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is the mic placed in close proximity to a 15" woofer to make the measurements in air? i.e. Is generating the tone in air part of the error involved with measuring this low?
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The mic(s) were placed near field using Sonnies
behemoth subwoofer and his ECM8000 microphone that was specially calibrated professionally down to 5Hz. Believe me, his mic is accurate to 5Hz. Certainly, using measurement data to derive corrections is fraught with problems though. Where the response changes rapidly, if you have positional differences in the mic elements, you can get errors. We placed a lot of effort in getting a response that was as smooth as possible to obtain as little error as possible. We make no guarantees. If you need a guarantee, you have to have your mic calibrated.
Once the calibrate files were all created and normalized, we loaded them in REW to test several like models to see how they compared to the ECM standard (which is quite accurate since it was calibrated). They all tracked fairly close and so as far as the ones we tested they were good to use, but there's no guarantee that your meter is the same. But it will be close enough for home use. It's not worth even bothering about a few dB, since a foot of listening position change will be at least that much change....
The Galaxies tracked the ECM fairly good down to about 7Hz and then were erratic below that. I think your fairly good down to 10Hz though..
brucek