Three sound level meters (SLM) during calibration.
Calibrator is a Type 1 Larson Davis CA250,
emitting 114.0 dB at 250 Hz.
Larson Davis model 820 on the left (yellow band)
In the photo, the following readings can be seen:
model 820 yellow = 114.2 dB
model 812 green = 114.1 dB
model 812 red : 113.7 dB
When making a sequence of measurements, a calibration check and an adjustment (if necessary) of the instrument is made at the beginning of the measurement activity. The Calibrator and the SLM microphone are allowed to stabilize at the same temperature before calibration.
At the end of the measurements, calibration is checked again.
The inaccuracy of the measurements will be at least the difference between the beginning and ending calibration values.
A calibration history of all adjustments is made. Changes are minimized and adjustments to the
instrument are made only when the tolerance is exceeded allowing for reasonable instrument drift.
Reasonable values are ±0.2, ±0.5, or ±1.0 dB for Type 1 measurements. The Larson Davis models 812 and 820 automatically record a history of calibration that is printed out.