Remember as well that you can subtract out the noise of your preamp. As mentioned it should be small anyway, but measure it with a dummy load equal to the mic output impedance. Square the total noise, and subtract the square of the preamp noise with the dummy load. Take the square root---that will be the mic noise alone. If you like, correct for the thermal noise of the dummy termination knowing the resistance and the bandwidth.

As JDB points out, the capsule amplifier and other noise sources in the front end will be far larger than thermal noise in the output impedance of the line driver. Actually "theoretical minimum noise" for a mic with a mythical noiseless capsule amp doesn't have to be thermal noise of the magnitude of the impedance either, for that matter---one can contrive circuits to look like a given Z over a useful frequency range without having full thermal noise of the equivalent resistor*. However, it would be a wonderful world if such considerations were relevant for microphones.

*I got into this behind a few glasses of wine at AES once, when Jürgen Wahl was presenting the Neumann Solution D, and asserted that regular mics were limited by the thermal noise of their impedance magnitude. My comments were not well-received, possibly owing to the presence of some Neumann execs in the audience :razz: