At 6:14 PM -0700 9/22/05, Dan Dugan wrote:
<snip>
>
>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
Ouch, I do grasp a little more, but I'm headed to the microbar.
Thanks.
>
>-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.
>
Rane Note 148 says -10dB =3D .4dB noise from the pre. -7dB is the
number I'll try to remember as its just less than 1dB. Rob D.
________________________________________________________________________
________________________________________________________________________
"Microphones are not ears,
Loudspeakers are not birds,
A listening room is not nature."
Klas Strandberg
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