Driver impedance/inductance relationship question.

[ErnieM]

I've been trying to understand why some drivers have very flat extended impedance curves with more typical inductance figures while others have low inductance but more typical impedance curves.

For example:

-The 18Sound 15MB1000 has a flat impedance curve up to 10k and is 1.4mh @ 1k.
-Some BMS woofers have figures around .7mh but have larger/longer voicecoils than the 18Sound. Impedance curves look fairly standard for large prosound drivers.

Is it simply a matter of motor geometry and what the desingner chooses to optimize? If anyone can shed some light on this it would be greatly appreciated. Thanks.

 

[eriksquires]

At the very least, you need to include the Re in your calculations. Sometimes this gets you close, but it's actually more complicated:

speakers - Where can I find specific examples of parameters for a loudspeaker circuit model? - Electrical Engineering Stack Exchange

 

[Ron E]

I've been trying to understand why some drivers have very flat extended impedance curves with more typical inductance figures while others have low inductance but more typical impedance curves.

All that matters for inductance is the impedance curve. Since you don't give concrete examples it is hard to answer your question. I spent my valuable time looking for data and found these...

http://www.eighteensound.com/Portals/0/PDFs/15MB1000.PDF
http://www.bmsspeakers.com/fileadmi...ymium/bms_15n840_2011-04_neodymium_woofer.pdf

These two woofers have roughly equivalent inductance. The way the Le is stated is different between the woofers, and this is the main difference. The scales on the impedance are completely different, which is why the 18 sound impedance looks flatter above resonance.

So, n00b mistake on your part, it appears.
Erik seemingly attempted to answer a different (unrelated) question...

Inductance is not a single number despite marketing departments trying to make it so. It is also not 2 numbers at 1k and 10k, it is really kind of a useless spec, for the most part.

FWIW, inductance varies continuously with frequency. It is highest at low frequencies and reduces as you go up in frequency. Its value and how much it reduces and what the shape of the curve looks like, depends on motor geometry and construction details. The reduction of inductance with frequency is due to eddy currents in the pole piece. The one thing that makes a big difference is shorting rings or a copper or aluminum cap on the pole piece. This can reduce the inductance to nearly nothing because it offers a very low resistance path for eddy currents and acts much like a one turn inductor 'in parallel' with the voice coil inductance.

 

[head_unit]

I've been trying to understand why some drivers have very flat extended impedance curves with more typical inductance figures while others have low inductance but more typical impedance curves...

Yes, to state the above in a different way, just because there is a [voice] coil of wire in there, doesn't mean it behaves like an inductor, because other stuff is going on. One AES paper, I believe by Lipshitz and Vanderkooy, described it as being more nearly a "semi-inductance."

I couldn't find that paper, but did stumble on this
https://www.klippel.de/fileadmin/klippel/Files/Know_How/Literature/Papers/Voice_Coil_%20Impedance_04.pdf
because the "inductance" can also vary with power level.

 

[ErnieM]

Thanks for the detailed answers! Yes it does appear that I didn't look very carefully at the graphs. Should've waited till the pain killers wore off .

As far as prosound drivers are concerned it looks like BMS won't disappoint.