BL distortion prediction / analysis in subs - Page 3 - diyAudio
 BL distortion prediction / analysis in subs
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diyAudio Member

Join Date: Oct 2011
Please note when exporting flux data from FEMM, the (Normal flux density) curve must be used:

The exerted force felt by the coil depends on the direction (or angle) of the flux. This is the flux that is perpendicular to the coil. The straight is the tangential.
So, sqrt(perp^2 + tang^2) = magn of flux density and, angle = atan2(perp,tang)

Here is a short proof:
in the 1st image below the coil is put in a field where the angle is not perpendicular, therefore BL is just 0.22, in opposed to the 2nd image where the angle is more perpendicular, now BL is ~4.0. The next images show the directions when only the magnet/coil is enabled, respectively.

(this is how the force roughly looks like: )

Next time i will share the application and the details, how to use it etc..
Attached Images
 3_femm.png (13.4 KB, 106 views) 3_flux1.png (38.2 KB, 105 views) 3_flux2.png (31.3 KB, 106 views) 3_flux3_magnetonly.png (6.5 KB, 104 views) 3_flux4_coilonly.png (5.1 KB, 104 views) 3_flux6.png (14.1 KB, 105 views)

 7th February 2013, 07:04 PM #22 diyAudio Member     Join Date: Jun 2008 Location: N.E. Ohio Nice work. DDD coil: __________________ Regards, Dan
diyAudio Member

Join Date: Jan 2011
Quote:
 Originally Posted by NEO Dan Nice work. DDD coil:

Yup. That's it alright. Been a while Since I fixed up that WGTi.

One thing I saw with that simulation of the DDD motor, is that the coils appear to be centered on each magnetic gap, where as in reality they are offset towards each other so that only the last 0.5" of each coil are within the physical measurement of each 0.5" gap. That's how it is for the mobile audio subwoofers anyhow.

Last edited by OscarS; 7th February 2013 at 08:29 PM.

 7th February 2013, 08:26 PM #24 diyAudio Member   Join Date: Oct 2011
diyAudio Member

Join Date: Oct 2011
Hi OscarS,

It's good to see someone who does reconing on subs.
There was a time when i was interested in getting bad/misadventurous subs and fix them. It's worth to do that. At least for their NEO structures.

Do you mean this way: ?

According to the simulations, the BL curve is not better. But i'm going to give it a try in FEMM, and see the real results of the evaluations.

Regards,
Akos
Attached Images
 4_jbl1.png (46.8 KB, 98 views) 4_jbl2.png (47.8 KB, 97 views)

 7th February 2013, 09:01 PM #26 diyAudio Member   Join Date: Jan 2011 The issue I see with your model is that you don't have enough space between your magnetic gaps. Notice how there is 0.53" in between the 1.4" long coils on the former. With this spacing it allows the coils to be "inner hung" without having any windings outside of the magnetic caps. Here is what I'm talking about: So to correctly model the W12 and W15 GTi subwoofers, you need to have the 0.5" long magnetic gaps 2.33" apart (on the inside). This places the gaps at the outermost end of each coil's last 0.5" worth of windings.
 7th February 2013, 09:48 PM #27 diyAudio Member   Join Date: Jan 2008 Location: Toronto and Delray Beach, FL Wonderful. Absolutely wonderful. Anybody read the book by the London student who tried to build a toaster from scratch (yes from copper ore, etc.)? Kind of a mess but a fun book. I have often wondered about a Darwinian dead-end: "dynamic" magnets. Today, easy to make large, clean DC to power the magnet. Could these be more powerful or otherwise better than permanent magnets? (I won't even joke about the value of super-conducting magnets like in MRI and other applications.) FWIW, when you are winding coils, you can create great motional feedback opportunities. THAT is the way to address distortion, not trivial improvements in pole piece geometry. Ben __________________ For several months now, nearly all my sub-woofer entries have been stalked by a certain humourless "guy" who never tires of lambasting me with long-winded screeds. Sad fellow. I worked at Bell Labs, Murray Hill, 1967-69.
 8th February 2013, 12:32 AM #28 diyAudio Member     Join Date: Jun 2008 Location: N.E. Ohio Wow. You guys should take your attitude somewhere else. Akos, I think you've done some great work, but I wouldn't go out of your way to provide anything, it would appear to be a waste of your efforts. __________________ Regards, Dan
 8th February 2013, 02:34 AM #29 diyAudio Member   Join Date: Jan 2008 Location: Toronto and Delray Beach, FL I believe there is currently some agreement that a forum with a light-handed Moderator can successfully control the crazies and nasties. B. __________________ For several months now, nearly all my sub-woofer entries have been stalked by a certain humourless "guy" who never tires of lambasting me with long-winded screeds. Sad fellow. I worked at Bell Labs, Murray Hill, 1967-69.
 8th February 2013, 03:58 AM #30 diyAudio Member   Join Date: Oct 2011 Thanks Dan, OscarS, the application gives the same results as FEMM. "FWIW, when you are winding coils, you can create great motional feedback opportunities. THAT is the way to address distortion, not trivial improvements in pole piece geometry." I've heard that they're going to use HgBa2 for their new motor structures. It doesn't require as low temperature as the previous ones. Why Modern subs have 2nd & 3rd harmonics of level around -30dB then? I assume they are well addressed. The fun thing is that you can find those curves that were evaluated in JBL papers too. Sure they should have been moderated too. And of course they were crazy, and nasty just because they had been searched for the cure.

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