"Garthrr" wrote in message

I'd like to try to clarify something for myself and any other dummies
like me who dont quite get this. I sometime read posts or articles
where the writer says that the noise figure of a particular peice of
gear (for instance, a mic) is less than something else (perhaps the
noise of the room or another peice of gear or whatever).
Someone might say (and they have) that 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. I take this to mean that they
are saying that the noise in the room sort of swallows up the mic's
self noise and nullifies (masks?) it. Lets say for example that a
mic has 22dBA self noise figure and that the room has 40 dBA ambient
noise. (excuse me if I am quantifying these figures wrongly) I get
the impression that the combination of these two noises added
together would not be 62dBA as would seem logical to me. If the
noises did add together in that way then I dont believe people would
say not to worry about a mic's self noise because the room is so much
noisier. Instead they would be saying that the mic's self noise is
just as important in the 40dBA room as it would be if the room noise
was 20dBA.

The correct way to add random uncorrelated noises is to convert the two dB
values into amplitude, and then take the geometric sum of the two.

I have a spread sheet that does this.

In your case the answer is 40.07 dB. IOW the contribution of the 22 dBA mic
noise is negligible.