Garth,

the noise figure of a particular mic is lower than the ambient noise in
the room and therefore the mic noise is a non-issue ... does the noise of
different links in the chain combine in a linearly additive way or is it
more complicated than that?

Great question. Noise does add, but when a little noise is added to a lot of
noise the louder version dominates. Noise from different sources does not
add the same way as static sound. For example, if you add two sine waves of
the same level, frequency, and phase, the sum is 6 dB louder than either.
But adding noise of the same level from two different sources is only 3 dB
louder. Or thereabouts - I'm not Mr. Math Guy. The reason is because the two
noise sources are not correlated.

More relevant to your question is what happens when you add two noise
sources when one is 10 dB louder than the other, or 20 dB louder, etc.
Again, I'm not a math guy so I don't have the formula in front of me (though
it's a very simple formula). But even 10 dB is enough of a difference that
adding them won't make the result much greater.

So in your case where the mike's noise is 22 and the room is 40, the sum is
like 40.1 or something. Certainly not 62! Another factor is the frequency
distribution. Depending on where you live, room ambient noise is often made
up of low frequency rumble as well as broadband hiss, unless you have
forced-air ventilation. Electronic hiss tends to have has more high
frequency content. So that too affects how the levels combine.

--Ethan