Diaphragm charging question

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I am reading Sanders' book on electrostatics, and have been looking at the many posts here and have a question that I haven't seen come up.

Please bare with me as I am trying to understand it.

If the ESL is being built in a push/pull format, why is the diaphragm only charged on one side? Wouldn't coating both sides and charging both sides increase the effectiveness of the rear stator in helping the front stator to pull or push the diaphragm (given that every design charges the front of the diaphragm, but not the back)?

Has anyone made an ESL with the back side of the diaphragm coated and charged?

OK, let me have it.
There was a technical article in the February 1956 issue of Wireless World on this topic.
They author claimed that distortion free operation for large diaphragm excursions could only be obtained if the two sides of the diaphragm were charged thru separate coatings that were insulated from each and fed bias voltage thru independent high resistances.

In the Letters to the Editor in the March 1956 issue, Peter Walker explained that if the capacitance through the diaphragm, from one coating to the other, is large compared to the capacitance between the coatings and adjacent stators then the operating conditions approached those of a single coating on one side of the diaphragm fed through a high resistance. Another letter provides the equations showing why separation of charges on either side of the diaphragm has no advantage over a single conductive side with the same net charge.

Another thing to consider is the mass added to the diaphragm from a second coating. For best high frequency response, you need to keep the moving mass as small as possible.


  • ESL_2coatings_WW1956_Article.pdf
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  • ESL_2coatings_WW1956_Letters.pdf
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The simples answer, I think, is that the diaphragm material is completely transparent to the electric field.

The electrostatic motor pushing the charges does not care whether the charges are on one surface, the other surface, both surfaces, or distributed through the bulk of the diaphragm. So long as they are attached to the diaphragm, and when they are move, the diaphragm moves with them.
or you could say the sheet of charge on one side only was "off center" by 1/2 the thickness of the diaphragm additionally divided by a factor of ( e_r - 1 ) of the plastic

in other words - by less than any tolerance anyone is likely to achieve
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