3) Diaphragm Stretcher Construction
by Mark Rehorst
There are a bunch of different ways to stretch a plastic film tight and you will see a few of them if you look around at DIY ESL sites on the web. I have tried the metal rods with screws at the corners and heat shrinking and my experience has led me to conclude that if you want to make speakers that are sensitive enough to use normal, 30-50W/channel power amps, the only way to get adequate tension on the diaphragm is the pneumatic stretcher that I invented about 18 years ago. It is so easy, fast, inexpensive to make, and so effective, that once you try it you will wonder why you ever tried any other method of stretching the diaphragm.
The pneumatic stretcher consists of a table with a bicycle tire tube stretched around its periphery. The table has a "lip" that borders it and the tube is stretched over that. The inside of the lip has double sided adhesive tape stuck to it. The tube's valve passes through a slot or hole in the lip that surrounds the table. My stretcher has pretty sharp corners and edges. I recommend that you round the corners and edges on your table when you make it. Also, the lip on my table is just taller then the uninflated tube. I recommend that you make the lip a bit taller- maybe 75mm (3 inches) or so. Trace the outline of one of your stators on the table so that you'll know where to apply glue to the film.
Here are a couple CAD renderings of the stretcher table. The green is double sided tape. The film goes on top of the table and is pulled to the underside and stuck to the tape. Inflating the bicycle tire tube stretched around the table stretches the film tight.
Photos of my table showing top and bottom corner.
You can see the air valve and the double sided
adhesive tape that goes all the way around the
periphery of the table.
The diaphragm is stretched by laying it on top of the stretcher table, smoothing it flat, then folding it over the tube and lip of the table and securing it to the tape that lines the inside of the lip. Once the film has secured to the tape all around the lip, you attach a bicycle tire pump and inflate the tube. It will stretch the film VERY tight. You can stretch it so tight that it will break so be careful. You want it to be almost, but not quite, tight enough to break. You will have to experiment to see how much stretching your film can take. Unfortunately, the pressure in the tube is not high enough to register on a normal bike tire pressure gauge (I am using a Blackburn TP-1 pump with attached gauge), so you can't easily use the air pressure as an indicator of the stretch.
Stretcher with film before stretching.
This shows the film attached to the adhesive tape
inside the lip of the table, before stretching.
Once the film is stretched tight, there will be no wrinkles. You apply some Scotch Grip 4693 to the film where the insulator will be attached, apply some to the insulator/stator assembly, wait a few minutes, then set the insulator/stator down on the film. The two will bond instantly. It takes 24 hours for the bond to achieve its ultimate strength, but after a few minutes it is strong enough that you can cut the diaphragm/insulator/stator assembly free.
The next step will be to coat the diaphragm with a high resistance coating. I like Licron. Just mask off the insulator to protect it from the Licron and spray it on. Let it sit for an hour or so to dry and it will be safe to handle. Peel off the masking tape, and complete the assembly by attaching the other half of the driver.
I recently performed some tests that demonstrate that the table provides uniform tension. I drew a series of lines on the table with a green pen, then attached a piece of diaphragm film, inflated the tube just enough to take out most of the wrinkles, then traced over the lines on the film with a black pen.
Table with film just tight enough to remove wrinkles. I traced over the green lines on the table with a black pen, so the black lines are on the film.
Here the film is stretched tight. You can see that the black lines on the film are shifted outward from the green lines on the table. The film is stretched almost perfectly uniformly even though you can see the tire tube is bulging in different places around the perimeter of the table. That is the beauty of this stretcher- the flexibility of the tube and fluidity of the air inside it ensure that the stretch will be uniform everywhere.
Here is close-up of the upper right corner of the table with a scale to show how far the black lines have been stretched away from the lines on the table. This was about as far as I could stretch my film before it burst.
Here is a close-up of the lower right corner of the table with the scale.
Here is what happens if you give it a little too much air pressure...
I also stretched a film tight, then released the air pressure to see if there had been any permanent deformation of the film. I stretched one as tight as I could without breaking it (about 4 mm stretch measured at one corner of the film) and found that when the pressure was released, the lines on the film matched the lines on the table again indicating that even at that maximum stretch, the film is still operating within its elastic limits. I am using 6 um thick polyester film called Lumilar, made by Toray in Japan. You may get different results with different film.
I recently received email from a guy named Geert Vijncke in Belgium. He has used a pneumatic table that is 1350 x 600 mm to make speakers that are 1250 x 250 mm. He had to use two bike tire tubes glued together to stretch that large, but apparently it has worked well for him. Here are some pictures he sent to me.
1) Principles of ESL operation
2) Materials Required
3) Diaphragm Stretcher
4) ESL Driver Construction
5) Support Frame Construction
6) Bias Power Supply Construction
7) ESL Bibliography and Links