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I'm currently working in a charity shop.

We have a range of hi-fi units, radios, CD players etc.

We also have a range of speakers to go with them. Some are marked with their nominal impedance, others are not.

I am generally able to determine the recommended speaker impedance range for a particular hi-fi either because it is marked on the unit or I can find the specification online from the make and model number.

In many cases however there are no markings on the speaker to help me. These speaker units are as supplied with a particular hi-fi so may contain more than one raw speaker together with associated internal crossover circuitry. In some cases I know the manufacturer of the hi-fi as I have say a 'Sony' label on the speaker unit but no indication which model it was intended for.

Is there a simple way for me to measure this? I am assuming it is not as simple as the DC resistance.

I can imagine placing a resistor in series with the loudspeaker and a small, say 1 volt, signal generator across the pair by measuring the, true AC rms, voltage either side of the resistor I can calculate the impedance. Is this the best option or is there a simpler one?

If this is the best option what frequency range would I need to measure over and which figure should I take?

The intention is that the charity shop can sell a second hand system with suitable speakers even if not the ones that were originally part of the system.

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  • \$\begingroup\$ Are these speakers mounted inside boxes, or "raw"? You may have to contend with a crossover network hidden inside a box that would colour an AC measurement. DC resistance does give a good clue, even through a crossover network. For example, a raw 8-ohm speaker often has DC resistance in the 6-to-7 ohm region. A crossover network may add a small DC resistance. \$\endgroup\$ – glen_geek Nov 22 '19 at 19:22
  • \$\begingroup\$ Edited to clarify. \$\endgroup\$ – Warren Hill Nov 22 '19 at 19:35
  • \$\begingroup\$ an AC bridge is the canonical way to measure this, but in most cases a dc resistance measure and add 30% will get you close. \$\endgroup\$ – Jasen Nov 22 '19 at 21:01
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The impedance of a loudspeaker depends on frequency, crossover network, how many drivers it has, etc.

If you measure it with an ohmmeter, you'll get value which will be the DC resistance of the woofer, plus any coils in the crossover lowpass filter which are in series with the woofer.

A 8 ohms woover will usually have 6-7 ohms DC resistance, add 1 ohm for the crossover coil and wiring, so you'll probably get 7-8 ohms on your multimeter. Maybe 6 ohms if the woofer is unfiltered, or if this is a one-driver speaker without filter.

If you get half that, it's a 4 ohms speaker.

That's a bit rule-of-thumb-y but it'll work in most cases.

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