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Old 3rd March 2011, 11:13 PM   #1
Spatz is offline Spatz  Germany
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Default New design for segmented ribbon tweeters

Hi everyone,

I found a new design for ribbon tweeters:

To maintain wide dispersion (or the sound of ribbons), ribbons must be slim, but to maintain a high SPL and low usable frequency ribbons should have a large area, usually meaning long ribbons or wide gaps with low magnetix flux.

By segmenting a wide gap into 3 smaller gaps by inserting two additional lines of magnets, you can get high dispersion at high frequencies, a pretty good flux and enough surface area to give the ribbon a low crossover:

Click the image to open in full size.

This a 243 mm long ribbon, the magnets are two lines of 120x10x10 mm with a 3 mm gap between them to dampen the lowest bending wave of the ribbon. The gaps are 28, 10 and 28 mm wide. The iron parts have a width of 30 mm (left/right) respectively 36 mm (upper/lower). There is a 10 mm gap between the magnets and the iron parts, to have some space for mouting the ribbons. The ribbon areas are 130 cm² for the midrange and 22 cm² for the tweeter. that should be enough, its similar to a 170mm midrange and three 25mm tweeters. Overall depth is 10mm.

A simulation with N40-neodym-magnets and pure iron gives this result, with N42 magnets, B should be a little higher:

Click the image to open in full size.

Left is the wider gap, right the smaller one, B on the plateaus is 0.47 Tesla.

If you want to build one, I'd recommend gluing this whole thing on a suitable faceplate, without a solid mount the magnets in the middle could get loose. Heres a sketch with folded ribbons for the midrange and straight ribbon for the tweeter (straight ribbons cause less distortion, but are more critical at "large" excursions):

Click the image to open in full size.

Of course you could try other lengths of the overall construction as well, but if you make the ribbon longer, you should make the outer magnets larger and if you make the outer magnets larger, you should make the iron parts wider as well. I have a simulation with twice the ribbon length, doubled magnet size and doubled iron width. B is then around 0,56 Tesla then.

Feel free to use this idea for your own projects and post a comment here!

Bye,

Spatz
Attached Images
File Type: png RibbonSimulation.png (51.7 KB, 1086 views)
File Type: png RibbonGraph.png (25.4 KB, 1052 views)
File Type: png RibbonSketch.png (55.6 KB, 595 views)
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Old 5th March 2011, 08:14 AM   #2
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>with a 3 mm gap between them to dampen the lowest bending wave of the ribbon

this is a complete nonsense ...


Problem No.1 : Glueing the (middle) magnets on one side only is too risky when you consider long-term stability and corrosion of neodym magnets , the attracting forces between the only 10mm spaced apart magnets are quite high and there is no iron on their back to hold them in place ...

Problem No.2 : Unsymmetrical field distribution across the gap (more pronounced in the outer gaps) , this makes torsional deformation of the ribbon (rocking mode) ...

Problem No.3 : Very unsymmetrical field distribution along the depth of the outer gaps , this stresses the ribbon mechanically and can lead to long time failure ...

Problem No.4 : The current in the outer ribbons will induce eddy currents in the iron return path structure and due to the inherent asymmetrical layout of your triple ribbon structure it raises the same problem like "Problem No.2" but this time the force vectors on both sides of the ribbon are reversed , also the force difference could be much higher than "No.2" .....

Problem No.5 : Even if you solved "Problem No.2" there is still an uneven field distribution across the width of the gap , RAAL found a solution named "Equafield" , I think it contributes significantly to the high power stability performance of their designs .

Problem No.6 : Unavoidable close acoustic coupling between your triple ribbon structure leads to modulation of the center ribbon output . This is the price to pay for wider HF radiation ...

Simulating a virtual depth of 10mm does nothing in FEMM , which is still a 2D code . Optimizing the field distribution in all directions needs a true 3D code , or you have to live with a few percent of errors .

Designing high quality Ribbon Transducers is not so easy as it seems , I have some serious doubts that any DIY design could stand against a well developed commercial product .

Sorry for the bad news - J-Marv
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Old 6th March 2011, 06:23 PM   #3
Spatz is offline Spatz  Germany
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Marvin!

Your language looks so normal in English... what's wrong with you?

The gap was tested by "LineArray" from Dipol 08 Lautsprecher sowie neue Vollbereichs-Flchenstrahler Modelle and he said it had a pretty good effect... so, why not? I think you must remember this thread, as you gave your 1,5 cents there as well.

Problem #1: Just a mechnical problem, the right glue should do it, maybe an additional brace in the back of the tweeter should give the last bit of stability. Do neodym magnets corrode by themselves?

Problem #2: Field distribution across the gap is flat, an by "across" I mean from iron part to iron part, and by "flat" I mean a straight line... so no rocking mode here...

Problem #3: Field over depth is not flat, yet symmetrical, more like a gaussian curve. 10% decrease in B is at +/- 4mm, that should be enough.

Problem #4: Those eddy currents, if they should turn out as a problem, should be reduced by using not a solid iron, but transformer sheets.

Problem #5: Same as in #2: the B-curve is flat all across the gap.

