[ProAudio] Microphones question

[Dan Lavry]

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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