These are either dB"A" or dB"C" meters. The difference in A or C weighting is that the noise is measured and adjusted on a third octave band basis by either an A-weighted curve or C-weighted curve, essentially different values for each third octave band frequency. After the A or C adjustment a log sum of the frequency range (specified on each meter specs) is performed. The number that is shown on the display will be the log sum value.
If someone has a dBA meter and another person has a dBC meter the answers will be different from the same noise source, in addition some meters have different frequency ranges the adjustments are performed on so another different answer.....
The dBA or dBC weighting were established as a means to measure noise environments as the ear hears them and to determine hearing damage due to exposure.
Too much blah blah blah acoustics but if people start comparing numbers knowing the background information A or C weighted and frequency range determined over would be helpful.
What's really cool (at least for an acoustics guy) but expensive, would be a third octave band analyzer.
Real Time Analyzer and Sound Level Meter with 1/3 Octave Band Filters