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Old 26th January 2012, 11:19 PM   #21
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Originally Posted by Few View Post
The opposing magnets flatten the field lines. Does this matter and is it worth the extra cost and complexity? Several well regarded manufacturers use purely single-sided designs so that suggests things can be made to work well with a simpler system. The results with the opposing magnets look enticing, though.
Your FEMM solutions reminded me of one other reason full range single ended planar magnetic designs may tend to have the tweeter section off to the side. At low frequencies approaching diaphragm resonance modes, the IM distortion would be the highest for a tweeter section placed in the middle of the diaphragm where the membrane displacement is largest.

The Wisdom Audio driver you mentioned seems to solve this problem by using individual diaphragm sections for the central tweeter(750hz - 20kHz) and the two outer panel areas(80Hz - 750Hz).

Quote:
They also use two-conductor alternative to Magneplanar's single-conductor per magnet gap approach.
Looking at your FEMM results again, it looks to me that multiple conductors spread in the gap would not be nearly as advantageous for single-ended magnet systems as for symmetric front-and-back arrangement used by Wisdom and BG. If I am seeing the image correctly, looks like they have 4 or 5 conductors in the tweeter gaps.

Oh, speaking of BG, thought you might be interested in the patent for their Neo10.
Figure 3 shows 4 conductors per magnetic gap.
Patent US8031901 - Planar speaker driver - Google Patents
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Old 27th January 2012, 01:25 AM   #22
Few is offline Few  United States
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Interesting thought regarding intermodulation distortion. I don't yet have a sense of what the frequency of the fundamental resonance of the diaphragm will be. I know I'm not telling you anything you don't know when saying that the fundamental mode is the one that puts an antinode where the central tweeter would be and would therefore be the most problematic. I've been thinking I could keep that fundamental resonance frequency quite low and not excite it much with music because:

1) I wouldn't need to use high diaphragm tension to counteract the DC offset exhibited by biased ESL diaphragms. The low tension would mean a low resonance frequency.

2) I'll be gluing Al strips on a very thin mylar diaphragm, so the added mass should drive resonance frequency down.

3) I was hoping to cross over to a column of open baffle woofers somewhere in the 300-500 Hz range, above the range I hope to be able to force the diaphragm resonance into.

My intention to try low tension is also motivated by Capaciti's findings that a cleaner decay is obtainable by minimizing diaphragm tension and using damping material around the diaphragm edges. Your post raises the point that I should be prepared, at a minimum, to place strategically some diaphragm damping or clamping to be sure a central tweeter section doesn't get over-driven. Thanks for provoking that thought.

My view of the Wisdom system left me with the impression that there were just two conductors per gap in the woofer section. I'll definitely take another look. If your point was that multiple conductors will spread the conductors into regions where the field lines aren't oriented advantageously, I certainly agree. That's why I was going to stick with one or two conductors that I could fit into the "hot spot" of the magnetic field. If I've misunderstand your intention, please let me know.

Few
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Old 27th January 2012, 01:48 AM   #23
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Originally Posted by Few View Post
Interesting thought regarding intermodulation distortion. I don't yet have a sense of what the frequency of the fundamental resonance of the diaphragm will be. I know I'm not telling you anything you don't know when saying that the fundamental mode is the one that puts an antinode where the central tweeter would be and would therefore be the most problematic. I've been thinking I could keep that fundamental resonance frequency quite low and not excite it much with music
Yeah, this would certainly be more of a problem for a full range design than a hybrid like you have in mind where you design for resonance placed well below crossover. But, I'm sure you know that the dipole woofers placed beside the ribbon will have no problem exciting the resonance through acoustic coupling. I think your thought that some strategically placed damping may be in order is a good one. Note that all sections of the Wisdom drivers had some damping applied to them.


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My view of the Wisdom system left me with the impression that there were just two conductors per gap in the woofer section. I'll definitely take another look.
Looked to be 2 conductors per gap in the woofer sections, 4 or 5 per gap in the tweeter.


