Last night I read up on the RS Digital SPL meter and it's issues. surprise surprise, for 50 bucks, it does not measure uniformly across the entire spectrum. Many folks have done some comparisons btw the RS model and calibrated lab equipment and have come up with a correction table (below). They tested many units and found that this issue is relativly uniform for the entire line and it has to do with the the electronics inside the unit, not the mic. This does not replace an RTA and a professional but I thought this was pretty interesting..
There are several sources of info on this subject.. here is one of them http://www.audioxpress.com/magsdirx/...a/koya2811.pdf You can google radio shack spl meter correction to find more..
All versions are screwy.. the newer models are worse, the digital model is the worst of the worst.. check out the graph on the PDF. The lightest line represents the digital meter. All of them have a bump in the 2 - 6 k range and varying ranges of LF response.
The good new.. the digital meter is +/- 2db accurate over all.