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Old 4th February 2012, 02:51 PM   #41
gootee is offline gootee  United States
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Quote:
Originally Posted by Few View Post

<snipped>

I fear that DC current-based measurements may lead us astray, depending on what we think we'll learn from them. Resonances within the driver's bandwidth (or even outside that bandwidth if driven acoustically, as Bolserst pointed out) are likely to lead to larger displacements than the rest of the frequency range and DC measurements won't reflect that effect. I think AC signals will be necessary to measure displacement limitations. I do agree an adjustable diaphragm-to-magnet distance would be handy. I hope to build a system with interchangeable spacers for that reason.

Thanks again for the links.
Few
Concur.
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Old 23rd February 2012, 12:08 AM   #42
Few is offline Few  United States
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I've come up with a promising way of stacking Fiskar rotary cutting blades, spaced with fender washers, with the purpose of cutting conductors with uniform width and spacing. A whole array of ten parallel conductors can be cut in one swipe. The prototype is ugly but quick tests suggest the approach has merit. Too bad the rotary blades are priced as if they were made of platinum.

Does anyone know how wide the Magneplanar quasi-ribbon midrange drivers are in, say, the 3.7? I haven't been able to dig that up despite multiple web searches. I'm not intent on duplicating their dimensions but they would provide useful baselines.

Also, the online owner's manual for the 3.7 doesn't specify the crossover frequencies. Does anyone happen to know what they are?

Thanks for any info you can provide.
Few

Last edited by Few; 23rd February 2012 at 12:10 AM.
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Old 27th February 2012, 01:15 AM   #43
gootee is offline gootee  United States
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Have you looked on the planar speaker asylum?

I'd guess 0.1 inch.
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Old 28th February 2012, 01:01 AM   #44
Few is offline Few  United States
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I'm looking for the width of the entire midrange diaphragm. I'm guessing it's something like 3" but that's based on absolutely nothing.

I've posted a similar question at the Planar Circle but haven't gotten any nibbles yet.
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Old 28th February 2012, 01:47 AM   #45
Few is offline Few  United States
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This interesting example of a central tweeter in a planar magnetic system just popped up in another diyAudio thread. I thought I'd cite it here in case others have an interest in the original point of this thread. It's clearly different from the tall narrow aspect ratio I'm pursuing, but their design suggests the intermodulation distortion discussed earlier in this discussion was deemed to be a concern.
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Old 29th February 2012, 12:19 AM   #46
gootee is offline gootee  United States
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Quote:
Originally Posted by Few View Post
I'm looking for the width of the entire midrange diaphragm. I'm guessing it's something like 3" but that's based on absolutely nothing.

I've posted a similar question at the Planar Circle but haven't gotten any nibbles yet.
The Planar Speaker Asylum (and Magnepan Users Group) is at:

The Planar Speaker Asylum

I think that just the QR foil tweeter section on my Magnepan MG-12/QR speakers is at least that wide (I'm TDY at the moment and can't check). I would have to guess that a mid-range would be wider. But I could be wrong.

Last edited by gootee; 29th February 2012 at 12:22 AM.
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Old 8th March 2016, 03:41 PM   #47
Edmund is offline Edmund  Netherlands
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Quote:
Originally Posted by Few View Post
In order to make it easier to compare the magnetic field's strength, orientation, and homogeneity for different magnet spacings I drew up a FEMM model with three different spacings. I'm still using NdFeB magnets that have 1/4" x 1/4" cross sections. The attached figure shows magnet spacings of 1/2", 3/8", and 1/4" (reading from the left to the right side of the diagrams).

I also calculated the absolute value of the component of the field strength that actually moves the diaphragm---the component of the field that lies parallel to the diaphragm. I assumed the diaphragm and conductors would be 1/8" above the faces of the magnets. The upper graph shows the result.

It's interesting to see the saddles in the graph for the 1/2" spacing case (left end of the diagram). That means that near the edges of the magnets the field lines aren't oriented in the right direction, but they're so closely spaced that the desirable component of the field is still largest there.

For the sake of argument, let's assume 1/4" wide conductors. My take on the upper diagram is that you're better off fitting two runs of those conductors above magnets spaced by 1/2" rather than a single run over the 1/4" gap. The field strength in the 1/2" case is more than half of what it is in the 1/4" case, and by doubling the conductor length you get double the force. There's also a larger radiating area. All this adds up to higher sensitivity and more acoustically open area behind the diaphragm. Can anyone confirm or shoot down this reasoning? I realize some method for ending up with the same impedance in both cases will be necessary in order for this simple analysis to apply. I'm also working with a fixed number of magnets, in order to compare approaches with similar cost.

