[ProAudio] Microphones question
Dan Lavry
dan at lavryengineering.com
Fri Jun 11 12:44:00 PDT 2021
My question is about mic output impedance, in relation to noise:
Both the mic and the micpre contribute to noise. The micpre generates
some noise voltage which can be measured by replacing the mic with a
short (0 Ohm). But there is also mipre generated noise current, which is
no problem for 0 Ohm, but real mics have some impedance...
At some point, it was decided to model a mic noise with replacing the
mic with 150 Ohm resistor. I am not proposing to change it, just trying
to understand why 150 Ohm.
The value 150 Ohm makes 1.568nV/sqrtHz (at room temp), so for 20H-20KHz
noise voltage of .225uV. Given that we are interested in noise power, we
can use the dBu scale to realize that the resistor itself sets a limit
on the noise floor at -130.9dBu. But say the impedance is 1K, then we
have -122.8dBu.
I assume that the resistor modeling is a simplification. I would be
interested in comments from the mic experts here.
Thank You
Dan Lavry
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