[ProAudio] Microphones question
dlj at josephson.com
Fri Jun 11 13:44:19 PDT 2021
150 ohms was the most common “nominal” microphone impedance for dynamic mics when these standards were being made. It's a convenient workflow to measure the output noise, reduced by the gain of the stage, to estimate the input noise. Then you can subtract the thermal noise of the resistor (the oversimplified version you describe is generally the method in use, with caution that the same weighting is used for all measurements or predictions) which allows you to state the equivalent input noise with whatever weighting and detector law is desired.
As Scott mentioned, microphone impedances vary hugely, model-to-model and versus frequency.
> On Jun 11, 2021, at 12:44 PM, Dan Lavry via ProAudio <proaudio at bach.pgm.com> wrote:
> 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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