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 SkySeeker 15th September 2007 07:52 PM

ESL Capacitance calculation

The formula for the capacitance of an ESL is given by:

C = (8.85E-12 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?

 ak_47_boy 16th September 2007 08:58 PM

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.

 Calvin 17th September 2007 09:22 PM

Hi,

basically it dosenīt matter which distance You choose!

The formula gives an idea about the capacitance value under nosignal conditions.

In a symmetrical ESL the distance between stator-stator is twice the distance stator-membrane. The capacitance between stator-stator is one half of that of stator-membrane. 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 stator-diaphragm capacitance. The low-impedance stators You can easily measure. This value is more interesting because it is the one that the amplifier sees (through the transformer).

jauu
Calvin

 SkySeeker 18th September 2007 09:49 AM

> The low-impedance 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?

 Calvin 19th September 2007 04:59 PM

Hi,

Yes,odf course :D
For the sheet metal stators I use the st-st-capacitance is nearly the same as calculated.

jauu
Calvin

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