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"A" weighting primarily measures frequencies in the range of 500-10,000hz. "C" weighting measures frequencies from 32-10,000, so essentialy "A" would be leaving out most of your low frequencies, such as those produce by a woofer or subwoofer, and "C" would add them in hence the reason why "C" is louder. It is measuring more sound. You should use "C" to calibrate your home theater. This info can be found on page 11 of your spl meter manual.
If you are getting over 70 on the meter before the test tones you must have something in your basement producing low frequency noise. If not your meter is defective.
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