ehsjr said:
[...]
Balderdash. The OP is *guessing* he has a carbon mike.
If he does not then what good is it posting "the answer"
(if one existed) concerning a specific carbon mike?
You mention 'the answer for "some" specific carbon mic'
There is no "the answer ...", for crying out loud. It depends
on external (to the mike) circuit elements: the load and the
source. "The answer" does not exist.
Here, let me try one more time. I'll ask a hypothetical question that is
different than the OP's, but that might be informative nonetheless. I know
it's hard to answer hypothetical questions, but some people manage okay.
Let's say I have a friend who thinks he has a carbon mic element - maybe he
got it out of a 1950s-era telephone operator's headset. He thinks it's
carbon because it's old and when he shakes it, it rattles. He offers to
give it to me.
Now, I don't have it yet. But I want to think a little about what sort of
circuit might get some audio out of it. So, on the hypothesis that it is
indeed a carbon mic, I imagine I will hook it up to a 1mA constant DC
current source. (Note that the AC impedance of a DC current source is
infinite. The load and source are thus specified, just as they were in my
earlier question.)
1. Roughly what AC voltage level might I reasonably expect to see developed
across it, when I speak into it (at, say, 75dB SPL)? Are we talking tens of
uV, or tens of mV, or ...?
2. Does the performance of this microphone technology depend on a particular
current being passed through it? Would, for instance, 10mA be better than
1mA, in terms of the linearity and dynamic range of the mic?
3. Would it be better (in terms of linearity and dynamic range) to apply a
constant DC voltage and measure AC current variation, or to apply a constant
DC current and measure AC voltage variation?
4. What circuit conditions would this element have typically been used in,
and in those conditions, what sort of AC signal would it typically develop?