Shunt/Stray Capacitance

[starz00700]

I have searched this forum, and seen the question posted as part of a thread, but I could not find the answer. Question: If you do not apply conductive coating to the part of the diaphragm which covers (attached directly to) the spacers, does this reduce shut/stray capacitance? Sorry for such a basic question, but as I assemble my panels, the answer will influence how I apply the coating. Thanks for any and all help here.

[chinsettawong]

Hi Starz00700,

I don't think that the coating has anything to do with the capacitance. I think that the capacitance comes mainly from the area where the stator is covered by the diaphragm.

Wachara C.

[Calvin]

Hi,

when You use a high-resistance coating the diaphragm basically is ´switched´ out of the electrical circuit. As such there is no capacitance between diahragm and stator. The electrical connection to the diaphragm is just to keep the membrane charged so that the electrical field can act upon it (think of Electret-diaphragms which don´t need any electrical connection at all).
Stray capacitance is a capacitance value measured between the two stators (or stator-diaphragm in the case of a low-resistance coating of the diaphragm). It is the part of the area which can´t contribute to sound but still functions as a capacitance. Roughly it is the summed up area of spacers. Since the dielectric constant of the spacers is usually higher than that of air the capacitance value can reach comparable values like the usable capacitance of the free vibrating membrane area. Since the overal capacitance value defines the impedance of the panel, the design and drive-demands for the audio tranny and amplifier are also influenced.
You can´t avoid stray capacitance but You should try to keep it small.

The reasons to not coat the complete membrane area are:
- a few mm around every spacer the membrane is restricted in stroke and can´t contribute fully to the SPL.
- the rims of contact between spacers and diaphragm are prone to flashovers and leakage. Sparing out the coating around these rims reduces this phenomen


jauu
Calvin

[starz00700]

Thanks Wachara for the quick reply, and Calvin for your very thorough explanation. I will not coat the area over/around the spacers, not to reduce stray capacitance, but for the reasons Calvin outlined. Thanks again! Jerry