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Re: noise

Subject: Re: noise
From: Rob Danielson <>
Date: Thu, 22 Sep 2005 14:48:21 -0500
At 11:38 AM -0700 9/22/05, Dan Dugan wrote:
>Rob Danielson, you wrote,
>
>>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.
>
>One of the problems with mic specs is that people use different
>scales and you have to convert to compare. The mV/Pa is typical. dBu
>for sensitivity isn't; Shure and Sennheiser reference dB re 1V. Rane
>is converting to dBu because they're trying to make it easier for us.
>
>>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?
>
>Any dB figure except gain is ambiguous unless the reference is noted.
>dBu is shorthand for 0 =3D .775Vrms. dBV is re 1Vrms. dBm is re 1
>milliwatt, a power level, not a voltage level, so it depends on the
>impedance. It's gone out of style because we don't impedance-match
>audio connections any more.

Thanks. Okay, so:

"dBu" =3D  decibels referenced to "0" or .775 volts rms.
"dBV"  =3D  decibels references  to 1 volt rms.

and the "-32 dB" you used is "-32dBV"?

To determine the EIN value for a mic (for seeing whether its self
noise is a couple of dB higher than the pre), I'd need to be able to
convert mV/Pa into dbV right?  Can one do this with pre calculus math
skills?  I'm guessing, as a nature recordist, that the conversion
involves the .225 volt discrepancy.  Is there a short cut like the
2.2dB addition you did below?  Rob D.

reinserted for reference:
>>
>>The noise output of a mic with 75dB S/N and -32dB sensitivity (very
>>hot!) would be -(75+32) =3D -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 =3D -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
>





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