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Old 2nd June 2002, 08:00 AM   #11
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Default stretcher

I used a rectangular pneumatic stretcher table of the sort described on my web page (photo attached). The picture shows the bottom side of it and there is some diaphragm material stuck to the tape on the inside of the rim to keep the tape clean. You can see some of the tape along the bottom edge of the rim on either side of the valve stem. Very simple, and very effective!

The large speakers were built using only a heat gun to shrink the film and I am not happy with the result. I get MUCH higher tension from the stretcher table and I don't risk melting a 98% complete speaker's diaphragm. The high tension means I can really crank up the bias voltage and get good sensitivity from the speakers.

The PVC pipe frame speakers have drivers that are 2' wide and 4' high. Too big for a single driver unless you apply some curvature to stiffen the whole thing up (THAT is the real reason Martin-Logan curves the drivers- curving them has little effect on the beaminess). If I were going to do it again, I'd use a pneumatic stretcher. Bicycle tire tubes will stretch quite a lot!

MR
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Old 2nd June 2002, 07:32 PM   #12
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Mark,
Your table looks like a good idea. I may want to try it for my next batch (sigh). I remember reading about it in your paper, but I thought it wasn't a very good idea at the time. My stretching table does have a little bit of a problem with pressure points. Sometimes my stretching will cause parts of the Mylar to rip, while other parts are much looser.
Thanks for your picture.
-Dan
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Old 3rd June 2002, 09:43 PM   #13
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The stretcher you used is a variation of the metal bars with screws that Roger Sanders used in his Speaker Builder (or was it Audio Amateur?) articles many years ago. I tried it and discovered that since the metal bars will flex, you always end up putting lots of tension on the corners of the film and very little at the center. This can be seen when the diaphragm fails because of over tightening -it is always split along the corner-to-opposite- corner axis.

The bicycle tire tube tends to equalize the tension across the whole diaphragm because as tension increases in one area, the air in the tube will be pushed to an area with less tension. The result is very high, more or less even tension across the whole surface.

The pneumatic stretcher would allow all sorts of interesting fooling around- you could built it right into a speaker and then adjust the tension (air pressure) to look for audible effects of resonance, and etc. while the speaker is playing...

MR
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Old 3rd June 2002, 09:55 PM   #14
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Default one more thing

Set-up time...

It takes just a little practice to set the film up right on the stretcher so that you get a completely wrinkle free result. Once you have mastered the technique (<10 minutes) you can throw a new diaphragm down and stretch it tight in under a minute. no clamps, no fooling around.

You spread the film out on the table surface, getting as smooth as possible, then start wrapping it around to stick to the tape, starting near the center and working your way toward the corners. You don't need to apply any more muscle than necessary to simply get the film pulled over and stuck to the tape. Don't worry about small wrinkles - they won't be there when you're done.

Try it once and you'll never fool around with clamps, heat guns, or other Rube Goldberg contraptions again.

MR
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Old 4th June 2002, 01:55 AM   #15
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Mark,
Thanks for the input.
What type of tape are you using? It seems that the tape will have to hold as much tension as you are putting on the membrane. People have suggested plain ole masking tape, but I don't know what they are talking about. Maybe its the humidity, but masking tape holds about as much as a post-it note.
-Dan
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Old 4th June 2002, 03:28 AM   #16
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Default tape

I use some stuff that is sticky on both sides. I bought it in Japan, but it is just household stuff. You can use carpet tape.

It doesn't have to hold the entire tension applied to the film- that is why the tape is placed inside the rim of the table- the film bends over 3 edges which greatly diminishes the tension seen by the tape. It's like wrapping a piece of string around a piece of pipe and pulling on it. It takes almost no effort at all to keep the string from slipping, even if you pull real hard on the opposite end.

MR
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Old 4th June 2002, 04:01 AM   #17
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Mark,
Ahhhh! I did not realize that the Mylar was wrapping around as such. This sounds like a much better way to tension. Carpet tape sounds like a viable option. Of course, I'll have to build the rig first...
A problem with my stretching rig is the mylar's slipping. Sometimes it will creep out from under the plexiglas. Obviously this could be fixed by using many more clamps, but I exhausted my supply at what I thought was just enough. So, I didn't bother to get anymore.
More questions: What do you use as a flat surface behind the Mylar to grind in graphite? Do you do so while in the stretching rig?
Thanks
-Dan
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Old 4th June 2002, 01:30 PM   #18
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The top side of the table has a sheet of smooth plastic laid over it. I apply the graphite to the film while the film is under tension. The stator assembly that I glue to the stretched film is the side with the diaphragm electrical contact.

There is a small hole all the way through the table to allow air trapped under the diaphragm to escape, otherwise, you get a nice little pillow when you stretch the film.

MR
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Old 4th June 2002, 10:23 PM   #19
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More stupid questions.

If I was to build a table for such a stretching rig, I realized that it would have to be at least 21"x37". Where do you get a tire that is that big? It would have to be 116" in circumference. A 26" tire is only about 80" in diameter. Would it stretch enough?
Thanks

-Dan
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Old 5th June 2002, 01:28 PM   #20
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Sure. Bicycle tire tubes are very stretchy. Go out to your garage and find an old one and try it.

MR
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