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Old 5th June 2006, 05:42 PM   #1
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Default stretching Mylar for curved ESLs, How?

Hi folks,

I have just finished a small curved wire esl.
I made a curved table top for stretching the Mylar, but I can't get rid of the large wrinkles running along the length direction. I tried to remove those wrinkles by very gently pulling across the width direction but this caused the Mylar to touch the back-stator. It seems I needed too much of those width-tension to remove those wrinkles.
I am now making a kind of jig for a more uniform tension, but don't know if this will work.....
Are there any tips and tricks?

I also thought about heatshrinking. Would it be possible to attach just a little part of the Mylar on the table top, than heat shrink (tension in the width direction should not occur since only a narrow strip is treated with heat, the other part is covered), than the next strip, etc?

Thanks in advance,
MartinJan
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Old 5th June 2006, 09:05 PM   #2
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Default Shrink direction

Mylar is a biaxially oriented film (as are mor films used for ESL's) which means that when you heat shrink them they will shrink equally in both length and width directions. So I think that to obtain the results you wish you will need to tension using stretch only in the length direction of your panel around the curvature that you want with just enough lateral tension to keep things wrinkle free. Best regards Moray James.
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Old 6th June 2006, 03:23 PM   #3
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Hi MartinJan,
I think if you make a flexible frame like of sheetmetal,
bigger in size than you need it for you ESL, glue the stretched membrane to it,
and then bending it to the curved stator and gluing it together,
you will be right.
Best regards
Frank
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Old 7th June 2006, 01:05 AM   #4
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Quote:
So I think that to obtain the results you wish you will need to tension using stretch only in the length direction of your panel around the curvature that you want with just enough lateral tension to keep things wrinkle free.
Absolutely true. Keep the tension only along the long straight axis. There is a potential problem with curved single diaphragms, and it is that the surface will tend to cup inward if there is any tension laterally around the bend. It has been reported that this will reduce headroom and increase distortion due to the unbalanced forces in the gap. Look at the diaphragm reflection in any Martin Logan and you will see the cupping effect of even a low lateral tension (unavoidable). This is why almost all ML models have to use cone woofers to avoid distortion at frequencies where excursions would be higher. By contrast, the segmented flat panels in a Sound Lab curved array can thunder out bass without cones, but the costs are higher with multiple panels.
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Old 7th June 2006, 07:34 AM   #5
Calvin is offline Calvin  Germany
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Hi,

curved stators have advantages as well as disadvantages.
A clear disadvantage is the manufacture, especially the stretching and mounting of the membrane. Even ML used to hand-stretch the diaphragm. As I understand nowadays they use a method by which the front stator is layed on a flat peace of diaphragm (very probabely tensioned hard at least in the long direction and slightly tensioned in the short direction!!??) and sucked to the stator inside by vacuum. Seems to me a clever solution.
As a DIYer You will most probabely have to stuck to the old method of using a mechanical stretcher.
I´m using one with 2 curved elements at the short sides to which I glue the membrane with doublesided sticky tape. The elements are then pulled apart. The key is to use as much tension in the long axis as the diaphragm can stand. >150Hz resonance with 3.5µm and up to 250Hz with 12µm (ML used this stuff) should be reached. Then even a strong pull along the long axis of the diaphragm doesn´t lead to excessive cupping. A good film -original Mylar C or S or Hostaphan RE will allow for 2-4% of lengthening when pulled before the material starts to ´flow´. Still You need supports in the short direction. With a flat panel You may design for such a width depending on d/s that You don´t need any horizontal support. But with curved panels horizontal supports are a must -therefore its of not much use to design curved stators slim. They can be quite wide. The needed support is achieved by accordingly positioned horizontal spacers.
Forget about heatshrinking with these kind of stors. The only solution I can think of is a specialized film. There is a manufacture of Hostaphan that only heatshrinks in machine direction app. 2% but has no shrinkage in transverse direction. But its more expensive and harder to get than standard PET-film.
To glue the membrane onto the stator I recommend using instant bondage material like double sided sticky tape. I just take the tensioning frame with the stretched membrane and lay it freely on top of the inner stator. The weight of the frame pulls the membrane nicely and evenly onto the tape. You have to be very careful because the edges of the stator can easily puncture the membrane. So take care that the membrane settles onto the tape slowly, completely and at once. Rubbing the glue points or rolling over them fastens the membrane. After You reached this point its of no harm any longer when the membrane gets punctured ´outside´ the stator. You can cut it out anyway now -thow I prefer to have it hanging around this manner for 2-3 hours because the bond between tape and mebrane strengthens over time- before I cut the stator loose.

jauu
Calvin


the key is to use as much tension along the long axis as the diaphragm can stand.
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