Rob Danielson, you wrote,
>"dBu" =3D decibels referenced to "0" or .775 volts rms.
>"dBV" =3D decibels references to 1 volt rms.
Yes. Both these units refer only to electrical audio levels,
referenced to standard levels that are .225V or 2.2dB apart.
>and the "-32 dB" you used is "-32dBV"?
The original poster just said the sensitivity was -32dB, and I
-assumed- it was referenced to 1V because major manufacturers use 1Pa
and 1V for zero.
>To determine the EIN value for a mic
An input has an EIN. A mic has an equivalent Sound Pressure Level for noise=
.
>(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 think so; I cheat by using the Shure Sound Reinforcement
Calculator, a cardboard slide rule. But that calculator references
sensitivity measurements to 1uBar (that's 74dB SPL)! It's a morass, I
tell you. After making many mistakes, I've drawn in another arrow on
the slide rule 20 dB to the left for 10uBar(=3D1Pa). BTW the "Bar" in
microbar has to do with the air pressure of ye olde planet earth--so
we're back to nature.
The reason for using 1uBar is that 74dB is a typical SPL of a talker
at a mic, so the sensitivity rating done that way gives you a figure
that immediately tells you what voltage you're going to see from your
mic in a typical application. It all makes sense, but my head hurts.
The math: dB in voltage levels is 20 * log(10) of the ratio of the
levels in Volts.
20 * log((mV/1000)/1), I think. Of course using 1V for the reference
makes it unnecessary to divide at all.
I've uploaded the little Excel spreadsheet I use for this--it's
actually two calculators, voltages to dB and dB to voltage ratio.
I've locked all the fields except the data entry fields. The (20)
figures are for voltages or pressures, the (10) figures are for
power. Don't ask.
http://www.dandugan.com/downloads/Ratio_in_dB.xls
>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?
No, that relates to the difference between the dBV and dBu reference
levels. Sorry to make your head hurt, too.
-Dan
p.s. the mic's noise output needs to be "more than several" dB above
the preamp's EIN to make the preamp's noise irrelevant. Around 10dB
to make it add less than .1 dB, I think.
-dD-
________________________________________________________________________
________________________________________________________________________
|