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15th September 2007, 07:52 PM  #1 
diyAudio Member
Join Date: Aug 2007

ESL Capacitance calculation
The formula for the capacitance of an ESL is given by:
C = (8.85E12 x A) / d Where C is in Farads, A in square meters and d in meters. My question is, what should "d" be? The D/S spacing? but the diaphragm is at constant charge. The audio signal is applied to the stators. Thus should not "d" be the distance between the stators? The books I have seen use D/S, but that seems wrong to me. Is there an explanation why D/S should be used? 
16th September 2007, 08:58 PM  #2 
diyAudio Member
Join Date: Sep 2006

An ESL is a very complex load, i think that formula is just a good approximation.
Capacitance increases with area and decreases with spacing. But the spacing is always changing during operation. Edit: There would be stator/stator capacitance also but it would be small and most likely insignificant. 
17th September 2007, 09:22 PM  #3 
diyAudio Member
Join Date: Nov 2004
Location: close to Basel

Hi,
basically it dosen´t matter which distance You choose! The formula gives an idea about the capacitance value under nosignal conditions. In a symmetrical ESL the distance between statorstator is twice the distance statormembrane. The capacitance between statorstator is one half of that of statormembrane. This is equal to a series connection of two identical caps....the value is halved. But since the diapgragm is usually coated with a high ohmic value material, it is difficult to measure the statordiaphragm capacitance. The lowimpedance stators You can easily measure. This value is more interesting because it is the one that the amplifier sees (through the transformer). jauu Calvin 
18th September 2007, 09:49 AM  #4 
diyAudio Member
Join Date: Aug 2007

> The lowimpedance stators You can easily measure.
> This value is more interesting because it is the one > that the amplifier sees (through the transformer). Exactly, that is why I was surprised to see the D/S capacitance used in the books. (Sanders, Wagner, Fikier) Have you ever compared the measured and calculated capacitances of your speakers? 
19th September 2007, 04:59 PM  #5 
diyAudio Member
Join Date: Nov 2004
Location: close to Basel

Hi,
Yes,odf course For the sheet metal stators I use the ststcapacitance is nearly the same as calculated. jauu Calvin 
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