I hope the graphs make some sense...

Few
Hi Few I hope you are still here...
Can you clarify what you mean by the two conductor VS one conductor situation. How can the spacing be different, is there a picture?
I asked myself the question about multiple conductors VS a single conductor.
What exactly happens? If it is a serial conductor the impedance will get bigger
which might be good, the mass will get bigger too.
What about the driving force? Compare one conductor VS a quadruple loop which gives four conductors over the same length of magnets.
What will happen now with the driving force?

Another question, how will an iron plate influence the field strength of the magnets and what if we make -your- magnets 1/8 inch thick?


Edmund
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Old 8th March 2016, 04:20 PM   #48
lowmass is offline lowmass  United States
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I have built a number of prototypes in the past to work these ideas. A tru curved panel plainer magnetic,a segmented mutiple flat sections curved panel, two way with MRT mounted between two bass panels, and two way with MRT mounted centered on the same diaphragm as the bass panels. These have all been built using foils from 4 micron to 24 micron and mylars the same. Yes Ive been a busy boy
The best was the two way with the MRT mounted in the center between two bass panel.

The curved panels with no xovers have their merits ( espscially the 4 micron version BUT in the end the compermises to bass performance, directivity, freq response, sensativity etc IMO are trumped by a well design 2 way where the indavidual drivers can be optimized better. These drivers are well behaved and with simple xovers and concentric MRT the presentation is very close to single driver but better in a number of ways so it wins in the end. BTW this layout was consistantly less "bright" than the typical layout as in Apogee with MRT to the side reguardless of freq response tayloring.

BTW just a heads up, the higher mass diaphragms such as Apogee and Magnaplainer very much need the acustic resistance of the magnets and perf steel out back. About 12 % open area works well. Its a way to damp the diaphragm.

Last edited by lowmass; 8th March 2016 at 04:24 PM.
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Old 9th March 2016, 07:30 PM   #49
Few is offline Few  United States
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Quote:
Originally Posted by Edmund View Post
Hi Few I hope you are still here...
Can you clarify what you mean by the two conductor VS one conductor situation. How can the spacing be different, is there a picture?
I asked myself the question about multiple conductors VS a single conductor.
What exactly happens? If it is a serial conductor the impedance will get bigger
which might be good, the mass will get bigger too.
What about the driving force? Compare one conductor VS a quadruple loop which gives four conductors over the same length of magnets.
What will happen now with the driving force?

Another question, how will an iron plate influence the field strength of the magnets and what if we make -your- magnets 1/8 inch thick?

Edmund
Hello Edmund,
I'm still on the right side of the turf, as the old saying goes. I don't have a drawing handy (I can make one later if it turns out to be necessary). It's been awhile, but I think the point was that I could use closely spaced magnets with a single conductor in the magnet gap, or use a wider magnet gap that would accommodate two conductors. The magnetic field strength is smaller in the wide-gap case but you make up for it by having a BL product (field strength times length of conductor) that is twice as large. In fact, I had concluded you more than make up for it for the two cases I was comparing.

In general the driving force depends on the product B x L as described above. Using parallel rows of magnets, widely spaced, reduces B (the magnetic field) but makes room for more runs of the conductor, which increases L. You have to look at both parameters to decide where the sweet spot is for your application.

To get the impedance of the speaker within a useful range you can use various combinations of conductors in serial and parallel. Since posting those old posts my attention has turned to cutting conductors with a Silhouette computer-controlled cutter. I'm planning to stack vertically several shorter drivers, each of which has somewhere between 3 and 5 conductors in the magnet gap. I'll then use parallel and series connections of drivers to reach a target impedance.

Of course there's more to worry about than B and L. There's the directivity pattern, which is determined primarily by the driver's dimensions and whether it's used open-backed or mounted in an enclosure, the power handling, the sensitivity, the bandwidth... Lots to think about!

Few
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Old 9th March 2016, 08:47 PM   #50
WrineX is offline WrineX  Netherlands
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there is a reason why magnepan uses seperate wires and places the tweeter section close to the spacers. since there is not much movement there, and it allows for thicker magnets to be used in that section to get higher efficiency without slapping into the magnets. (and thus limit the width needed to create a certain amount of ouput)
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