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Old 9th December 2007, 09:32 AM   #11
diyAudio Member
 
Join Date: Nov 2005
Location: Southern Germany
Hi Few,

yes i mean that different tension is applied for width and length. The only way to avoid the impact of tension on the longer side is to tension a mylar sheet which is 3 times longer than the height of the ESL itself. But this is very uneconomical.

I think nobody would prefer this, so you need tension on both directions. It is nearby impossible to predict the time based creeping of the molecules, so i had to go the long trial and error way. My personal experience tells that the final behaviour, especially at fundamental resonance, is very dependent from the tensioning ratio of a specific ESL-geometrie.

e.g. if the tensioning procedure is at its optimized level, fundamental resonance show Q-factor lower than 1,5 and even the suckout in the upper bass range is less significant.


Capaciti
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Old 9th December 2007, 12:16 PM   #12
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Hi Capaciti,

How can an inexperienced DIYer apply the dual tension approach? A bicycle tyre stretcher, for example, would automatically tension equally in all directions.
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Old 9th December 2007, 09:30 PM   #13
gvy is offline gvy  Belgium
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Hi,

In my opinion, this works indeed good.

And because I can place 2 stators next to each other, they have te same tension too.

Click the image to open in full size.

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Geert
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Old 9th December 2007, 11:48 PM   #14
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Wow,
Where did you find an inner tube that big? Betcha those panels sound good!
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Old 10th December 2007, 12:19 AM   #15
gvy is offline gvy  Belgium
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Well,

It was a really big bike

No serious,
I simply cut two inner tubes open with a knife and glued them together to one long tube with rubberglue. You know, the one you use to repair your inner tube in case of a hole.

It worked fine. I just had to find a trick to do the glue job well.
I put one rubber tube end in the other and layed over the above.
Then I put glue on the inner and outer and let dry the glue.
Then I rolled back the outer one over the glued inner and pressed it together like you do when you repair a tire.

I build 4 panels (hybrids) for my home theater.

Before I used a mechanical stretching table.
With the pneumatic table you can easily get a very even tension on the membrane.

You just have to be carefull not to go to far, because it can create a massive tension force (and you don't want your mylar to tear)

I also learned to lay the mylar very loose and with a lot of wrinkles on the table and let the tube and air do de job.
The first time, I tried to get the membrane already a bit tensioned and without wrinkles on the table. That wasn't a good idea, because once I started pumping , there was no room anymore for the membrane to search it best position and in some corners the tension got directly to high, while in the other, there were still wrinkles.

So if you start with a very loose membrane, it finds very easily its way to divide the tension very evenly. A tube can expand very much.
On some places, the tube gets in a real big bubble, but that is OK. The air divides that way a really even tension on the membrane.


Geert
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Old 10th December 2007, 07:11 AM   #16
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Hi Geert,

Perhaps I'm missing something, but I thought Capaciti specified different tensions for length and width, whereas the bicycle tyre approach gives one uniform tension in all dimensions? So could someone please show me a practical way to get different tensions in different dimensions? Thanks.
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Old 10th December 2007, 11:22 AM   #17
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Quote:
Originally posted by Capaciti
My personal experience tells that the final behaviour, especially at fundamental resonance, is very dependent from the tensioning ratio of a specific ESL-geometrie.
I suppose this would depend for a large part on the material used. If you had for example a TD/MD of 40/60 this would require different tensioning as with 40/40. Correct?

Did you find a predictable relation between geometry / film properties and the tension ratio? Like a rule of thumb where to start?
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Old 10th December 2007, 03:48 PM   #18
EdwinR3 is offline EdwinR3  Belgium
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Hi wolf-moses

Quote:
So could someone please show me a practical way to get different tensions in different dimensions? Thanks.
I have build a stretching jig that allows just that.
It concains to 2 rails, one one a long side and one a short side. Each rail contains short blocks in MDF that can slide sideways. All the blocks are linked together with a rubber band.

Each rail can be pulled to stretch with a number of threated wires (one turn is a displacement of 1 mm).

I put double sided tape on the rubberstrip and use this to fix the membrane. The rubber strips on de rails can get longer while beeing pulled aside when the other rail is displaced.

The complete jig was made from one panel of MDF 18 mm (+ rubber strips of 6 mm thickness).

After prestretching the film to remove the (few) wrinkles I have by fixing the membrane, I measure the distance between the double sided tape and calculate how many turns I must apply to get a preset elongation.

This jig permits to use a different ratio in lenght and width, and works quite well.

For the moment I have used it a number of times with a film called WINWRAP of 9 micron. It was ment as a test to check the consistancy of the rest of my frames. It has been playing rather well for six months, and has stabilised quit well. I hope soon to put the real Mylar to use.

I'll try to find some photo's we must have made once while tensioning.


Edwin
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Old 10th December 2007, 09:58 PM   #19
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Hi Folks,

sorry, up to now i did not find a way to predict the rigth tensioning depending on the panel geometrie, but you need to go the trail and error way.

It took me years to find the right one for my current serie which is 20cm in width and 137cm in height for the mebrane area. Even there is different tensioning along the height in order to accomodate for the different mass load of air (more on the bottom less towards the top of the speaker.

By optimizing the excisting panel i have been able to improve maxoutput by about 3dB, having identical frequency response. At the very beginning of development the membrane touches the stators at specific locations first, when reaching maxexcursion, while other sections showed reserve. Now the whole membrane area reaches its limits very consistent.

Capaciti
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Old 10th December 2007, 10:29 PM   #20
Few is offline Few  United States
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Hi Capaciti,
Thanks for your continued contribution to this thread. When you say:
Quote:
Even there is different tensioning along the height in order to accomodate for the different mass load of air (more on the bottom less towards the top of the speaker.
do you mean there is more tensioning at the bottom of the panel, more air load on the diaphragm at the bottom of the panel, or both? My guess is that there is more of an air load at the bottom of the panel (due to floor reflections) so the membrane excursion is less in that region. This would allow the diaphragm tension to be less where the panel is closest to the floor.
Is this what you have found in practice or am I misinterpreting something?

-Few
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