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Old 8th March 2016, 01:20 AM   #1
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Red face Diaphragm charging question

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.
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Old 8th March 2016, 01:35 AM   #2
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the diaphragm is so very thin and the charge so very high that the diaphragm does not with any significance impact the charge value on the non connected side. So the simple answer is you don't need to make it more complex you have just as many volts of charge on both sides to do the job.
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Old 8th March 2016, 01:53 AM   #3
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Ok, that makes sense. Thanks.
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Old 8th March 2016, 03:02 PM   #4
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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.
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Old 8th March 2016, 07:02 PM   #5
golfnut is offline golfnut  New Zealand
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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.
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Old 8th March 2016, 07:06 PM   #6
jcx is offline jcx  United States
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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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Old 8th March 2016, 07:32 PM   #7
bengel is offline bengel  United States
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Good question, I was kind of curious about this myself.
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