I measured my 8 ohm speakers with a multimeter and the figure was spot on 5 ohms. But there are all the band-pass filters included. I don't want to take the elements off just to measure the resistance..
1600 posts does not mean I am an "audioguru" I am academic that codes and does digital stuff..
Hi there misterT,
I am happy to hear that you took Nigel's advice and measured at least one speaker very recently. As you found, it has DC resistance that might be higher than you originally thought. But also please be aware that if there are any other things in parallel to that speaker that they can lower the measured resistance as well. I think you already saw this. But the speaker itself should measure close to the stamped value unless of course it is stamped wrong, which is always a possibility.
For example, less than 5 minutes ago i measured the DC resistance of a speaker i use from time to time. It is stamped as "8 ohms". It measures 7.4 ohms with the multimeter that uses a DC current to make resistance measurements. It is a lone speaker with no crossover network or any other components.
Although there are similarities, speakers are not motors. Motors can have low resistance because their coils work closely with the special metal alloy cores that they are wound on. The core acts with the wire to increase the field which then acts with the permanent magnet. The speaker on the other hand has no armature core. That is necessary in order to allow high compliance to the drive signal. That means that the speaker, although working on the same basic principle, only has it's winding to act with the permanent magnet, but no core to help increase the field. That in turn means that there must be more turns of wire in order to develop a force that is great enough to move the cone. Luckily, the cone isnt too hard to move, but at higher frequencies it still has to have enough force to get it moving quickly. That requires a lot of turns, so the DC resistance necessarily goes up. If we added a core to the speaker we would have a more motor like construction, but then the mass would be so great that it would not be able to handle a higher frequency audio signal without requiring huge current levels and we'd have to take into consideration the losses in the metal at higher frequencies too.
Also keep in mind that there will be a basic tolerance for the DC resistance of a speaker. An 8 ohm speaker DC resistance spec will not be as accurate as say an 8 ohm 1 percent resistor. There is going to be some tolerance just in the spec itself without even considering any AC quantities yet. That means it may be possible to actually find 8 ohm speakers that measure anything from roughly 6 to 12 ohms, but that's just an illustration of what we might find and not a real life range that i would count on. An incorrectly marked speaker would measure just about anything from 2 ohms up to maybe 32 ohms.
Another interesting thing...you could also measure your headphones. These things have a really wide range of possible DC resistances. From maybe 8 ohms up to 64 ohms or something like that. Mine measure around 32 ohms and that's what they are supposed to be. Think of how much wire they must use, and how thin it is. The thin wire means the coil can be close to the magnet, which increases force.
Also, i happen to have a 32 ohm speaker around somewhere. It does measure pretty high DC resistance like 30 ohms.