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Old 20th May 2012, 09:49 AM   #11
Zen Mod is offline Zen Mod  Serbia
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tnx man .
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Old 20th May 2012, 12:22 PM   #12
Magura is offline Magura  Denmark
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I have a similar project going. It's been muffballed lately though, due to time constraints.

I have made a motor similar to yours.

I have opted for 6.5", but with a +/-5mm excursion, to get the full spectrum.

The only way as I see it, to get up to 20kHz, is to make a light and stiff cone.
My solution to this, has been to go the composite route.
I have currently made molds for the spider, the surround, the VC, the phase plug is also made, and the mold for the cone. Lacking so far is a suitable basket, and a couple of days work.

Thanks for posting your project, it sure is interesting, and you seem to have the skills required to pull this off.


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Old 20th May 2012, 12:37 PM   #13
Hentai is offline Hentai  Romania
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The reason for this basic shape of the magnetic structure, as i can understand it, is that the middle pole piece, the core of the field coil saturates very easily. It has to be very thick. Mine is 80mm in diameter and the shrinks to create the magnetic gap.
In the rest of the circuit i looked out for signs of saturation but also didnt want to add too much iron because of cost and weight. Below is a picture with the bottom plate thinner and flat. You can see there is trouble there so i made the common truncated cone shape.
Click the image to open in full size.

More on Voice Coil:

The magnetic gap is 12mm in height and this is the VC's height limitation as well. At first i was looking into a 9 mm height vc but the xmax figure was unacceptable. A thinner wire tho allows me to reduce the height to 6 mm to give the theoretical +/- 3mm of excursion. I made the pole piece 1 mm above the top plate to make a more gentle slope on [B] curve in case this excursion is exceeded. Now i think it could've been 2mm as well or maybe better.

When calculating the voice coil parameters at first i was obsessed with low mass. I was even thinking of a single layer of such thin wire if it was possible. It presented too many issues tho both electrical and mechanical.
With a single layer i would have a small BL value, high DCR and the voice coil would be too fragile for a 12 incher.
So i wondered with such a thin wire how much mass can i save by not going multilayer? It turns out that gain in mass has a too small effect on efficiency compared to the increase in BL.
Of course one needs to consider this gain in mass in addition to the cone mass. The chart bellow compares the two efficiency values for different cone mass. As you can see the cone has to be very very light, has to have a mass very close to that of the wire to make a difference. Once the weight lies mostly in the cone its best to put as much wire as possible in order to get high BL.
Click the image to open in full size.

One more aspect i needed to clear. The actual motor force figure. DCR opposes current so it's not enough to have a high BL i needed to factor in DCR. Looking at efficiency formula the figure [BL^2/Re] stands out. And it makes sense as this figure is the actual motor strength figure. This is a constant for every loudspeaker even the ones with dual voice coils. In dual voice coils BL can change depending on how you wire the coils but Bl^2/Re remains constant.
Considering all this i came to a 16ohm nominal voice coil made from 2 paralel coils of 14.6m each. One can eliminate the parallel windings by using thicker wire.

6mm Hvc

Zn---Len---Rdc---Obs--------------BL------Mass---BL^2/Rdc
16---7.33--14.09--Single layer------10.995--0.36---8.580303971
8----7.33--7.04---2 Layer Parallel---10.995--0.72---17.16060794
16---14.6--14.09--4 Layer 2 S 2 P---21.99---1.44---34.32121588

Bellow are some charts i made when doing calculations for this specific 0.13mm ECCA wire.

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Attached Images
File Type: jpg saturation.jpg (56.1 KB, 33 views)
File Type: jpg VC_efficiency.jpg (137.9 KB, 36 views)
File Type: jpg VC_multi_BL_DCR_mass.jpg (123.5 KB, 28 views)
File Type: jpg VC_single_BL_DCR_mass.jpg (138.9 KB, 24 views)
File Type: jpg wire_mass_vs_length.jpg (102.4 KB, 32 views)
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Old 20th May 2012, 12:49 PM   #14
Hentai is offline Hentai  Romania
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Quote:
Originally Posted by Magura View Post
<snip>
I have opted for 6.5", but with a +/-5mm excursion, to get the full spectrum.

