This is the crucial part of the project, the “bass section” and “tweeter section’s membranes. I put them between quotation marks because they are neither, the cutting frequency is 1000 Hz (or is it 500 Hz…?), and the filter is of a very low slope, so they overlap a considerable amount with respect to the frequencies.
The membrane is at the moment the most time requiring part of the project, as it is designed. I will give, at the end, a series of possibilities as to how this could be shortened. It is
also the part that requires most care, patience and “abilities”, but these will develop during the building and design.
The membrane consists of three parts, aluminium foil, a plastic support membrane and adhesive glue, that binds the two.
The plastic support in the original project was an oven polyester bag slit along the edges, giving you a strong, very much, non-stretchable film at a very low cost. The negative sides were the limited size, the crease at the middle, that can even have a discontinuity cut and the relative thickness of the film (and some oven bags are humidity sensitive to my horror!).
I have chosen Mylar Film, something I was not able to find in Chile, but as I casually was in Buenos Aires, Argentina, I bought in the most unlikely shop, in a very adequate width of 12 micrometers (micra), 1 m. width and 15 m length, material for a big amount of loudspeakers… Some builders do not get to stuck up on the width or weight of the materials used on the membrane, with the argument that “the weight of the air moved by the membrane is so high that a few grams more or less on the membrane will be of no effect, I have no way of supporting or counterargument this, I chose the lighter and thinner materials, that give you a very flexible and non rattling membrane, a problem that arises when you have thick aluminium and a thick polyester material.
Here in Chile, and after I ripped the second membrane I was making (!!!%$$~@, booo hoooo), I was forced to find some more membrane material for the first full size prototype, so I looked
hard and found what can be a very good solution for those that can’t find Mylar film, stickers…. Stickers, in Chile at least, are mostly made of Mylar film with adhesive gum on the backside.
I found they sold the prime material for stickers in 1 x 1 meter sheets, just the right side for the project’s loudspeaker membrane, and then you need not worry about the sticking of the aluminium, the mylar already comes with it.
What about the aluminium foil on the polyester membrane? How do we get that and how do we stick it on? What width must it have? How do we cut it?, etc.
I’ll start with the glue. In the original project the aluminium foil they use is a foil used as thief alarm conductors for windows, a 30 micra wide foil, that included glue on it’s backside.
In my design, I was required to stick the aluminium foil on the Mylar, as some other designs do too. I recommend you use a 3M, Super 77 Spray adhesive, this permits a thin glue layer that is strong and durable. You can use other glues but remember to dilute them well, so you can spread them on easily, and thinly.
With respect to the foil, some have chosen to use aluminium foil, cut in long bands. I have chosen an easier route, I have used a special paper that includes a thin aluminium foil on waxed paper, here in Chile, this paper is used to pack chocolates here in Chile…. I do not know if this is universal… This paper has the characteristic that the foil comes off the paper if you
heat it and liquify the wax. Thus you can leave a very thin foil (aprox. 12 micra) on the polyester and eliminate the heavy paper.
How to place the membrane on the frame
This bi-layer permits an easier cutting of the membrane design. You cut the design directly on the foil, avoiding cutting through the base paper, thus the design is held together by it.
What about the cutting, you have to leave a 1 mm. space between each conductor! You may then say, “How are we to cut so precisely!”
Easy: You use a double sharp bladed “knife”, we call them in Chile, cardboard knives, sharp blades, wide and easy to handle, you put two together with a coin between, or something 1 mm. wide, scotch them together and start cutting! With care, obviously, but deep enough to cut well through the foil, once you do so it is easy to pull off the thin 1 mm. wide aluminum foil string from the waxed paper, you must be careful doing this, if the foil isn’t completely cut you may pull a piece off the conductor too, slowly, slowly, that’s the way.
In my opinion, this is the easiest way.
- Membrane material: Mylar Film 12 micrometers (0.012 mm)
- Alu-foil: approx. 12 micrometers (0.012 mm) Total width: 0.024mm
- Membrane is not corrugated, just stretched out as is.