Some questions about diaphragm tensioning:
I have 12” x 48” perforated metal panels (gross size) and 6-micron Hostaphan film to be attached with 1/16 x 3/4 3-M foam tape at the perhphery and 2 (maybe 3) ¼” wide vertical support strips.
My plan was to heat-shrink the film but then I read a thread here advising against heat shrinking Hostaphan and recommending instead to mechanically tension the film to 1 ½ to 2 percent elongation but there were no instructions about how to do it or measure the elongation. I’ve also seen a bicycle tube tensioning jig described but, again, there was no method of determining the correct tension. Anyway, I’m confused about how to tension my diaphragms correctly and I’d appreciate any advice on methods, jigs and measuring that would apply to my panel size and diaphragm material.
Thanks all!
Charlie
I have 12” x 48” perforated metal panels (gross size) and 6-micron Hostaphan film to be attached with 1/16 x 3/4 3-M foam tape at the perhphery and 2 (maybe 3) ¼” wide vertical support strips.
My plan was to heat-shrink the film but then I read a thread here advising against heat shrinking Hostaphan and recommending instead to mechanically tension the film to 1 ½ to 2 percent elongation but there were no instructions about how to do it or measure the elongation. I’ve also seen a bicycle tube tensioning jig described but, again, there was no method of determining the correct tension. Anyway, I’m confused about how to tension my diaphragms correctly and I’d appreciate any advice on methods, jigs and measuring that would apply to my panel size and diaphragm material.
Thanks all!
Charlie
Seek, (or in this case, search) and ye shall find...
thread 1
thread 2
thread 3
There are some great photos of stretching jigs and other ideas in there (perhaps these are the threads you've already read.)
You can measure % elongation by using a permanent marker or small tab of tape to mark reference points on the limp diaphragm. Stretch the diaphragm until the distance between diaphragm marks changes the desired amount (~1-2% of their original separation).
Or, monitor the diaphragm's resonance frequency as you stretch it. It'll have a high Q so it's pretty clear what the frequency is. You can just tap the diaphragm or its frame to excite the resonance. I think I read someone suggesting dropping a ping pong ball on it as another approach. It shouldn't matter how you do it as long as you can see the resulting oscillation.
You might consider dots of silicone rather than long vertical spacers. Lots of folks report good results with that approach. I haven't tried it yet but plan to use it on the speakers I'm building.
Few
thread 1
thread 2
thread 3
There are some great photos of stretching jigs and other ideas in there (perhaps these are the threads you've already read.)
You can measure % elongation by using a permanent marker or small tab of tape to mark reference points on the limp diaphragm. Stretch the diaphragm until the distance between diaphragm marks changes the desired amount (~1-2% of their original separation).
Or, monitor the diaphragm's resonance frequency as you stretch it. It'll have a high Q so it's pretty clear what the frequency is. You can just tap the diaphragm or its frame to excite the resonance. I think I read someone suggesting dropping a ping pong ball on it as another approach. It shouldn't matter how you do it as long as you can see the resulting oscillation.
You might consider dots of silicone rather than long vertical spacers. Lots of folks report good results with that approach. I haven't tried it yet but plan to use it on the speakers I'm building.
Few
Thanks Few,
I'm gonna try the inner tube jig (looks easy). I can borrow a vernier caliper from work to measure elongation using marks applied before stretching.
I'm much intrigued by your suggestion to use silicone dots. I noticed that Sheldon used foam tape dots in his design with 1/8" stator/diaphram spacing. Mine will use half the spacing that Sheldon used so I will need a bunch more dots to prevent driving the diaphram into the stators. Also, my stators are less flat than I'd like so maybe I need more dots anyway. Even so, dots should be more efficient that strips by virtue of leaving more driven area-- I'm wondering whether I need two or three rows of dots and if I should align or offset the dots between rows.
I'm thinking 1/2" foam tape dots might work for me... is there an advantage to using silicone over foam tape for the dots?
I'm gonna try the inner tube jig (looks easy). I can borrow a vernier caliper from work to measure elongation using marks applied before stretching.
I'm much intrigued by your suggestion to use silicone dots. I noticed that Sheldon used foam tape dots in his design with 1/8" stator/diaphram spacing. Mine will use half the spacing that Sheldon used so I will need a bunch more dots to prevent driving the diaphram into the stators. Also, my stators are less flat than I'd like so maybe I need more dots anyway. Even so, dots should be more efficient that strips by virtue of leaving more driven area-- I'm wondering whether I need two or three rows of dots and if I should align or offset the dots between rows.
I'm thinking 1/2" foam tape dots might work for me... is there an advantage to using silicone over foam tape for the dots?
Roger Sanders has a rule of thumb that for good stability the stator spacing to support spacing should be 1:100 or less. So If you've got 1/16" diaphragm to stator spacing, your max distance to a spacer should be no more than 6.25" If you are doing a line of dots like I did, don't forget that the largest distance is actually a diagonal.
Sheldon
Sheldon
Update
Thanks for the info guys.
