The process of checking a noise measuring instrument by applying a signal of known level and frequency to verify its operation in the field. Any drift from the nominal reading is usually corrected prior to measurements starting by means of a change to the displayed level shown by the meter.

Some standards require that the measurement instrument be also checked at the end of a measurement to verify that no significant drift has taken place during the measurement run. Over a typical 8 hour workplace exposure noise measurement a drift from start to end of more than 1 dB may be considered to be significant and the cause should be investigated. Reasons for the drift in the sensitivity of the instrument could be large fluctuations in the local environmental conditions or a fault in the microphone capsule. Calibration can also apply to an air sampling pump to verify the flow rate before and after a measurement so that the total volume of air drawn through the sample filter is known correctly.