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David Flinn
The Audio Circuit

Introduction

This section provides information on how to make membrane material conductive for use in electrostatic or ribbon loudspeakers.

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Information

Diapragm Coatings

Ormecon Lacquer, Conductive Coating for plastic film (Marc Schroeyers)

Conductive Lacquer (Jonas Karud)

Elvamide (The Audio Circuit)

Tin Dioxide >(Jukka M. Mononen)

The Original Quad Diaphragm Coating, A Direct Replacement (by Gary Jacobson)

Review of Gary Jacobson's new original Quad Diaphragm coating (August 2000, by Ben Openshaw)

Mark Schapper (June 2003): I have had success using graphite on 6 micron Mylar to obtain (so far) very stable conductivity in the range.5 - 4 nS. The technique involves rubbing powdered graphite in with a dry tissue pad until a resistivity of ~50 - 100 KOhm is measured from any point to any point on the membrane. I use the Fluke Model 19 digital multimeter that can measure these very low conductances. It is the only unit I've found at anything like a reasonable price that can do this! The coating is then rubbed down with a tissue lightly moistened with Shellite. After some work, the conductivity falls within the range .5 - 4 nS.

The resulting coating appears very stable(1 year to date) and it is as near to massless as you can get, unlike some of the inks. I have used it on arrays working above 200Hz. Provided you place the film on a backing with some give in it (I use clean paper) and you ensure there is absolutely no dust or dirt between the backing and the film, this process does not damage the film. The procedure is performed before the membrane is glued into the panel, so that if you mess it up, you simply start again on a new piece of Mylar. I have 6000m of the stuff - enough to practise on!

Questions and answers

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Pedro Lauridsen Ribeiro (October 2003): Dear sirs, I'm having a serious problem in my attempts at building an electrostatic loudspeaker (ESL) panel. I live in Sao Paulo, Brazil, and here the air's relative moisture is considerably higher than in temperate weather countries such as US, Netherlands ans so on. This makes electric discharges between the membrane and the stators much more frequent (that's also why in tropical countries the storms have a great deal more lightnings), and makes it impossible to raise the mean potential between stators (50 cm x 50 cm wide) above 1,000 volts. The resulting audio power is too tiny to my prototype panel to be effectively used as a regular loudspeaker.

I'd like to know if anyone has ever met such a problem, and if and how it's possible to circumvent it. Roger Sanders's book, which is my main technical resource on the subject, says nothing about air moisture problems. I'd appreciate if you forwarded this message to the builders in ESL Circuit, so someone could provide me with an answer.

Thank you in advance for the attention and help. Regards, Pedro Lauridsen Ribeiro

Rob Mackinlay (November 2003): Hi Pedro, Martin-Jan's suggestion will be ok assuming you use a coating that is not humidity dependent to make it work. Encasing the panel in dust covers to provide a dry micro-climate will work to a degree but airborne moisture will still penetrate through the film at a molecular level. Soaking up the moisture with silica gel or other hygroscopic compounds may dry it out too much.

The other downside of this is that you now have dust covers colouring the sound. Not a desirable thing (IMO).

It's possible that the humidity is causing the coating to become far too conductive which will allow arcing from stator to coating to be a noticeable event as the full capacity of the diaphragm will be discharged into the arc. Have you tried a highly resistive coating, say >2000 megohms/square?? If you wish for us to send a small sample of such coating to you, please contact us via enquiries@eraudio.com.au

We have used this coating in very humid areas such as coastal China, Philippines, Singapore etc with excellent results. Best regards Rob

Martin-Jan Dijkstra (October 2003): Dear Pedro, This is just an idea which I haven't tried; ESLs like Quad use protective membranes at both sides to avoid dust etc coming into the ESL. Maybe you can use such protective membranes (thin as possible ; 1 to 3 micron) to protect against humidity. Assembling the ESL should occur under dry conditions. In addition you can make an airchannel to a amount of hygroscopic salt (eg CaSO4) to remove the moisture constantly. Of course the construction has to be sealed, so plastic materials are first choice and wood is not. If it is sealed well enough you may have to replace the hygroscopic salt only once. Good luck! Please contact The Audio Circuit if you can provide an additonal answer.

Jonas Karud: During my years of ESL-building, there's always been one problem overshadowing the others; How to make a lasting, slightly conductive coating on the membran, so high in resistivity that no currents can flow on the membrane surface, thus eliminating arcing and the need for insulated stators.

There is also the benefit of less distorsion plus the fact that You can choose very thin films as membranes. If We DIY:ers shall take the ESL-development further, we must find the formula for this conductive compound! The compound exists. The QUAD 63 uses it in a very high resistivty coating on a 2um (!) film. The BIG QUESTION is; How is it done?

Update: Sheldon D. Stokes is not selling his indium coating, due to difficult applying techniques, graphite is to low in resistivity, even when its wiped off. I'm on the trace to a very promising compound with a resistivity at around one GigaOhm/square, that dries to a very thin durable film, must do long-time testing before presenting it to the ESL-circuit.

Barry Waldron (December 2000): Electrostatic loudspeakers can be designed to operate either in a '¯Constant Charge'¯ mode, or non. The latter has been used in tweeter panels, and in the early days, single ended speakers. (those having a single stator. It is my understanding of CS operation that a minute amount of current must circulate, lest the device not operate.

Recent experiments with various coatings and insulations has shown that high resistance, high voltage, high dielectric stators generate a much stronger arc resulting in large holes in the diaphragm; whereas, low(er) resistance coatings, high voltage, high dielectric stators generate an arc with considerably less destructive force. For speakers that are not expected to operate in the bass frequencies it would seem prudent to use graphite or other lower resistance media. I hope this is of help. Barry Waldron. Please contact The Audio Circuit if you can provide an additional answer.

MartinJan Dijkstra (December 2000): I am worried about the durability of the soap layer on the mylar membrane. I have used ''Driehoek'' liquid soap with glycerine in it. It seems also to attract more dust than a graphite coated membrane. May be I have used too much soap. Who can tell me more about soap as a conductive layer? Please contact The Audio Circuit if you can provide an answer.

John Doe (December 2000): Who has experience with liquid soap as electroconductive coating? I am specially interested in stability aspects.

Barry Waldron: Liquid dish detergent has been applied to diaphragms off and on for years. It has advantages and drawbacks. First, it is opaque, being neither translucent or transparent. Second, its appearance, depending upon how it was applied, leaves either brush marks or '¯finger painting'¯ streaks. It provides a high resistance and is suitable for low frequency operation. Soap adds more mass than other substances, and, it must be used with a low resistance bus for two reasons. One, soap can corrode the diaphragm contact resulting in panel failure. A low resistance bridge keeps the contact away from the chemicals in the soap Two, in order to guarantee a rapid and equal charge recovery time over the entire surface area, the low resistance loop transmits the voltage from the diaphragm contact to all points around the perimeter equally.

Stability seems to be excellent. The material does make it difficult to remove dust from its surface, and, it can easily flake off if rubbed. I hope this is of help. Good luck. Barry Waldron. Please contact The Audio Circuit if you can provide an additional answer.

Specifications

Dupont Elvamide Ć‚Ā®, data sheet (PDF 1.4 MB)