I'm in need of some technical info... or possibly some pointers on where to go to find some information. Is there some way I can calculate how much background noise will effect the reading I get from my SPL meter? i.e. if the background noise was 55dbl... and I measure the acoustic output from a fan as 70.6 dbl, what would be the effect of the reading if the background noise was 60dbl? 65dbl?