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Old 19th December 2012, 10:49 PM   #221
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Martin, ok i didnt realise that was how you were doing it, i did use that technique early on in my build but switched to just using the cryo i linked to in my previous answer and with any cryo adhesive its best to use a primer for the mylar,its up to you which one you prefer i just found it a bit messy with the contact glue also i overlapped the mylar with the rubber more than the factory amount just to insure a more secure fixing about 10mm.
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Old 19th December 2012, 11:49 PM   #222
Few is offline Few  United States
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This has been a very informative thread. Thanks to all the contributors.

The one thing that keeps nagging me is that when a surround is fixed to a cone loudspeaker it's important to match the impedances so that the waves propagating from the voicecoil through the cone and out to the surround don't reflect back any more than necessary. Attaching mylar to therabands through a glue joint just "feels" (sorry, I don't have any numbers to back this up) like a recipe for an impedance mismatch. Perhaps it's a non-issue if the theraband surround is used for low frequency drivers. Maybe the increased linear compliance gained is a net good for bass response. At higher frequencies (midrange and above), though, my gut says that this approach may be less attractive.

I don't mean to imply anyone has said the theraband surround should be applied to supertweeters, but I thought I'd raise this in case those with hands-on experience have some clarification they can offer. Does anyone have experience that can clarify the frequency range over which this approach is most effective? I'm accumulating parts for a planar-magnetic speaker and I'm trying to nail down the final design. The question of how best to treat the edges of the diaphragm is still keeping me up at night.

Thanks,
Few
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Old 20th December 2012, 01:26 PM   #223
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I am no expert but i would imagine this is more of an issue with standard drivers rather than a large flat planar bass diaphragm as the sound is not emanating from the centre and moving outwards .
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Old 21st December 2012, 04:42 AM   #224
Few is offline Few  United States
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Yeah, I considered that but it seems to me that the force is still applied to the central portion of the diaphragm (where the conductors and magnets are) and not to the edges. There will still be waves propagating outward from the regions where the force is applied. What happens to the waves when they reach the edges?

The propagation rate of those waves through a diaphragm is finite, whether it's a rigid cone or a more flexible planar magnetic diaphragm. As a result, when a wave starts at the conductive traces and later reaches the diaphragm's edges it'll be reflected to an extent that depends on the nature of the edge termination. To frustrate diaphragm resonances I think the reflections from the diaphragm edges need to be minimized. That, in turn, requires that the impedances of the diaphragm and surround be well matched and that the edges be lossy.

I would expect a thin mylar diaphragm to be inherently lossy compared to a rigid cone. If I'm not off base, that would help reduce diaphragm resonance problems. The mismatch between the mylar and theraband impedances is likely to be much less than that between mylar and some completely rigid clamping so a theraband surround is likely to be an improvement over the more typical rigid clamping. When significant diaphragm displacements are required (at low frequencies) the extra compliance of theraband compared to mylar is likely very helpful.

My focus has been on higher frequencies, say 300-500 Hz and above, where diaphragm displacements are less of an issue. Capaciti's experiences with his ESL development show that lossy edges and lower diaphragm tension can significantly clean up CSD plots compared to results from the more common rigidly clamped edges and maximized tension found with ESL diaphragms. I'm trying to extrapolate from those findings to planar magnetic drivers.

To that end, I'm exploring ways to dissipate diaphragm vibrations by absorbing them at the edges (with less focus on maximizing linear diaphragm displacement). I don't mean to suggest the theraband approach is a bad one---it looks great, especially at low frequencies. I'm just asking about the bandwidth over which it's most usefully applied.

Few

Last edited by Few; 21st December 2012 at 05:09 AM.
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Old 21st December 2012, 09:39 AM   #225
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Quote:
Originally Posted by harlee View Post
Hi,
Great work there with the repair.

Is it necessary to have the tweeter ribbon if the bass panel(Jamebos) can play up to 20K?
As much as I take my hat off to the effort taken to fabricate the panel speakers you refer to above (Jamesbos), they in no way consitute a step towards hi-end sound reproduction. They'd be lucky to reach outside of their 1khz-6khz operating range..
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Old 21st December 2012, 09:57 AM   #226
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Thumbs up Analysis Rebuild

Brian T -

Congrats on your meticulous rebuild ! No matter that you took a year to do it, the point is that you got there in the end - AND with what I believe to be a solid and lasting job.
Well done !

Now go listen to some music
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Old 21st December 2012, 10:21 AM   #227
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cheers, yes is was a bit protracted the rebuild ! but got there in the end i am now a fully paid up member of the Cyanoacrylate society not to mention the dozens of rubber samples i got,my girlfriend thought i had a rubber fetish! Has been a steep learning curve but the results have made it worthwhile, now i know the intricacies of putting these together i might reverse engineer a larger version in the future.
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Old 21st December 2012, 11:34 PM   #228
gootee is offline gootee  United States
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Quote:
Originally Posted by ApogeeLover View Post
As much as I take my hat off to the effort taken to fabricate the panel speakers you refer to above (Jamesbos), they in no way consitute a step towards hi-end sound reproduction. They'd be lucky to reach outside of their 1khz-6khz operating range..
Where did you get the "1 kHz to 6 kHz" figures?

Jamesbos has reported measured frequency response figures that cover a much broader range.
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Old 23rd December 2012, 04:25 PM   #229
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Originally Posted by gootee View Post
Where did you get the "1 kHz to 6 kHz" figures?
.
Errrmm..... I used the auditory sensors either side of my head ?? lol
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Old 23rd December 2012, 09:43 PM   #230
gootee is offline gootee  United States
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Understood. I'm not saying that you are incorrect but I guess that means that we would need more-complete data, since there's no way to know what the input source was doing, and what the output/input ratio was, for each frequency.

He has put forth so much effort, and has been so generous with his posts, here, that it would be a shame to have his results dismissed so casually, even if you are correct. I, personally, have learned a significant amount from his posts, and greatly-appreciate his efforts and his presence here.

Regarding better data, I wish jamesbos would download the free ARTA software, or something like it, and acquire a measurement microphone, and collect some real frequency-response data from his speakers. There's always room for improvement and he must be running out of new things to try, anyway. Some real measurements would also enable him to tell us what has worked well and what has worked less well, of what he has tried.

In trying to think of things that might improve the frequency response, one idea that comes to mind is to try making the panels larger, at least 22 W x 60 H inches. Also, note that Magnepan uses one vertical section of each panel as a tweeter, and uses the rest as either a combined mid/bass driver or as two separate drivers, for mid and for bass. The tweeter section is tall and thin, and uses foil (whereas their older models used wire for the mid/bass, although the newer ones use foil for all three). On my Magnepan MG-12/QR speakers, the tweeter section's magnets are closer to the mylar than the ones in the mid/bass section, since there is less travel of the mylar for the tweeter. Segmenting the panel does, of course, require the use of crossover circuitry, which jamesbos has successfully avoided, so far.

Another idea would be to take two (or maybe three) existing full-range panels per channel, and use a crossover to make one of them treble-only, and use one or two for mid and bass only. Even if the speakers were all already capable of reproducing the full range, using crossovers like that should significantly improve the QUALITY of the sound reproduction. Even with my Magnepan MG-12/QR speakers, if I use an amplifier that can drive 400 Watts into 4 Ohms, it is possible (at extremely high output levels) to cause the bass to modulate the treble and mid outputs enough that they sound distorted. I'm not sure if this also occurs at lower levels but just isn't clearly noticeable then, or not. But separating the panels completely should eliminate the possibility.

Cheers,

Tom

Last edited by gootee; 23rd December 2012 at 09:47 PM.
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