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Old 4th February 2013, 06:15 PM   #1
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Default BL distortion prediction / analysis in subs

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.
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Old 4th February 2013, 06:41 PM   #2
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Quote:
Originally Posted by flowwolf View Post
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?
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Old 4th February 2013, 07:04 PM   #3
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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
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Old 4th February 2013, 07:20 PM   #4
NEO Dan is offline NEO Dan  United States
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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.
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Regards,
Dan
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Old 4th February 2013, 07:22 PM   #5
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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.
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Old 4th February 2013, 07:30 PM   #6
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Quote:
Originally Posted by NEO Dan View Post
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.
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Old 4th February 2013, 07:38 PM   #7
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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
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Old 5th February 2013, 05:52 AM   #8
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Quote:
Originally Posted by weltersys View Post
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.
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Old 5th February 2013, 09:49 AM   #9
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Quote:
Originally Posted by flowwolf View Post
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 :-)
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Old 5th February 2013, 02:12 PM   #10
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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:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.


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:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.


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.
Click the image to open in full size.
Click the image to open in full size.

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

...
Attached Images
File Type: png motor01_flux.png (8.4 KB, 128 views)
File Type: png motor01_fluxpos.png (8.4 KB, 127 views)
File Type: png motor01_fluxneg.png (8.4 KB, 129 views)
File Type: png 2_flux 38.png (41.9 KB, 130 views)
File Type: png 2_blcurve 38 10mm.png (43.8 KB, 130 views)
File Type: png 2_blcurve2 38 14mm.png (47.6 KB, 129 views)
File Type: png 2_blcurve3 45 10mm.png (42.2 KB, 128 views)
File Type: png 2_blcurve3 45 14mm.png (43.5 KB, 128 views)
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