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Old 6th January 2012, 08:00 PM   #1
Few is offline Few  United States
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Default Planar magnetic with central tweeter?

I haven't seen any tall and narrow commercial planar magnetic systems that use a central tweeter section flanked by midrange sections, all sharing a common diaphragm. I'm envisioning something analogous to ESL systems that use resistors to progressively limit the bandwidth of the signal sent to sections of the diaphragm as the distance from the tweeter increases (except using capacitors instead of resistors). My aim is directivity that doesn't vary strongly with frequency. Have I missed the examples where this has been done or is there a reason it's a bad idea? I'm considering a one-sided (non-push-pull) design mimicking the quasi-ribbon midranges in Magneplanar speakers, but using NdFeB magnets.

Few
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Old 8th January 2012, 03:03 AM   #2
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Hello Few,

The only example I can recall of a quasi-ribbon using a concept like you describe is the Neo8 PDR.

"...the patent pending PDR(progressive drive and radiation) technology allowing for a significantly wider horizontal dispersion above 8 kHz. The PDR technology provides progressively increasing excitation force from the periphery to the center portion of the diaphragm, while creating frequency dependent acoustic dampening and absorption across the diaphragm...."


It is interesting that the idea has not been used on a larger scale transducer similar to ESLs. Perhaps the problem is that the thin/light aluminum conductors required for mid-tweeters to be able to reproduce 20Khz, are not be able to handle the current and resulting heat needed for low frequency reproduction. So, you wouldn't be able to segment a quasi-ribbon the way you can an ESL.

BTW, I think your ladder network would need to be made from inductors, not capacitors as you mentioned.
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Old 8th January 2012, 04:22 AM   #3
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Temperature might be a concern but it'll probably be trickier just to get everything else to work out.

There are several considerations for choosing the foil thickness, width, length, and resistance. I re-derived the equations relating all of those, and did a bunch of planar driver layout-type sample calculations, which are posted in the Planar Speaker Asylum thread at the following URL:

RE: "If we replaced the wire with foil that had 1/4th the mass and 4X the resistance per inch" ... - tom_gootee - Planar Speaker Asylum

Selecting "all" at the top should also show the other posts in that thread, where I did significant related calculations.

As far as the heating effects are concerned, it should be simple to calculate. The total power dissipated would just be iČR for the resistance of the whole conductor. So the power dissipated per inch should just be iČR/Length in inches. I can't remember offhand how to calculate the constant factor to get temperature rise, from that. But it is on line, for things like copper PCB traces. You'll probably have to convert any equations you can easily find from copper to aluminum, or ask in one of the physics forums.
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Old 10th January 2012, 03:05 AM   #4
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Thanks for the responses.

It sounds like the PDR approach has some aspects in common with what I described but also some differences. The idea is to drive the center of the membrane harder than the edges and use acoustic absorption (foam or felt) to reduce the high frequency radiation from the edges. Do I have that right? If so, late night thinking (always dangerous) suggests the high frequency absorption would have the same effect as the inductor ladder (thanks for catching my capacitor mistake) approach I described. It would just limit the bandwidth acoustically instead of electrically. I recently saw another company doing the same thing. I'll have to figure out who that was.

The part about driving the center of the diaphragm harder than the edges at first had me thinking the diaphragm would act as if it were curved (like a Martin-Logan ESL) but that can't be right. The diaphragm might approximate a section of a cylinder when driven, but the concavity of the cylinder would switch as the diaphragm vibrated. It wouldn't have the fixed concavity of a truly curved diaphragm. I guess I'm not really sure what's achieved by the "progressively increasing excitation force."

Thanks also for the comments on conductor mass and current capacity. Since I'd be building from scratch the strength of the magnetic field is not a fixed quantity. I believe Magneplanar does not use NdFeB magnets, although I'd be happy to be corrected on that if I'm wrong. If I'm right about that then I'd be interested in figuring out whether increasing the magnetic field provides a useful amount of wiggle room on the mass or current capacity fronts. Thanks for the link that supplies many of the pertinent equations.
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Old 11th January 2012, 03:37 AM   #5
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I would think that going to neo magnets, which Magnepan does not use, would give a huge increase in wiggle room(s)!

Thinking (late-nightedly) about membrane convexity or concavity, I guess that theoretically you could use superimposed DC voltages to push or pull the conductors (and thus the membrane) into other-than-flat shapes. And, for example, it seems like you could curve the magnet array and then force the membrane to conform, with proper clearances, only when the superimposed DC voltages were present.
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Old 12th January 2012, 03:19 AM   #6
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Hint: Using DC can also be advantageous when you are positioning the conductors and for holding them in place while the adhesive cures.
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Old 12th January 2012, 03:31 PM   #7
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Few: have you had a look at the Sumo Aria? This was designed and manufactured by Highwood Audio in Calgary Alberta Canada in the late 80's early 90's. Superb dispersion, we measured 10 KHz at rt angle to the diaphragm edge we used a 1.25 inch voice coil to drive a large Mylar diaphragm. Think of it as a tweeter with the worlds largest suspension. IMO they did a much better job compared to a Quad ESL63 with about the same radiating area. Very simple and easy load. Zero components in the signal path. Best regards Moray James.
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Old 12th January 2012, 04:34 PM   #8
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Quote:
Originally Posted by gootee View Post
Hint: Using DC can also be advantageous when you are positioning the conductors and for holding them in place while the adhesive cures.
Clever trick


@Few
I had a thought while driving home last night of an acoustic reason why segmented quasi-ribbons may not work like segmented ESLs. It concerns % open area and the resulting response of each of the ribbon segments. Before I bother going any further, can you confirm what typical open area is for the back plate of the single ended quasi ribbon? I remember examining an MMG some years ago and was surprised to find only rows of very small holes between the magnets. Estimated open area for the MMG backplate would be <10%. Is this typical?
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Old 13th January 2012, 06:53 AM   #9
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Bolerest: The small holes that you see in the back plate help provide damping on the diaphragm. Best regards Moray James.
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Old 14th January 2012, 12:27 AM   #10
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Yes I am familiar with the Sumo Aria, but of course not as familiar as you are. Once I'm done not building my planar magnetic speakers and have finished not finishing my wire-stator ESLs, maybe I'll start (but not finish) a project to duplicate the Aria. Sounds like fun! The Aria seems like an elegant idea for sure. How was the impulse response?

Bolserst: I wouldn't have said that 10% open area is typical based on photos I've seen of planar magnetic systems but I haven't done a careful study. I'm surprised to hear that's what you saw, in fact. I was planning on much more open area (50% or a bit more) and then trying some felt or wool or one of the other usual suspects to absorb some of the high frequencies in the back wave and damp some of the cavity resonances. The openings in Bohlender-Graebener drivers look larger than the ones you've described

Click the image to open in full size.

and pictures I've seen of the old Apogee woofer sections showed more open area, if I remember correctly (I can't seem to find the pictures even though they were posted here on diyAudio). In any case, I realize you're talking about high frequency drivers.

My goal in the present project is not to try to curve the membrane. I want to use driver width to control directivity on this one. Nonetheless, interesting ideas are always...interesting, and of course, appreciated.

Few

Last edited by Few; 14th January 2012 at 12:37 AM.
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