# Rough SPL Calibration using microphone sensitivity and Multimeter



## Chester (Feb 19, 2007)

Hello, I have been looking into using a multimeter to measure the voltage coming off of my microphone, with knowledge of the sensitivity of the microphone, presumably, I can have a fairly good calibration of the SPL once the voltage is known and related to the sensitivity:

my question: since it is an XLR connection, should I be measuring one 'side' of the balanced signal (hot to ground), or would both sides (hot to cold) be providing the 'correct' voltage? I presume it would be "hot to ground" since hot-cold is more used for removing noise from the line... Any advice?

My microphone is a DBX RTA-M, I am using the calibration file posted on this: http://www.audiobanter.com/showthread.php?t=74286 page, the RTA-M is supposed to have a relatively tight manufacturing tolerance. It quotes a sensitivity of 8.1395 mv/Pa (in the images of the frequency response), which by the calculator on this site http://www.sengpielaudio.com/calculator-transferfactor.htm (the last one at the bottom of the page) yields a sensitivity of -61.788045 db (Ref 74 db SPL) which is in line with the specs which state -63 db (+/- 3 db)... so I think that the noise floor of the mic (the quietest it can pickup) would be ~12 db? 

I realize that this is not necessarily the optimal way to measure the spl of the room, however I have already paid for my multimeter  I think it would be a nice addition to REW if there were an applet/area to input the microphone parameters so one could get a rough estimate (not as necessarily as good as calibrating with a SPL meter) what the spl of the mic is.

Suggestions/comments welcome (as always)!


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## Kal Rubinson (Aug 3, 2006)

Assuming you have the necessary preamp for the mic, feed it to your PC and use a freeware program for the measurements: REW or TruRTA.


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## Chester (Feb 19, 2007)

Right, I am using REW (which is why I posted in the REW forum) 

I am trying to correlate the output of the microphone (in millivolts across the xlr terminals coming off the outputs of my phantom power supply (ART Phantom I)) to the SPL in the room based on the sensitivity of the microphone... the preamp is actually *in* the microphone, which is powered by the phantom power supply.

Once I know that 'x' millivolts of output (at 1khz) means 'y' db spl, I can enter that number into REW once I plug the microphone into my PC and am running REW


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## Kal Rubinson (Aug 3, 2006)

Chester said:


> Right, I am using REW (which is why I posted in the REW forum)
> 
> I am trying to correlate the output of the microphone (in millivolts across the xlr terminals coming off the outputs of my phantom power supply (ART Phantom I)) to the SPL in the room based on the sensitivity of the microphone... the preamp is actually *in* the microphone, which is powered by the phantom power supply.
> 
> Once I know that 'x' millivolts of output (at 1khz) means 'y' db spl, I can enter that number into REW once I plug the microphone into my PC and am running REW


Borrow an RS SLM. Of course, I would not recommend that you buy one (and return it within 30 days).


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## Chester (Feb 19, 2007)

That would never be an option for me  I live ~10 miles from a Radio Shack and go by there all the time...

I mainly wanted to do it this way because I am a DIY kinda guy  Plus it is a learning experience


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