The only way as I see it, to get up to 20kHz, is to make a light and stiff cone.
My solution to this, has been to go the composite route.
I have currently made molds for the spider, the surround, the VC, the phase plug is also made, and the mold for the cone. Lacking so far is a suitable basket, and a couple of days work.
Hello Magura,

I'm glad you joined this thread, i read about your project on Zen Mod's forum.

Indeed, i too, place a high importance on low mass and i hope petals will make my cone rigid. So far i haven't done much work on suspension system. My goal is a fs around 50hz with a Mmd of 24g. But i will clear on this next week most probably. Im making a tool to help me create the cone and i can estimate a mass value then.

Keep in touch.

-Andrei
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Old 21st May 2012, 12:24 PM   #15
Hentai is offline Hentai  Romania
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Copper-clad aluminum wire has arrived.

Click the image to open in full size.
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Old 21st May 2012, 04:39 PM   #16
Magura is offline Magura  Denmark
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Where did you find that?

......in smaller quantities that is.

Magura
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Old 21st May 2012, 06:32 PM   #17
Hentai is offline Hentai  Romania
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I got it all the way from China directly from factory. It's very hard to get indeed. The trick is to ask for sample.
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Old 21st May 2012, 06:53 PM   #18
Zen Mod is offline Zen Mod  Serbia
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I presume that Metaldanske knows how to use PM button

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Old 24th May 2012, 01:47 AM   #19
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Hi Hentai,

I enjoyed reading your field coil education. Constructing an execeptional cone, surround, and suspension has driven many engineers mad, so stock up on good California red wines now.

It is possible to construct an underhung radial NdFeB speaker motor with over 2T gap flux by using a magnet-to-magnet topology across the gap. When one works through the total T/S parameter requirements for an optimum speaker+cabinet, this high gap flux creates a very low Qts which typically requires horn loading to get extended bass, or at least a 4-way design to compensate for the limited driver bandwidth. The engineer has to balance efficiency vs. bandwidth vs. cabinet effects(like high phase delays from horns).

Do you have personal experience with beaming/lobing effects from using a 12" full range? Controlled directivity speakers which use a high efficiency 12" midbass typically cross over to a 90x40degree horn at 900Hz or a 60x40 degree horn at 1200Hz. The Levinson Daniel Hertz is one example of how a high efficiency 12" is used with controlled directionality 60x40 horn tweeter. Do you have any back up plans that use your 12" speaker in a 3-way design?
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Old 24th May 2012, 10:05 AM   #20
Hentai is offline Hentai  Romania
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Hello LineSource,

Yes the mechano-acoustic part of the loudspeaker is the hardest to make. The phenomenons are too complicated to predict accurately. Red wine sure comes in handy .

Depending on final Mmd i want Qes between 0.15 to 0.25. The overdamped motor coupled with the low Mmd should result in a sensitivity rise with increasing frequency. In a bass reflex alignment you will need to lower the current through the field coil to achieve a better tonal balance. In horn however this tilt in response can be extremely beneficiary, especially in a FLH and BLH (kinda like the Westminster), as it will lift the lower sensitivity of low and mid band to match the high frequency instead of just using xover to cut the high band.

There is much to be said about beaming and about whizzer's role in the whole picture. There isnt much data on the development of these components either. But lots of opinions and tested units. In a few hours when i will get home i will post some simulations results. I used Axidriver to try and simulate beaming of the cone. Unfortunately there is no option to add whizzer cone so i made separate simulation. Of course the issue of combining the two is another problem in itself.

You can get high flux density with neo magnets but its pretty hard to work with pre-magnetized neo and metal parts
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