# CM-140 cal file and C-weighting



## bertdw (Jul 6, 2010)

Hello everyone. Please forgive me if this question has been answered before; I couldn't find it.

I'm trying to use the CM-140 manually, without Room EQ Wizard, by adding the correction values in the REW cal file and the standard C-weighting curve to my measurements. According to this post by member LPS:

"If you intend to use the CM-140 to record /analyse the sound externally, you will need to take the A or C weighting into account, plus the microphone response (this is what the "calibration files" are for).

This seems (to me) to indicate that I should add the two correction values together, so at 20Hz, for example, I would add 13.52 dB to the meter reading. However, in the Room EQ Wizard Help Index, Mic/Meter Settings section, it states:

"Select the C Weighted SPL Meter check box if you are using a C weighted SPL meter as the input to REW, measurements will then be corrected to remove the low and high frequency roll-offs of the C weighting characteristic. *If a cal file is loaded the correction will only be applied outside the frequency range covered by the cal file.*"

I'm confused by the statement in bold. The cal file seems to cover the entire audible frequency range, therefore I should not add C-weighing to the cal file. Should I add the two correction values together, resulting in a 13.52 dB correction at 20 Hz, or add only the 7.32 dB from the cal file? Thank you very much for your time.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Welcome to the Forum, Bert!

Anything you see regarding the CM-140 meter and REW only applies for using the meter with the REW program. If you’re taking manual SPL measurements in your room using frequency-specific test tones, none of that applies. In that case, just add the dB value from the calibration file for the test tone that you’re taking a measurement of.

Regards,
Wayne


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## bertdw (Jul 6, 2010)

Thanks for the speedy reply, Wayne. I suspected something was wrong when my corrected measurements showed a rising response above 10kHz. They look better now. 

Also, thanks to those responsible for the cal files and the general help I've found at this site so far. I hope someday I'm able to return the favor.


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