Sound level meters measure sound pressure level and are commonly used in noise pollution studies for the quantification of almost any noise, but especially for industrial, environmental and aircraft noise. However, the reading given by a sound level meter does not correlate well to human-perceived loudness; for this a loudness meter is needed.

The current International standard for sound level meter performance is IEC 61672:2003 and this mandates the inclusion of an A-frequency-weighting filter and also describes other frequency weightings of C and Z (zero) frequency weightings.

Sound level meter: Hand held instruments for the measurement of an employees noise exposure. Frequency analysis within Sound Level Meters can also be used to determine the correct selection of hearing protection

http://en.wikipedia.org/wiki/Sound_level_meter