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If your point was that multiple conductors will spread the conductors into regions where the field lines aren't oriented advantageously, I certainly agree.
Yes, that's what I was getting at.
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Old 27th January 2012, 03:24 AM   #24
Few is offline Few  United States
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Good point about the acoustically driven resonance. Dang. I guess I'll have to orient the woofers perpendicular to the planar magnetic diaphragm (and aim their dipole node at the listener) I guess damping/clamping will be the plan.

My SketchUp diagram shows a different spacing of conductors than what I'd likely aim for. I was being lazy when drawing. I was actually thinking I would have a small gap between the two conductors in the same gap, and a larger gap between conductors over the magnets where the force wouldn't be in the right direction. I still need to play with the constraints of impedance, BL product, radiating area, magnet cost, expansion of the universe...man what a way to make a living.

Thanks for catching the flaw in my reasoning.

Few
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Old 28th January 2012, 02:34 AM   #25
gootee is offline gootee  United States
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Quote:
Originally Posted by Few View Post
Interesting thought regarding intermodulation distortion. I don't yet have a sense of what the frequency of the fundamental resonance of the diaphragm will be. I know I'm not telling you anything you don't know when saying that the fundamental mode is the one that puts an antinode where the central tweeter would be and would therefore be the most problematic. I've been thinking I could keep that fundamental resonance frequency quite low and not excite it much with music because:

1) I wouldn't need to use high diaphragm tension to counteract the DC offset exhibited by biased ESL diaphragms. The low tension would mean a low resonance frequency.

2) I'll be gluing Al strips on a very thin mylar diaphragm, so the added mass should drive resonance frequency down.

3) I was hoping to cross over to a column of open baffle woofers somewhere in the 300-500 Hz range, above the range I hope to be able to force the diaphragm resonance into.

My intention to try low tension is also motivated by Capaciti's findings that a cleaner decay is obtainable by minimizing diaphragm tension and using damping material around the diaphragm edges. Your post raises the point that I should be prepared, at a minimum, to place strategically some diaphragm damping or clamping to be sure a central tweeter section doesn't get over-driven. Thanks for provoking that thought.

My view of the Wisdom system left me with the impression that there were just two conductors per gap in the woofer section. I'll definitely take another look. If your point was that multiple conductors will spread the conductors into regions where the field lines aren't oriented advantageously, I certainly agree. That's why I was going to stick with one or two conductors that I could fit into the "hot spot" of the magnetic field. If I've misunderstand your intention, please let me know.

Few
I think that it is only (or at least mainly) that there could be more excursion in the center than at the edges, which would modulate a tweeter more than if it were placed near an edge. I am also thinking that, maybe, looking at it like it's a classic "elastic membrane vibration" situation, with nodes and anti-nodes, etc, "should" not be too relevant to this application.

The foil strips will not be vibrating like strings would, with waves superimposed on them, such that a wave equation or something predicted by the calculus of variations should be used to best-describe their motion. Nor will the membrane (except maybe a little, but only in the most-basic "single" mode, I think). No, I think that this is (or should be) a fundamentally different type of system, for the most part, and for your purposes, mostly (except maybe for basic resonance-type problems).

Think about how the membrane is driven: It is not the classical single-point forcing function (or initial displacement) of vibrating elastic membrane theory. Essentially, the electric current is the same, everywhere, at any instant (at least in terms of audio frequencies). Therefore, the entire membrane (more or less) will be forced to move forward and backward, all at the same time, except that the edges are constrained.

In some ways, this has much more similarity to a vibrating truly-rigid plate than it does to a vibrating elastic membrane. In fact, if strong-enough forces were induced on the current by the magnetic field, and the edges were not constrained, then it would be almost identical to a vibrating completely-rigid plate, because the membrane basically could not bend at all and would always stay perfectly flat/planar, if all points on it were controlled with exactly the same magnitude of force and thus had the same displacement, at all times.

So, for the most part, we would want to hope (if the magnetic field were strong-enough) that all points on the membrane would move in unison, with identical displacements, except near the edges. But, of course, because the edges are constrained and the membrane is slightly elastic, the displacement might continue to increase as we look farther from the edges. But we should hope and would like to be able to expect that there will be no other wave-like structures in the displacement itself, e.g. at any harmonics of the driving signal's frequency components, and that the elastic effect wouldn't become a dominant characteristic.

