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Re: noise

Subject: Re: noise
From: Eugene E Dorcas <>
Date: Tue, 20 Sep 2005 19:18:39 -0700 (PDT)
Thanks Dan,
 
I am an elec eng also but I've always been involved in the computer logic 
world.  This audio stuff is new to me.  thanks a million.
 
Gene

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. 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.)



"Microphones are not ears,
Loudspeakers are not birds,
A listening room is not nature."
Klas Strandberg 
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