In Reply to: SPL meter levels posted by abraham on October 02, 2002 at 16:24:47:
One of them has a flat frequency response, while the other basically mimics the freq response of the human ear (less sensitive at frequency extremes). I forget which is which, but if you switch between A and C while listening to music, the flat response will give a higher reading, especially when there is significant bass.
"A" weighting does not measure the bass frequencies, i.e. the meter is intentionally desensitized (rolled-off) for frequencies < 500Hz, I think, as someone else said.
"A" weighting is typically used to measure noise levels that will harm the human ear on the job. The Radio Shack manual has a table in the back regarding the Gov'ts safe dB levels of sound per time period.
If you want to measure music levels for calibration purposes or whatever, you use the "C" weighting scale. Even this scale doesn't accurately record the levels of lower bass frequencies, but at least their not ignored and rolled-off like the "A" setting.
I believe A-weighted begins its low frequency rolloff at about 500Hz. As for the high frequencies, I don't remember if the mic's rolloff is what affects the high frequencies or if it's due to the A-weighted characteristic. C-weighted measurements are supposed to measure the full frequency spectrum, but are of course limited to the response capability of the mic.
Here's a link to the Radio Shack SPL meter. If you go to the "controls" section, it tells you the A-weighted and C-weighted features and responses.
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