It might be nice if we could use a non-elastic membrane with an elastic area only near the edges, or, ideally, maybe, an unconstrained non-elastic membrane, driven all the way to the edges. (I suppose that with a typical real membrane there might also be some good way to lower the driving forces closer to the center, to make the membrane stay more planar.)

I'm just making this up as I go and everything I've said could be wrong. And even if not, reality probably includes some combination of "rigid" controlled planar motion and classical elastic membrane motion, for planar magnetic types of speakers.

At any rate, except for lower cost or greater simplicity, I can't see any reason that the tweeter should be on the same membrane as anything else, at least not without some mechanical isolation mechanism. It would just be one more way to get distortion. And for anyone trying to make a full-range planar setup, it would also be a limiting factor for low-frequency amplitude, as it is with my Magnepan MG-12/QR speakers, now that I have enough power to drive them to extremely-loud levels without amplifier clipping (400 W/ch @ 4 Ohms): The bass begins to significantly distort the tweeter before anything else as bad happens.

Cheers,

Tom

Last edited by gootee; 28th January 2012 at 02:46 AM.
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Old 28th January 2012, 03:50 AM   #26
a.wayne is offline a.wayne  United States
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Just dawned on me that the big Mbl speaker has a central tweeter..Ok not a regular planer magnetic , but ...
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Old 29th January 2012, 04:43 AM   #27
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Good point about the acoustically driven resonance. Dang. I guess I'll have to orient the woofers perpendicular to the planar magnetic diaphragm (and aim their dipole node at the listener) I guess damping/clamping will be the plan.
I didn't mean to imply the situation was unworkable, just that you needed to keep it in mind.
You might check out how Martin Logan dealt with this issue in their Statement 2 hybrid ESL.
Adding an air gap between your planar baffle and your dipole woofer column will help.
Adding a rearward projecting baffle board on the inner edge of the woofer column will as well.
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Old 29th January 2012, 04:53 AM   #28
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Originally Posted by gootee View Post
Think about how the membrane is driven: It is not the classical single-point forcing function (or initial displacement) of vibrating elastic membrane theory. Essentially, the electric current is the same, everywhere, at any instant (at least in terms of audio frequencies). Therefore, the entire membrane (more or less) will be forced to move forward and backward, all at the same time, except that the edges are constrained.
Hello gootee,

It seems no matter how uniformly an edge clamped membrane is driven, it still tends to do what membranes like to do when excited with transients...form nodal patterns. The lower order odd modes which tend to be dominate, have the center part of the membrane moving considerably further then the edge.

I have measured them on ESLs:
ESLs have bad decay plots

They were also easy to see with a strobe light when driving a planar like the MMG.
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Old 30th January 2012, 01:14 AM   #29
Few is offline Few  United States
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I didn't mean to imply the situation was unworkable, just that you needed to keep it in mind.
That's how I took it; the melodramatic response was all in fun. There will be at least some air gap between the woofers and planar magnetic so I'll first determine whether there's a problem and if there is I'll seek ways to cut down the pressure gradient or clamp the diaphragm, or some combination.

Thanks for providing the link to your ESL mode experience. I was going to try to dig that out as evidence that the drum head modes really do matter but I'm glad you beat me to it. It's too bad resonant systems are so easy to make resonate!

Few
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Old 30th January 2012, 01:25 AM   #30
Few is offline Few  United States
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At any rate, except for lower cost or greater simplicity, I can't see any reason that the tweeter should be on the same membrane as anything else, at least not without some mechanical isolation mechanism.
For me lower cost and greater simplicity are significant drivers of the design. I also want to maximize the symmetry of the system, especially those parts that radiate short wavelength sound, so that it behaves as much as possible like a single full-range unit.

The central tweeter that was the point of this thread is intended to achieve a cylindrical wave front analogous to the spherical wave fronts generated by speakers from Thiel, Quad, KEF, and quite a few others. There seems to be growing consensus that off-axis response is more important than previously appreciated (likely to be true especially in my VERY live listening room) so I want to ensure the on-axis and reflected off-axis sounds are as similar as possible. If the tweeter is off to the side instead of centered it'll be harder to ensure the reflections mimic the on-axis sound.

Few
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