# Understanding SPL offset (UMIK-1)



## Waschwasch (Oct 28, 2015)

Hello,

I am using the UMIK-1 (Calibration file 700-9345) and Win7 for a student project and want to comprehend the final step of SPL computation in REW. I understand the steps for frequency- and time-weighting (dBA/C/Z, Slow/Fast). 

I located "HKEY_CURRENT_USER\Software\JavaSoft\Prefs\room eq wizard" in the registry and found the key SplLastOffset, which corresponds to LZPeak in my tests. This should correspond to 0 dBFS (full-scale). Now I've mapped the input level (Win7, dB and regular) to the key for several values (see attachment). You can right-click the input level meter in Windows to change it to dB, in case you didn't know. At +24 dB digital gain I reach an offset of 100.673 (SensFactor = -.673 in cal. file) and at -14 dB 140.673 (Also notice the outlier at -31 dB!) . This makes sense because a higher gain should lead to a lower full-scale value. Adding the digital gain in dB and the REW SPL offset for various input levels yields a constant ca. 125 dB. 

Now my question is: Why 125 dB? Can I derive this from the UMIK-1 specification?

Side-note: My UMIK-1 is shipped with +18 dB Gain (which is accounted for in the calibration file, hopefully).


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## JohnM (Apr 11, 2006)

The cal file sensitivity figure is the dBFS input reading for 100 dB SPL with the input volume set to 100%, which is 24 dB of digital gain. The UMIK internal gain (18 dB in your case) is taken into account in the cal file sensitivity figure, so your UMIK with the internal gain set to 0 dB would need a sensitivity figure of -18.673 dB.


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## Waschwasch (Oct 28, 2015)

JohnM said:


> The cal file sensitivity figure is the dBFS input reading for 100 dB SPL with the input volume set to 100%, which is 24 dB of digital gain. The UMIK internal gain (18 dB in your case) is taken into account in the cal file sensitivity figure, so your UMIK with the internal gain set to 0 dB would need a sensitivity figure of -18.673 dB.


Hello John,
thank you for your reply. 

Where did you get the 100 dB SPL info? Is that an industry convention? Or did miniDSP disclose it to you?


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## JohnM (Apr 11, 2006)

That's from info provided by miniDSP.


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