At 4:22 PM -0700 9/20/05, Dan Dugan wrote:
>Eugene E Dorcas wrote:
>
>>Can you calculate the output noise from other specs such as
>>signal-to-noise ration and sensitivity??
>>
>>For instance: If the sensitivity is -32dB and the SNR is 75 dB,
>>what would the output noise be??
>
>Mic signal-to-noise is measured from 1 Pascal, which is 94dB SPL.
>
>If the SNR is 75dB, you can calculate the self noise of the mic:
>94-75 = 19dB SPL (probably the SNR was spec'd in dBA, if so it would
>be 19 dBA).
>
>The sensitivity is more relevant to the question of whether that mic
>would get its own noise above a particular preamp's input noise
>level, called Equivalent Input Noise (EIN). If it does by more than a
>few dB, then you can get the best performance from the mic.
>
>http://www.rane.com/note145.html
>
>The noise output of a mic with 75dB S/N and -32dB sensitivity (very
>hot!) would be -(75+32) = -107dBV. dBV because the sensitivity is
>measured re 1 Volt (1Pa/1V).
>
>0dBu is 2.2dB below 1V (don't ask), so the noise output voltage of
>the mic in Haleakala crater would be -104.8dBu.
Professor Dugan, I signed up for the course in Audio Measurement
Reference Levels, again. Charts 3 and 4 here
http://www.rane.com/note148.html show mic sensitivity as -dBu and as
mV/Pa. Seems not to be a simple arithmetic relation/conversion. What
is the "-32 dB" sensitivity measurement you are working with
referenced to or need it be? Rob D.
>That would be way
>above the noise of even a commonplace preamp (like EIN = -125dBu). A
>mic with 10dB lower output than yours, like the Shure WL183, is still
>hot enough to make the noise of a cheap preamp irrelevant.
>
>Another confounding factor would be that preamp EINs used to be
>measured in dBm, and some still use that measure. Let's not go there,
>either. Any real audio engineers here, please check my figures.
>
>-Dan Dugan
>(I'm not an engineer, but I play one at work.)
>
>
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