You will need to occasionally calibrate your microphone to be confident in its readings. You may already have an acoustic calibrator or pistonphone, or you will have purchased one with your meter. This will need to be professionally calibrated once per year with a certified calibration laboratory. If your acoustic calibrator has a valid certificate, you can calibrate the 97x meters in the Function > Calibration > Calibration by Measurement part of the meter’s menu. For the 958 series, usually Channel 4 is the sound channel. Enter the level of the tone being emitted from the calibrator (usually 94dB or 114dB – check the manual for the instrument) and place the calibrator firmly on the microphone. Take care not to carry this procedure out with high background noise.
If you need a formal instrument calibration carried out, which is recommended every 18-24months, please contact email@example.com with your requirements.