meter and take voltage and current readings then use arcane formulae and long forgotten mathematics and eldritch incantations (whew)!!

Thanks guys

Pete

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Wow, is it really that simple??!!

Yep, in this case, it is.

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