Diaphragm / adhesive material choices?

[moray james]

I have built a number of panels over the years and for the most part used the origonal Mylar diaphragm material used by Acoustat. This is a biaxially oriented shrink mylar. While made by Du Pont this film is not listed on thier product lists. This film is used as a mylar backing material for video and audio tapes. This is a fairly heavy weight film which is strong and easy to use. The film does however tend to streach and get loose over the years. Carefull retensioning with a heat gun works well to restore tension to this material.
I have used some other Du Pont products in the Clysar range with success. I was wondering what materials others have used and with what sucess? I have samples of some very low micron thickness films some so fine that when you handle them you are amazed at how thin they are only to discover that you are actually holding three or four layers. These films are cool because they are so light but they are not much fun to work with.
I am also wondering what the current day preference is for adhesive transfer tapes with which to mount diaphragms with? I would be interested in knowing of the experiences of others here. I have used 3M transfer tapes in the distant past but have lost my reference material. For the most part I was used to using a non solvent contact glue made by Le Page's called "Presstite Green". This contact is very user friendly and is solvent free (water soluable) and sticks to just about everything. I used it to do fast prototype work and like it alot. This is sold in Canada but I am not sure about the States. This is very strong and has a very long open time and allows for repositioning. Once you burnish the contact area it is down for good.
I look forward to reading any and all of your comments here. Thanks and best regards Moray James.

[I_Forgot]

I've never trusted heat shrinking to give sufficient tension or to hold it for long. I like to use high bias voltage to get maximum output from the drivers. A few experiments I did with heat shrinking the film I have resulted in insufficient tension. Maybe other films shrink better.

Anyway, it's easy enough to mechanically stretch the film, so that's my preferred method. Back when I was first trying to figure out how to make ESLs and digging up patents and other info I found there were two methods people were using to stretch diaphragms. There was Quad's way, a complex mechanical device with a bunch of fingers pulled apart under spring (?) tension, and there was the amateur ESL builder's way of making a rectangular frame with screws at oblique angle at the corners. I tried the latter method and found it poor at best, so I came up with my own device. A bicycle tire tube stretched over the rim of a small table. I lay the film on the table and wrap the edges and stick to double-stick tape on the inside of the table's rim. Then just inflate the tube. The flexibility of the tube allows the air to evenly distribute the tension in the film- places where the film is tight won't stretch much and will force air to go to where the tension is lower.

I have recently been experimenting with double sided adhesive tape to hold the diaphrgams instead of contact cement which I used to use all the time. Contact cement is quite permanent, but a pain to apply. Tape is easy to apply, but I don't know how long it will last, so one of my ESL-63 drivers has a taped diaphragm. I am using the tape that is used for plastic window film (the stuff you put on and shrink tight in the winter to reduce heat loss through windows). I decided to try that tape after finding that the same tape that has been on my diaphragm stretcher for 15 years is still sticky and works fine. If the tape will hold for that long in a driver I'll be happy. It sure grabs and holds the diaphragm film tightly.

A long time ago I saw some pictures of Martin Logan speakers being assembled and it looked like they were using some sort of foam tape as the spacers in the center area of the speakers. I hope it isn't that crappy stuff that is used for hanging pictures, etc. That stuff usually dries out and crumbles within a year or two.

I_F

[SY]

There just aren't many films available in the thin gauges required. And other than Mylar or Clysar (my fave), they're mostly engineering polymers which are quite "crinkly" in sound (e.g., Kapton).

I'm sort of the opposite in approach- I don't trust mechanical tensioning!

For adhesives, there's still nothing better than the 3M VHB line. I've used a pile of 467.

[I_Forgot]

Can you bias a heat shrunk diaphragm to 5 kV?

My film- Toray's version of mylar called "Lumilar" needs about 5mm between the diaphragm and stator if I heat shrink it and bias it that high. With mechanical tensioning (like Quad uses/used), I can get that spacing down to 2-3 mm, making the driver MUCH more sensitive/efficient.

The only potential downside to mechanical tensioning is the possibility of getting it too tight and having it split. I don't think you can get nearly that tight with heat shrinking...

I_F

[moray james]

I Forgot thanks for the cool tensioning idea. I may just give that method a try. You are right that you cannot tension as tightly with shrink film. However it does make it easy to repeatably tension panels especially for DIY guys.
Not sure that I understand why you were haveing troubles with the Toray film I suppose that with the heat shrinking the additional spaceing was necessary due to diaphragm instability which goes away with the higher tensioning obtained when you use your jig to streach the film tight. Breaking the diaphragm into smaller sections might just solve the instability problem allowing the use of heat shrink tensioning.

Sy: thanks for the nod regarding the 3M467 transfer tape. I am curious about which Clysar film you like best. I have a number of different Clysar films in stock and can pull them out and see which versions I have. Thanks for the input.

[SY]

mj, I used an HPG grade, which I print-treated myself. The tradeoff is that while the tensioning properties are excellent, it's easier to tear than some of the other grades. I wasted two or three diaphragms before I got a good feel for it.

[I_Forgot]

Originally posted by moray james
I Forgot thanks for the cool tensioning idea. I may just give that method a try. You are right that you cannot tension as tightly with shrink film. However it does make it easy to repeatably tension panels especially for DIY guys.
Not sure that I understand why you were haveing troubles with the Toray film I suppose that with the heat shrinking the additional spaceing was necessary due to diaphragm instability which goes away with the higher tensioning obtained when you use your jig to streach the film tight. Breaking the diaphragm into smaller sections might just solve the instability problem allowing the use of heat shrink tensioning.

That's it. If I heat-shrink only the tension isn't sufficient to keep the diaphragm centered and I have to lower the bias voltage, or reduce the unsupported area of the diaphragm, or increase the stator to diaphragm spacing, all of which reduce the sensitivity/efficiency of the driver.

If you monitor the air pressure on the stretcher you get pretty close to the same tension from driver to driver. The best technique I have come up with for using the stretcher table is to lay the film on the surface, and fold one corner under and stick it to the tape. The go to the diagonally opposite corner and do the same, then do the other two corners. Then go to the middle of each side, pull the edge of the film under and attach it to the tape, and work out toward the corners. Just connect your air pump/compressor and pump up the tube. Go slowly- you can easily stretch the film to bursting.

Once the film is stretched, you can set a preglued stator down on top of it. When the glue/tape is set, relieve the pressure in the tube and use a fresh razor blade to cut the film around the edges of the stator.

The picture shows the bottom side of the table (the top is pretty boring!). You can see the double stick tape that lines the inside of the rim of the table down near the air valve. The film is stuff I leave attached after mounting a diaphragm. It protects the tape until I need the table again.

Sy: thanks for the nod regarding the 3M467 transfer tape. I am curious about which Clysar film you like best. I have a number of different Clysar films in stock and can pull them out and see which versions I have. Thanks for the input.

What is this tape and where do you get it?

Thanks,

I_F

[SY]

I "inherited" a roll of 467 in sheet form, but you should be able to ge it (or the equvalent) from any 3M distributor. It sticks fearsomely to nearly anything.

[I_Forgot]

Originally posted by SY
I "inherited" a roll of 467 in sheet form, but you should be able to ge it (or the equvalent) from any 3M distributor. It sticks fearsomely to nearly anything.

Any experience with how long it will hold a tensioned diaphragm?

I_F

[SY]

467 has exceptionally low creep and amazing bond strength. The tension limitation will come from the diaphragm, not the adhesive.