Sound level calibrators are hand-held devices that emit audible tones of very accurate levels of frequency. The calibrator is fitted over a sound level meter's microphone and the reading is either checked manually by the user or automatically by the meter depending on the type used.
The sound level calibrator will provide a fixed level and frequency that the instrument can be referenced against. In order to work effectively, the calibrator will be attached to a sound level meter (a meter used for acoustic measurements). The reference level provided by the calibrator is used by the sound level meter to verify that it is measuring within a set tolerance. If the instrument sees that the frequency level is outside of the desired tolerance it will indicate an error.
Calibration of a sound level meter is important because it allows the user to see that the sound level meter is giving you the correct level against the reference provided by the calibrator. Secondly, most regulations require that a sound level meter is calibrated before it is used.