If you are willing and able to open up your Radio Shack SPL meter, and are able to solder, here is a site with the docs on how to modify it for a flat response in the bass area. I happen to have 2 of these units, after modifying one, a side by side comparison showed that the unmodified unit was indeed measuring 2db below what the modified unit was showing. Now, you won't need to make the calculated adjustments anymore.
Modify the Radio Shack SPL Meter
Note that there are directions for the 2 different Analog models, as well as the Digital model. I'd recommend you get 10V components, makes it easier to fit back into the case.
