There are a couple of problems here.
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and then multiplied all of the amplitudes by -1
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REW mic calibration files reflect the actual response. In other words, if a mic reads 10dB low at 20Hz, then the cal file entry is:
20 -10. Your file is the opposite of this. Your entry at 20Hz for example shows 50 and it would need to be -50..........
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You will of course need to turn off the cal function when taking impulse response measurements
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The cal file is reflected in the frequency response and not applied to the impulse response measurement. This how you are able to take an old measurement and apply a new cal file to the frequency plot.
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To enable me to take A-weighted SPL measurements
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You may be confused. The intent of REW is to produce response measurements as if every mic/meter used was perfectly flat over the frequencies of interest. If we actually had a flat mic, we would not use a cal file and we would not click the C-Weight checkbox. The mic is already flat. That's in a perfect world.
When we use a "flat" mic such as an ECM8000 we still use a cal file to correct its top and bottom end to result in a response that theoretically (and practically) used a flat mic.
The same applies to a Radio Shack meter that only has a switch to filter its inputs to C-Weight and A-Weight. On REW we apply a cal file to remove that weighting influence in the measurements, so as to result in a response that theoretically (and practically) used a flat mic.
(note, if the Radio Shacks meter response tracked a C-Weight curve exactly, then we would not need a cal file, we could simply click the C-Weight checkbox).
So, if you have your meter set to A-Weight and you offset it in REW with a A-Weight cal file, you have essentially taken measurements with a flat mic.........
brucek