I have a 2x15 bass cab from the 70's, but am unsure of the impedance. I opened it up in hopes that the speakers would have some information in regards to this. They weren't even labeled. Is there any way I could measure the impedance?
Measuring impedance requires special equipment. Sencore used to make a nice one years ago, but they're quite expensive. I have access to one at work, but that does us no good here.

There are various methods described on the internet, but most require items of test equipment probably not available to you. Unfortunately, you may never know the answer to this question.
Quote by Bor3dToD34th
I have a 2x15 bass cab from the 70's, but am unsure of the impedance. I opened it up in hopes that the speakers would have some information in regards to this. They weren't even labeled. Is there any way I could measure the impedance?

Just plug a short cable into the cab and use a multimeter and measure the resistance across the tip/sleeve. Resistance is measured in Ohms. And tada impedance is measured in Ohms. There are some hairy math equations to go along with it but essentially resistance is a measurement for DC current and impedance is a measurement for AC current and guess what they are so close together it won't make a damn bit of difference if you use a DC multimeter unless your circuit required hair fine resistance/impedance tolerances. Since were talking amps and cabs here it won't make a damn bit of difference.
Quote by sparkeyjames
Just plug a short cable into the cab and use a multimeter and measure the resistance across the tip/sleeve. Resistance is measured in Ohms. And tada impedance is measured in Ohms. There are some hairy math equations to go along with it but essentially resistance is a measurement for DC current and impedance is a measurement for AC current and guess what they are so close together it won't make a damn bit of difference if you use a DC multimeter unless your circuit required hair fine resistance/impedance tolerances. Since were talking amps and cabs here it won't make a damn bit of difference.

This. It's not an exact measurement, but there is a relationship, so it's a fairly reliable way to check an unknown cab.
Water which is too pure has no fish - Ts'ai Ken T'an
Impedance varies with frequency anyway; quoted impedances are nominal so a multimeter measuring DC Ohms is just fine. Considering that most manufacturers are making 8 or 16 Ohm cabs with the odd 4 Ohm thrown in you should be able to tell which you have without too much difficulty. If you get anything wildly different you may have a damaged driver or bad wiring, in which case you will need to check the speakers individually. Come back if you need help with this.

Andi
Quote by sparkeyjames
Just plug a short cable into the cab and use a multimeter and measure the resistance across the tip/sleeve. Resistance is measured in Ohms. And tada impedance is measured in Ohms. There are some hairy math equations to go along with it but essentially resistance is a measurement for DC current and impedance is a measurement for AC current and guess what they are so close together it won't make a damn bit of difference if you use a DC multimeter unless your circuit required hair fine resistance/impedance tolerances. Since were talking amps and cabs here it won't make a damn bit of difference.

That's just wrong. The difference can be markedly different. All you can tell by doing that is ascertain that it can't be below the DC resistance. ie. it can't be a lower impedance than the DC resistance but it can be quite a bit higher.
If you have a known impedance cab you can run a constant signal tone generator into it (I use the test tone from my DX7 into the amp's input) and measure the AC voltage across the speaker. Then connect the unknown cab and measure the voltage across that. The ratio of the square of the two voltages you measure is the ratio of impedances. Do it with the amp switched to 4 ohm if it's a valve amp to be on the safe side of things.
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Last edited by Cathbard at Sep 4, 2010,
Quote by Cathbard
That's just wrong. The difference can be markedly different. All you can tell by doing that is ascertain that it can't be below the DC resistance. ie. it can't be a lower impedance than the DC resistance but it can be quite a bit higher.
If you have a known impedance cab you can run a constant signal tone generator into it (I use the test tone from my DX7 into the amp's input) and measure the AC voltage across the speaker. Then connect the unknown cab and measure the voltage across that. The ratio of the square of the two voltages you measure is the ratio of impedances. Do it with the amp switched to 4 ohm if it's a valve amp to be on the safe side of things.

Yes it's wrong, but unless this is a school project it doesn't really matter; my 16 Ohm 4x12 has a DC resistance of 13.9 Ohms, my 8 Ohm 2x12 is 6.8 Ohms, all of my combo's speakers are within 3 of 4 Ohms below the stated nominal value. I think it's pretty safe to figure what output to use in each case.

Andi
Since I was rewiring a cab this weekend, I decided to check the DC resistance of each of the 4 16 ohm speakers and had a range of 13.5 to 14.5. The DC resistance reads a little lower than actual impedance. It's a pretty reliable way to check.

Here's what you would expect:

3 to 3.5 ohms 4-ohm speaker
6 to 7.5 ohms 8-ohm speaker
13 to 15 ohms 16-ohm speaker

This does not necessarily hold true speaker systems outside the voltages we use for our purposes.
Water which is too pure has no fish - Ts'ai Ken T'an