Sheldon, I took your advice on going with all-wood over the MDF rather than just painting the woofer cabinets. Both "beam-splitter TL" cabinets are now finished and I think they look great; although somewhat imposing. This evening I soldered the wire leads to the stators, then sprayed on the insulating basecoat-- was gonna use a 2-part polyurethane for that but the cost was prohibitive so I ended up using latex house primer and sprayed on the entire gallon. I will follow that up with a coat of black and then a couple of clearcoats. My stators sheets are [shall we say] less than perfectly flat (especially the one I short trimmed and had to butt and tack-weld it back together) and there will be only 1/16 stator-to-diaphragm spacing so I might need closer than normal spacing between the dots to ensure the diaphram is centered between [uneven areas of] the stators. I will draw up a couple of sketches of the dot layout and post them for review before assemble the panels. The next task is making a tensioning jig for the 6-micron Hostephan.
http://imgcash4.imageshack.us/img411/3618/eslmonsterbox005kw0.th.jpg
Thanks for the info guys.
Sheldon, I took your advice on going with all-wood over the MDF rather than just painting the woofer cabinets. Both "beam-splitter TL" cabinets are now finished and I think they look great; although somewhat imposing. This evening I soldered the wire leads to the stators, then sprayed on the insulating basecoat-- was gonna use a 2-part polyurethane for that but the cost was prohibitive so I ended up using latex house primer and sprayed on the entire gallon. I will follow that up with a coat of black and then a couple of clearcoats. My stators sheets are [shall we say] less than perfectly flat (especially the one I short trimmed and had to butt and tack-weld it back together) and there will be only 1/16 stator-to-diaphragm spacing so I might need closer than normal spacing between the dots to ensure the diaphram is centered between [uneven areas of] the stators. I will draw up a couple of sketches of the dot layout and post them for review before assemble the panels. The next task is making a tensioning jig for the 6-micron Hostephan.
http://imgcash4.imageshack.us/img411/3618/eslmonsterbox005kw0.th.jpg
Just to echo and emphasize Sheldon's point, when planning out the dot spacing, bear in mind that you want to avoid having regions of your diaphragm that are more than ~3" (0.0625" * 100/2) from the nearest dot. This is just another way of saying you want ~6" dot-to-dot spacing, but it emphasizes the fact that the key parameter is the amount of unsupported diaphragm. My knee-jerk sense is that a hexagonal array of dots will be the most effective way to distribute the dots, but that's partly based on hunch and partly based on the way spheres can be most densely packed. The hexagonal array would be achieved by offsetting the dots, as you were considering. If you sketch that out, you'll see dots near the middle of the array have six nearest neighbor dots around them, forming a hexagon.
One advantage of silicone, as Calvin recently pointed out to me, is that the silicone is hydrophobic and may therefore be resistant to accumulating a partially conductive layer of moisture and gunk that electrically short circuits the diaphragm coating to the stators. My 3M foam spacer-based panels have gotten more and more humidity sensitive over the years. I can't say for sure that junk accumulated on the foam tape spacers is to blame, but the theory holds at least a little water (truly unintentional pun).
Others may disagree, but my inclination is to err on the side of using spacers that are too closely spaced, rather than too widely spaced, particularly if there's a concern about uniform diaphragm/stator spacing. I think you gain more in the ability to crank up the bias voltage than you lose in free diaphragm area by using this approach. I don't have empirical evidence to back that up, but that's one of the lessons I thought I learned from some of my experimentation.
If you come up with an elegant tensioning jig idea, but all means share it. I've been very impressed by some of the photos of people's tensioning jigs I've seen, but some of them involve more construction complexity than my first set of ESL panels did taken in their entirety! I'm still searching for the best combination of reproducibility, ease of use, and simplicity. I agree that the inner tube approach has a lot of merit. I've only tried it on a very small diaphragm (for ESL headphones) and haven't yet fooled around with a full scale model. The weights and pulleys approach may be the ultimate in reproducibility but since I'm not planning on going into commercial production mode I've had a hard time justifying that level of complexity.
Few
One advantage of silicone, as Calvin recently pointed out to me, is that the silicone is hydrophobic and may therefore be resistant to accumulating a partially conductive layer of moisture and gunk that electrically short circuits the diaphragm coating to the stators. My 3M foam spacer-based panels have gotten more and more humidity sensitive over the years. I can't say for sure that junk accumulated on the foam tape spacers is to blame, but the theory holds at least a little water (truly unintentional pun).
Others may disagree, but my inclination is to err on the side of using spacers that are too closely spaced, rather than too widely spaced, particularly if there's a concern about uniform diaphragm/stator spacing. I think you gain more in the ability to crank up the bias voltage than you lose in free diaphragm area by using this approach. I don't have empirical evidence to back that up, but that's one of the lessons I thought I learned from some of my experimentation.
