How much current do microphone transducers produce?

[crazydoc]

...BTW, the signal from the capsule is not that small, as you might think. In fact, it is pretty healthy.

I found this quote from a recent thread somewhat provocative. In trying to figure out how much current a capsule produced, I realized how ignorant I was of AC circuit theory.

By using the following parameters (which probably suffer from inaccuracies, as I pulled them out of my butt) I tried to calculate the current produced by a condenser capsule.

Diaphragm to backplate distance = 50um
Diaphragm maximum excursion = 8.5um @140 dBSPL (I actually got this from a book) which I rounded up to 10um and assumed the whole diaphragm moved (instead of max at the center).
Voltage across condenser = 50v

By calculating the change in capacitance with diaphragm movement and converting this to change in charge (Coulombs), then converting this to current, I came up with a ballpark figure of 10's of nanoamps, which does not seem like a very healthy signal to me.

Of course I expect I've made errors, both in arithmetic and theory. Maybe Marik, or anyone else, could further explain and correct how much current is produced by a condenser capsule.

And while were at it, how much is produced by dynamic and ribbon transducers? I'd expect these to be orders of magnitude more, as they require no further amplification (except impedance matching and signal balancing done by transformers) prior to the preamplifier.

 

[Marik]

Of course I expect I've made errors, both in arithmetic and theory.

Doc,

No, there is no error in arithmetics. Your calculated value is more or less what capsule will produce.
But in theory... don't forget, the capsule is not a current, but a VOLTAGE generator. Remember?--current drives Low-Z, voltage drives Hi-Z.
Becasue of capsules current is so small, we need very high input Z of the impedance converter. Heck! because of such a low current, you even can't measure polarization voltage at the capsule--the voltmeter will load the reading.

To illustrate what kind of voltage capsule produces, take a cathode follower circuit. It has a slightly below unity gain. Then we have a step-down (let's take a comfortabale to calculate value 4:1=-12db) transformer, so we reduced the signal by more than 12db, and the mic still has a nice output.