John,
You've explained it very well. I'm afraid much of the confusion comes
from mic manufacturers who don't define what their figures mean. And
I'm not going to mention weighting curves again here. (I just have)
:-)
I didn't use the "vacuum method" as this could cause disaster in
practice with a sealed mic capsule, but as a principle it works well.
> Happiness is when the sum of the mic output noise (in mV) and the
> preamp noise (in mV - actually the square root of the sum of the
> squares of the values) is less than 1 dB greater than the output
> noise of the microphone alone (or the noise of the preamp alone in
> the unusual case of a preamp that is noisier than the microphone).
I like your criterion of less than 1dB increase due to the preamp. In
other words, the mic noise should swamp the noise in the later stages.
With mic outputs greater than about 20mV/Pa, preamp noise should not
be a problem. If it is, you could be using a poor quality preamp with
an expensive pro quality mic which would be silly. :-)
You could say that the greatest noise you can hear (apart from
acoustic noise) should come from the microphone.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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