Single Driver Impedence

because its an inductor obviously

Hey Paul.

For a typical driver with a voice coil, yes. For reference, take a look at a typical example datasheet here:

https://www.scan-speak.dk/datasheet/pdf/18w-8542-00.pdf

I use a ScanSpeak datasheet because they include a couple of things you asked about. You asked if the impedance of a single driver varies. SS provides the impedance of the driver alone, in free air in the first chart with the blue line. A very typical mid-woofer impedance graph

If this was a simple inductor, as Kenjit offered, the impedance would be a simple line. The real answer is that the driver’s impedance is electro--mechanical. Perhaps the only device in audio where the impedance varies because of properties that are not actually part of the current flow. This includes properties such as suspension, enclosure, air pressure, magnet strength, etc.

The last part of that PDF is called Advanced Parameters. There you will find that to model the impedance of this driver you will need a number of components. The components in this model aren’t real, in the sense that the driver does not have discrete resistors, capacitors or a transformer but they are real in that the math will provide the right output when used to predict the impedance of the driver at any given frequency. On the left of the transformer are the properties of the coil, to the right, everything the cone, suspension and air bring to the equation.

This really is a pretty advanced model.  If we just care about impedance we can often model a single driver more easily as a coil, resistor, and resonant circuit, basically everything to the left of the transformer. When we make crossovers, we don’t even use this, instead we measured measure the driver as mounted in the cabinet, and forego attempting to decompose the model into components. It's more accurate that way.  Kind of like, you are better off using a marking board than a measuring tape. :)

It may further interest you to know that if you see the impedance of a ported speaker you will see 2 humps, not one. Think about that for a bit. We start with one hump, and then, by putting the driver in a cabinet we are significantly changing the electrical impedance! Even without a crossover. This is because how the air in the cabinet moves, and pushes against the driver affects the coil’s current flow, and therefore the apparent impedance as seen from an external amplifier. You could also change the shape of the curve by taping weights to the cone.

Lastly, this overall discussion applies to drivers with voice coils only. Other devices, like ribbon tweeters, AMTs, or ESLs are going to behave differently. AMT’s, for instance, behave almost entirely as pure resistors. The moving element of a ribbons is purely restive but since they often need a transformer to work may be much more inductive.

For more, I suggest you reach out to the multi-way forum on DIYaudio

Your asking if a full range speaker changes impedance through it's frequency range. Yes it does, and a well designed one changes LESS. That is why it is difficult with a conventional cone/voice coil driver, to be full range. Most that make the claim, don't cut the mustard, 20hz to 20khz, not even close. Normally lighter drivers in "MASS" work better for higher frequencies. The NO XO design, just doesn't work for me. Never has.

Passive Radiators give a double hump too, and a lot more versatile too. Not less expensive, or easier to get right, but more options, as far as quicker roll offs and deeper bass response with less effort.

I have a tough time with a two way. or No XO
4 way. Sub, Bass, Mids, and highs...That works for me..

I have to admit, I've heard a few, large electrostatic that were 2 ways, I was impressed.. ML and Sound Lab, a few.. 20-30 ohm systems in the bass region.. Transformers in lue of voice coils..

Regards

Thanks Erik and oldhvymec. Makes sense to this layperson.