On a sunny day (Sat, 04 Dec 2010 14:09:56 -0500) it happened Phil Hobbs
wrote in
:

Jan Panteltje wrote:
On a sunny day (Sat, 04 Dec 2010 12:32:34 -0500) it happened Phil Hobbs
wrote in
:

signal. If you rip an electret mic apart, you'll find that it is
metallized on both sides, and the signal is taken between the plates.

It really isn't a capacitance microphone, although its audio
characteristics are somewhat similar. The plates don't move together,
and there's no net field anyway, even if they did. The physics is the
change in the electric polarization due to strain in the material--i.e.
a poled piezoelectric.

Snipped PhD confusion

I do not think that is correct.
http://en.wikipedia.org/wiki/Electret_microphone
Scroll to the bottom, there are 3 types of electret mikes described.
The second one described has the electret film fitted to the backplate, where it does *NOT MOVE AT ALL*.
The third one has the film on the inside front cover where it does *NOT MOVE EITHER*.
That sort of nullifies your argument about strain, except on your mind of course.


insert clue here



We're now in violent agreement, I see.

Oh no.



I didn't mean "move in the same
way", but "move closer together", i.e. the spacing doesn't change, so it
can't be a capacitance mic. Clumsily put, admittedly, but the rest of
the argument should have removed the ambiguity. Electret mics are not
capacitance mics.


It all depends, apart from the issues of ego and losing face,
for a man it is better to admit he is wrong, simply, at times, a pussy will blow
smoke and go side-paths.
You show how much you know about piezo, but this was about electrostatics.
Apples and oranges.

Let's look a bit closer.

If you take 'capacitor mike' literally, as one MIGHT do,
then neither the capacitor mike, nor the electret mike, really uses the CAPACITANCE.
One could make a 'capacitor mike' by making a tuned circuit (LC) oscillator, and have one fixed and one moving membrane
form a capacitor, and the frequency would change under influence of the capacitance,
so under the influence of the membrane moving, so under the influence of air pressure changes, say audible sound
if it is in the air pressure changes occur in the right frequency range.
There is a square in there somewhere, so it would not be all that linear over a large movement.
Once could use a FM radio to detect this if the circuit was made to oscillate in the FM band.
Maybe the correct word here is 'parametric'.

Then there is the real 'capacitor mike' as we call it, and that is actually an 'electrostatic mike',
http://en.wikipedia.org/wiki/Microphone
where something moves RELATIVE TO (old Einstein ;-) something else, where one of the objects carries a charge.
This is the SAME in the 'capacitor mike' and the 'electret mike'.

The difference is that the capacitor mike needs an external supply for the voltage,
and the electret uses a polarised film, so needs no external supply, simpler, cheaper, better,
no hum, no filtering problems.

So for all practical purposes, and all theoretical purposes, we can compare a 'capacitor mike'
with an 'electret mike', as both use exactly the same mechanism.
Thank you for your attention and have a nice day.

PS
There are also electrostatic speakers, Quad comes to mind as a manufacturer.
Very much HiFi.
I have even listened to electrostatic headphones.

El Pante