Problem #6: Woofers near a ribbon should be even worse than a ribbon next a ribbon, plus this is no problem if you use a transconductance amplifier...

From what I read, FEMM simulations are pretty reliable. Do you have sources that state otherwise? What is a good software to simulate this ribbon in 3D?

Interestingly, there are many DIY-ribbons out there and most of their users are pretty happy...

Bye,

Spatz
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Old 10th March 2011, 11:42 PM   #4
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Hello Spatz,

my own ribbon tweeter design built in 2002 had vertical folds ...

For this reason it has very high stiffness along the longer
(vertical) axis, which a conventional ribbon does not have.

An audible bending mode occured as a consequence, which could
be mitigated to be inaudible by introducing a center gap to the
magnets, which was considerably wider than 3mm, i guess it was
double at least. That was done to weaken the field in the center.
A modification - or 'workaround' if you like - which worked very
well in that specific design to solve that specific problem.

When using the common horizontal folding or flat membrane
configuration, that problem will not occur and the gap is presumably
useless if not detrimental.



Cheers
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File Type: jpg BändchenNahGross.jpg (43.6 KB, 210 views)
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Last edited by LineArray; 10th March 2011 at 11:50 PM.
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Old 2nd June 2013, 08:31 PM   #5
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Quote:
Originally Posted by Spatz View Post
Marvin!

Your language looks so normal in English... what's wrong with you?

The gap was tested by "LineArray" from Dipol 08 Lautsprecher sowie neue Vollbereichs-Flchenstrahler Modelle and he said it had a pretty good effect... so, why not? I think you must remember this thread, as you gave your 1,5 cents there as well.

Problem #1: Just a mechnical problem, the right glue should do it, maybe an additional brace in the back of the tweeter should give the last bit of stability. Do neodym magnets corrode by themselves?

Problem #2: Field distribution across the gap is flat, an by "across" I mean from iron part to iron part, and by "flat" I mean a straight line... so no rocking mode here...

Problem #3: Field over depth is not flat, yet symmetrical, more like a gaussian curve. 10% decrease in B is at +/- 4mm, that should be enough.

Problem #4: Those eddy currents, if they should turn out as a problem, should be reduced by using not a solid iron, but transformer sheets.

Problem #5: Same as in #2: the B-curve is flat all across the gap.

Problem #6: Woofers near a ribbon should be even worse than a ribbon next a ribbon, plus this is no problem if you use a transconductance amplifier...

From what I read, FEMM simulations are pretty reliable. Do you have sources that state otherwise? What is a good software to simulate this ribbon in 3D?

Interestingly, there are many DIY-ribbons out there and most of their users are pretty happy...

Bye,

Spatz
N0. 6 ??...... I would probably Isolate the rear radiation of the tweeter from the mid's , but this co-location should have no problem crossed over below 4000 hz . As you said, even higher would still be better than most woofer/tweeter combos ever achieve.
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Old 3rd June 2013, 10:00 AM   #6
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If I recall my physics correctly , doesn't placing 3 narrow sound sources next to each other have the same effect as I wider sound source?
e.g. : 3 x1cmm ribbon dispersion = 1x3cm ribbon- ie : actually narrower...?
if you want wide dispersion and higher sound level maybe go to stacking one on top of the other - ie: Linesource..?


I could by wrong..
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Old 3rd June 2013, 10:50 AM   #7
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If I recall my physics correctly , doesn't placing 3 narrow sound sources next to each other have the same effect as I wider sound source?
e.g. : 3 x1cmm ribbon dispersion = 1x3cm ribbon- ie : actually narrower...?
if you want wide dispersion and higher sound level maybe go to stacking one on top of the other - ie: Linesource..?


I could by wrong..
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Old 3rd June 2013, 01:32 PM   #8
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Older thread... but imo, the problem is mostly vertical dispersion, not horizontal dispersion from a narrow & (relatively) tall line source. So, I have no idea what problem these folks were trying to solve.

Also, as you lay line sources (ribbons) side by side there are beaming effects WRT frequency left to right (horizontally).

Stacking vertically adds SPL but also creates lobing in the vertical plane since there is a finite distance between each adjacent vertical element, and the element(s) beyond...

The solution most often used is a tall ribbon for the tweeter. This obviates the vertical dispersion issue, increases SPL (more size) and if thin enough maintains a good horizontal dispersion.

Still, it's a compromise no matter what choices one makes.

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Old 5th June 2013, 09:55 PM   #9
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Quote:
Originally Posted by george a View Post
If I recall my physics correctly , doesn't placing 3 narrow sound sources next to each other have the same effect as I wider sound source?
e.g. : 3 x1cmm ribbon dispersion = 1x3cm ribbon- ie : actually narrower...?
if you want wide dispersion and higher sound level maybe go to stacking one on top of the other - ie: Linesource..?


I could by wrong..
No doubt, the wider the transducer, the narrower the beam width.
So the midrange pair would have a narrower beam width. However if you look at the beam width of most any transducer below a typical crossover point. (lets say 3500 hz) Beam width this low is typically not a problem. Off axis response probably rivals a 5 1/4" midrange. Now the dipole guys might have something to say about this.
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