measuring microphone self-noise

["jpbeale" jpbeale]

Not sure where to post this, would it go better on the micbuilders list?

I would like to measure the self-noise of some of my microphones. I
have a very rigid airtight enclosure which I suspect has high enough
attenuation to be useful, but the interior space is fairly small.

My question: should I expect the noise output from a microphone that
is closely wrapped in felt to be the same as the noise when there is
more open airspace around the mic, assuming in both cases negligible
external acoustic noise?  The only difference I can think of is
thermal convection currents, which I hope are too small to measure.
Are there any exotic effects I might be missing?