The ECM8000 is a very good choice but you will still require a mixer of some sort like this one as the ECM8000 has an XLR connector and you can not connect that directly to a PC.
Another less expensive option is to get a better SPL meter. The Galaxy CM140 can be found here and is much more accurate than the Radio shack meter. A calibration file is also available for it.
Another less expensive option is to get a better SPL meter. The Galaxy CM140 can be found here and is much more accurate than the Radio shack meter. A calibration file is also available for it.