I haven't done it in my car yet (still trying to figure out how to mount the amp) but I've done it at home. The method already described is correct, as for the setting you want, usually it is the C weighting. FWIW, the radioshack SPL meter tends to be fairly inaccurate below 80Hz, there is a spreadsheet floating around with corrections for the meter. At least there are for the analog meter and I have been told that they are the same exceot for the display.