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So the question is, how do you figure out where the meter trails off?
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You can either send the meter in for calibration or you can use a (less accurate) comparison method to derive the file from a known calibrated microphone.
We sent an ECM8000 to be professionally calibrated, then we did some rather painstacking near-field comparison measurements of that ECM8000 and a Galaxy CM140 (actually a whole lot of Galaxy CM140's) to derive the CM140 file we have on our site.
This was also done with the three Radio Shack models of SPL meter.
We feel they're all within a few dB. Close enough when measuring a sub.
Here's 3 ECM8000 and 3 Galaxy CM-140 meters compared using the cal files from our site.
If you feel you need it closer than this, you should have your own calibrated.
3ecm and 3galaxy.jpg
brucek