BL distortion prediction / analysis in subs

[flowwolf]

Hi guys, i'm planning to build my own speaker drivers for a 4-way open-baffle application.
The details are not so important as it is a typical open-baffle configuration like Linkwitz's and John K's, but
i'm struggling with the subwoofer's excursion since one subwoofer will be used per/baffle. the only important
thing, that it must handle around 14mm excursion as linearly as possible.
Well, i know that the best answer would be 'double the number of drivers', but have taken a look at a typical BL curve
and wasn't satisfied with the results.

Playing with FEMM for a while, checking the distortion levels was a must.
therefore i've made a simple application that evaluates the distortion and THD for a given BL curve generated by FEMM.
Let's take a look at it:

http://s5.postimage.org/jxxv44f47/app01.png

The app eats the BL curve and shapes a given wave, then the distortion is evaluated.
Here is (the already) optimized motor structure:

http://s5.postimage.org/vx9dbfkp3/motor01.png

Since evaluating the BL curve is a pain in the @ss, and FEMM is unable to simulate AC frequencies with permanent
magnetic fields, the BL curve is not entirely precise. I mean, shorting rings will be used, and by changing the
direction of the current in FEMM, the BL curve also changes. btw. the BL curve itself (mathematically) is an integral
of the flux density in regarding to the coil.

So, here is another app. that interprets the 'air gap vs flux density' curve generated by FEMM:

http://s5.postimage.org/xdkvtknlz/motor01_flux.png
http://s5.postimage.org/u9a7ws6tj/app02.png

http://s5.postimage.org/n7caal37r/app03.png

and generates the given BL curve for it. With this, basic distortion checking is simplified.

it is a simplified model, it doesn't take lorentz forces and so on into account so it is not a FEA.

there is more to this story, but i'm waiting for replies.

[weltersys]

Originally Posted by flowwolf

Hi guys, i'm planning to build my own speaker drivers for a 4-way open-baffle application.
the only important thing, that it must handle around 14mm excursion as linearly as possible.
Well, i know that the best answer would be 'double the number of drivers', but have taken a look at a typical BL curve
and wasn't satisfied with the results.

If you actually want to build speakers, I'd suggest getting a Klippel Analyzer like the manufacturers of speakers with excellent BL curves at high Xmax values use to help in the design process.

Why do you want to build your own drivers?

[flowwolf]

The Klippel analyzer is a clever/important one.

I don't want to compete with big companies there are a lot of good subs available, just interested in speaker building, that's all

Regards,
Akos

[NEO Dan]

Hey Art,
The OP is interested in the design side, the Klippel only reveals the end result.


Interesting and insightful stuff Akos.
I had never tried to reverse current but I have wound coils in "reverse" in FEMM.

[KaffiMann]

I am more interested in HOW you are planning to build your own drivers. I think your approach is interesting, as far as I can see you are trying to reliably simulate distortion/behaviour of a design before you make it, so it would be an interesting tool in adittion to a Klippel analyzer. Before/after.

I assume you will also take into consideration weight of movable parts and physical resistance.
Surface area of gap? Will you be able to adjust gap design in simulation?

Sorry if this seems trivial.

[weltersys]

Originally Posted by NEO Dan

Hey Art,
The OP is interested in the design side, the Klippel only reveals the end result.

He said he's "just interested in speaker building", if one doesn't measure end results one won't know if the design side worked as predicted.

There is a lot more to building drivers than the design side.

[flowwolf]

I'm going to pass the parameters to a loudspeaker driver engineer.
Yes, drivers cannot be made at home in a nasty environment. I'm more like a designer.

http://s5.postimage.org/wgegkpc3r/wire.png
http://s5.postimage.org/573367b0n/design.png

Regards,
Akos

[flowwolf]

Originally Posted by weltersys

There is a lot more to building drivers than the design side.

Well, then which subwoofers would you recommend me for low BL distortion @ hi xmax? prices around 300$.

maybe you have custom build subs. i would take a look at them then.

KaffiMann, what do you mean by reliably simulating distortion? It's not a FEA, so a simplified model won't give realistic results as it's doesn't take AC fields and so on. into account.

[KaffiMann]

Originally Posted by flowwolf

Well, then which subwoofers would you recommend me for low BL distortion @ hi xmax? prices around 300$.

maybe you have custom build subs. i would take a look at them then.

KaffiMann, what do you mean by reliably simulating distortion? It's not a FEA, so a simplified model won't give realistic results as it's doesn't take AC fields and so on. into account.

... What? You mean besides these?
AE Speakers Online Store

Yeah, I get the point about not being 100% realistic, but it seems like a good start :-)

[flowwolf]

I know about AE speakers, one of my favorites (well, I never heard them, but i'm sure in the results)

Let's take it longer.

You agree that this is just a simplified model. FEMM takes the permanent field of the coil into account, but - in my opinion - it does not matter, because it also changes with the direction of the current, and if you change it to an AC waveform it can be reduced / or almost completely eliminated by a shorting ring. This is what it's called flux modulation as described by others many years before, and also influences the field of the permanent magnet itself.

FEMM flux for DC coil zero positive & negative currents:





Let's ignore them now.
From our perspective, the distortion caused by other things like the spider, surround, or intermodulation distortions are irrelevant now.

Here is the sample BL curve which is generated for the test subwoofer above for a 38mm voice coil:





The prediction shows that it has a BL deviation of about 4% for 10mm and around 10% for 13-14mm. It follows the typical rule that BL deviation of 30% results in around 10% THD but this is an oversimplification. First the motor structure itself is somewhat optimized since it is an undercut & extended polepiece.

Next, I increase the length of the VC to 45mm:
The distortion figures are improved, 2nd and 3rd mainly. This is a common behaviour/solution but IMO not the best one.



(note here that the evaluation also depends on the quality, but i wasn't able to recognize any improvement above a certain point)

...