In my early days of electrostatic speaker repair I coated one side of the diaphragm only. Many people will tell you that you need only coat one side. There are many internet sites which say this. Also there are books available which describe how to make your own ESL speaker and these also tell you to coat one side only.
I will try to explain why many people suggest coating one side only and why I don't think this works in the ESL 57 treble panel. What has to be understood is that the music signal which is being applied to the stators is attracting/repelling the charge on the diaphragm not the diaphragm itself. It is only because the charge is held captive on the surface of the diaphragm that means the diaphragm will vibrate.
Consider a diaphragm with a coating on one side.
So, you have a charge on the surface of a diaphragm which is being driven by a musical signal. This charge is an electric field. As such it is similar to a magnetic field. If you put a piece of paper in front of a magnet it won't stop it working. This is the case with the diaphragm. The stator on the coated side of the diaphragm can directly see the coating and hence the charge. But between the rear stator and the charge is the diaphragm itself but this doesn't matter because the diaphragm is invisible as far as the electric field is concerned. This is fine as far as it goes and this is why most people only coat one side but with the Quad treble panel there is another consideration.
Anyone who has taken a bass or treble panel apart will notice that the Quad diaphragms are coated on both sides. I tried to come up with an answer as to why this was so. I could only guess that this was because the original Quad coating was applied so carelessly that it needed to be coated both sides to ensure the whole surface of the diaphragm was driven.
I now believe that there is an extra reason which I had overlooked. As soon as you apply the coating to one side only you immediately have an imbalance. You should after all do the same to one side of the diaphragm as the other to maintain balance.
I have learnt the following after repairing many diaphragms so this is all practical experience.
When I repair an arced treble panel I clean the burned area and re-insulate it with corona dope. This insulates the exposed area but the dope is such that it never quite hardens. It always remains soft to the touch and slightly sticky. I would then put the new diaphragm in the panel and try it out. It was not unusual to notice that when the high voltage supply was removed from the diaphragm you would get a nasty crackling noise. I always put this down to some minor leakage causing the diaphragm to crackle but I now realise that this crackling was the diaphragm pulling away from the area which has been repaired with corona dope. This meant that the diaphragm was hitting the back stator and sticking there until the high voltage supply was removed at which point the diaphragm would gradually become unstuck from the corona dope and thus cause the crackling. The question is why was the diaphragm hitting the back stator hard ? I now realise that this was being caused by the coating only being applied to one side.
Because the charge is an electric field any conductive material in close proximity with it will have a voltage induced onto it. Inside the treble panel the stators are only 0.75mm away from the diaphragm. With a conductive coating on one side only it meant that the front stator had a voltage induced on it and because these voltages were the same charge they repelled each other. The effect of this is to push the diaphragm hard against the back stator. The simple cure for this is to coat both sides of the diaphragm thus restoring balance to the panel.
The only question left is why do so many people only coat one side ? In the case of other ESL designs I suspect the reason is that the diaphragm is a sufficient distance from the stators as to make it impossible to induce a voltage on them. Don't forget that other designs including the ESL 63 use stator spacings of at least 6.5mm whereas the Quad treble panel has a stator gap of only 1.5mm. This means the stator to diaphragm distance is only 0.75mm so its not surprising that a voltage is induced. As to how other ESL 57 repairers manage to only coat one side and make them work is a mystery. Best to ask them.
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