If you come up with an elegant tensioning jig idea, but all means share it. I've been very impressed by some of the photos of people's tensioning jigs I've seen, but some of them involve more construction complexity than my first set of ESL panels did taken in their entirety! I'm still searching for the best combination of reproducibility, ease of use, and simplicity. I agree that the inner tube approach has a lot of merit. I've only tried it on a very small diaphragm (for ESL headphones) and haven't yet fooled around with a full scale model. The weights and pulleys approach may be the ultimate in reproducibility but since I'm not planning on going into commercial production mode I've had a hard time justifying that level of complexity.
Few
dots layout on 12x48 panel
Hello guys,
Please have a look at the dots layout and tell me what you think. The dots are drawn as 1/2" diameter foam tape. If I do go with dots (rather than 1/4 wide strips) I will come up with some means to stamp them out from the 3/4 width tape that I have. I have a bicycle tube diaphragm stretcher finished and the stators are coated so I'm ready to go as soon as I decide on strips or dots and the pattern to use. The pic shows 3 rows of offset dots about 3.5" apart. Let me know what you think.
Charlie
http://imgcash1.imageshack.us/img242/5008/scan0010br0.th.jpg
Hello guys,
Please have a look at the dots layout and tell me what you think. The dots are drawn as 1/2" diameter foam tape. If I do go with dots (rather than 1/4 wide strips) I will come up with some means to stamp them out from the 3/4 width tape that I have. I have a bicycle tube diaphragm stretcher finished and the stators are coated so I'm ready to go as soon as I decide on strips or dots and the pattern to use. The pic shows 3 rows of offset dots about 3.5" apart. Let me know what you think.
Charlie
http://imgcash1.imageshack.us/img242/5008/scan0010br0.th.jpg
update on tensioning
Just updating the tensioning issue:
I fabricated a bike-tube tensioning jig from 3/4 MDF board with 2" edge bands glued on, 15" x 51" to fit my 12x48 esl panels. I rounded the edges using a router with 1/2" roundover bit and used a 27" x 1.25" bicycle tube.
Prior to stretching the 6-micron Hostephan, I placed reference marks 6" apart using fine tip felt marker. I was going for 1 1/2% elongation but my first attempt broke the film at just under 1 1/2%. Since I didn't have enough film to make a second mistake I settled for 1% elongation and there was no problem.
It doesn't take a lot of air pressure in the tube to stretch the film to the breaking point. Pressurizing the tube is definitely best done with a hand-pump rather than from an air compressor. 1% elongation is reached when the distance between the 6" reference marks measures 6.062".
I used 1/16" stator to diaphram spacing with 3 3/8" spacing between the supports and the diaphram elongated 1%.
My ESL's are working great--- they easily reach quite high volume levels without distortion and without the diaphram hitting the stators.
Just updating the tensioning issue:
I fabricated a bike-tube tensioning jig from 3/4 MDF board with 2" edge bands glued on, 15" x 51" to fit my 12x48 esl panels. I rounded the edges using a router with 1/2" roundover bit and used a 27" x 1.25" bicycle tube.
Prior to stretching the 6-micron Hostephan, I placed reference marks 6" apart using fine tip felt marker. I was going for 1 1/2% elongation but my first attempt broke the film at just under 1 1/2%. Since I didn't have enough film to make a second mistake I settled for 1% elongation and there was no problem.
It doesn't take a lot of air pressure in the tube to stretch the film to the breaking point. Pressurizing the tube is definitely best done with a hand-pump rather than from an air compressor. 1% elongation is reached when the distance between the 6" reference marks measures 6.062".
I used 1/16" stator to diaphram spacing with 3 3/8" spacing between the supports and the diaphram elongated 1%.
My ESL's are working great--- they easily reach quite high volume levels without distortion and without the diaphram hitting the stators.
I was almost at 1 1/2% per the reference marks when the film broke and it didn't fail near my measuring marks --- It may have been an abrasion or weak spot on the film or perhaps I didn' have it taped-down evenly. In any case, I didn't have enough film to risk a second break so I backed off to 1%.
No apparent problem with the sound, though... I love it.
No apparent problem with the sound, though... I love it.
Thanks Charlie; what I meant to ask was whether you approached the 1.5% (or 1%) stretch points along both the long and short dimensions of the diaphragm simultaneously, and if so, whether that seemed to happen "automatically." In other words, if you loosely stretch the diaphragm over the inner tube, tape it down, and then inflate the tube, is the final tension the same along the long and short dimensions of the diaphragm?
Few
Few
diaphram on stretcher
Since my reference marks were in the width dimension only, I don't know what the lengthwise elongation was at failure. The attached photo link shows my second attempt with the diaphram on the stretcher at 1% [widthwise] elongation.
http://img183.imageshack.us/img183/4214/diaphragmonstretcherur0.jpg
Since my reference marks were in the width dimension only, I don't know what the lengthwise elongation was at failure. The attached photo link shows my second attempt with the diaphram on the stretcher at 1% [widthwise] elongation.
http://img183.imageshack.us/img183/4214/diaphragmonstretcherur0.jpg
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