Zub, on the difference between A and C weighting. SPL meters have filters which intentionally roll off the frequency response(no "correction" necessary)at certain frequencies and at certain rates. The idea is to give a result that more closely matches what humans actually hear, since hearing ability is less for very low tones and very high tones than for the mid-range, and this difference increases as the sound level gets lower(Fletcher-Munson curves illustrate this). The A weighting roughly corresponds to hearing at very low sound levels while the C weighting corresponds at high sound levels and is usually suggested for use when calibrating audio equipment at a fairly high level. The curves and table shown on p.4 of this
lab handout from Michigan Tech illustrate the differences. So, among other points, note that published "corrections" for the RS meter at very low frequencies don't in fact relate to meter inaccuracy, but the inaccuracy of our ears.
