Thursday, January 10, 2008

DIY Electrostatic Loudspeakers / ESLs - The Diaphragms

The diaphragm is the one of the most important part if not the most important part of any ESL build. It is the diaphragm's surface that will vibrate the air and thus produce music.

Firstly, I needed a flat surface to work on that is both large enough to hold the film along with its sticky tapes (what is this for ... read on) and more importantly this surface must be free from protuberances that may damage the very fragile polyester film. So I used a glass sliding door from my walk in wardrobe. Next I prepare the sticky tape and place it all along the edge of the work surface. I mark out the perimeter of the panels that will be glued to the diaphragm on the work surface using whiteboard marker (erasable). This gives me a reference point as I go about my tasks.

Next I rollout a length of polyester film over the work surface. I make sure that there is enough film to adequately cover the references that was drawn on the work surface earlier. I also make sure that there is enough overhang on all sides of the film for me to work with I leave an overhang of 5-10cm on all sides. At this stage I just attach sticky tape over the 4 corners with moderate tension just to hold it in place.
Here is another view of the as yet, un-tensioned diaphragm. Here is a close up view from one of the corners with the sticky tape attached.
With the diaphragm and surface prepared, I am now ready to tension the diaphragm up to the designer's specification. To achieve this, I use the supplied tensioning gauge which is really a simple spring weighing scale with a piece of machined aluminium attached to the end.
First take a piece of sticky tape, attach about 2cm of it to the aluminium plate of the tensioning gauge. Next attach another 2-3cm of the sticky tape to the polyester film. Now pull the tension gauge until the correct weight is achieved, in ER Audio ESL III's case it is 800 grams. Then immediately press the sticky tape down onto the work surface and release the rest of the sticky tape from the tensioning gauge's aluminium plate. This is a really ingenious way of tensioning the diaphragm for a home builder, and simple too! The only complain is that I have 60 more sticky tape to tension all round the diaphragm. The polyester film is only 3.5 microns thick and as such very, very fragile. Do not let anything sharp come in contact with the surface. In my case, the tensioning gauge slipped from my hand and onto the surface (I was sleepy, and it was about 2-3am). It caused a rip and made half of the film unusable. Instead of getting 4 useable panels, I had to settle with just 2 panels with this sheet of film.After about 30-45 minutes of tensioning, you end up with a properly tensioned diaphragm.Now that the diaphragm is properly tensioned, I am ready to attach them to the panels. I start by applying the superglue or more aptly cyanoacrylate on to the air gap spacers. I apply two beads/channels along the length of the spacer. The first bead/channel is 2mm from the inner edge of the spacer; the second is just off the center of the spacer. The difficulty that I faced is that the cyanoacrylate dries quickly, so when I am applying the second bead, the first bead is already drying up and so I have to reapply the first bead again. You may not have realised but dry glue is no use to anyone!! This step is important so that the diaphragm is suspended properly from the inner edge of the spacer so as to avoid rattles when the diaphragm is at work.Once the glue is applied, I quickly place the panels onto the diaphragm and press down firmly to ensure a good bond. In addition I place weights onto the panels to ensure good contact as the glue dries.... I leave it for 2 hours..... 2 boxes of electrolytic caps used as weights (20 x 100,000uf @80vdc meant for another project.... watch this space)After the 2 hours are up, I use a very sharp cutter to free the panels from the rest of the diaphragm. Here is a picture of the completed panel with diaphragm. Because the glue does not spread evenly, I am left with numerous pockets of air between the film and spacer. In addition there are several instances where the film did not bond to the spacer at all. In this case I have to touch it up by dripping more cyanoacrylate on to the affected areas. This gets messy and aggravating to get right!These pictures just does not show how challenging and aggravating this part of the build is, for me anyway. Here is a picture of one of the treble panels that was just about perfect.Touching up the not so perfectly bonded area is a matter of applying more glue to the un-bonded areas. Since cyanoacrylate is very runny, I just drip some of it in the area and the capillary effect ensures that the glue fills up all the un-bonded areas. Attached is a picture of the less than perfectly bonded areas. According to Rob at ER Audio, there are more than a few ways to attach the polyester film to the air gap spacers.... 2 methods that I could have used was the super fast drying super glue method and the snails pace 24hour curing polyurethane method. On hindsight, I should have used the slower method. Why?

The super glue dries really fast, and as such you have a very short window of time from when the super glue comes out of the bottle until you place weights on the panel. And there are a lot of things that must go on in the correct sequence in between those 2 points in time, for the film to adhere to the spacers. And as my friend Murphy has it, a lot of things do go wrong.

Using the polyurethane would have afforded me much more time to get things "perfect" before the film touches the spacer. But I was impatient, so I chose the super glue method. Time will tell if I chose wisely or unwisely.... if the film stays on and does not rattle mechanically, I should be home safe. If not I have plenty of polyurethane glue and more than 40 meters of 3.5 micron polyester film left.

Next The Diaphragm Node Points.

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