elektret mike amplification

Exactly, hence the struggle. :)

joe

About ten messages down this thread we just found out that the electret is *not* a fixed capacitor with a moving electret foil in between.

One side of the electret is metallised and forms one plate of the capacitor, the other side of the electret faces the opposite capacitor plate.

It can be that the electret foil moves due to the sound pressure, it can be that the electret is mounted on the backplate, while another metallized foil is moved by the sound. The latter gives more freedom in the choice of the moving membrane and more opportunity to optimise with regard to acoustic properties.

So it indeed works 'just' by (open voltage) U = Q.d/(eps0.A), with Q the charge not on the capacitor, but on the electret...

joe

The fixed plates are at opposing voltages. It's a push-pull kinda thing.

We're in violent agreement about that. It wasn't a mystery to begin with, because (as you say) it couldn't work otherwise.

Depends on the load. Do google "displacement current" some time--it's a good concept to keep in the toolkit.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

** Electrostatic speakers have two fixed, perforated plates that are driven with anti-phase voltages. The diaphragm is charged to a high voltage ( ie 5kV) with respect to both plates and the stored charge does not vary in operation.

The electric field strength between the fixed plates is proportional to the voltage difference and is quite linear. So the force on the diaphragm is always proportional to signal voltage at any usable displacement.

Although the diaphragm may be large, it experiences an even force applied all over its surface - similar to microphone diaphragms when exposed to sound waves arriving on axis.

.... Phil

Yes, and more so in the electrostatic loudspeaker than in the electret microphone, as the surface to circumference ration of the former is much larger than of the latter.

I guess that in the electret, the bending of the diaphragm leads to more non-linear distortion than the bending of the foil in the electrostatic loudspeaker.

joe

** But it takes enormous SPLs to displace the diaphragm of a condenser mic more than a tiny amount - and it is not frequency sensitive.

Like most speakers, electrostatic speaker diaphragms move far more at low f requencies and can run out of available excursion. At any audio frequency a nd at levels over 100dB SPL, the needed drive voltage becomes excessive and arcing is induced. Amplifier power must be restricted or the speakers cont ain overvoltage protection, as in the Quad ESL63 etc.

At least one maker ( Dayton Wright) used Sulphur hexafluoride gas in their speakers to overcome this.

.... Phil

Ok.

joe