Why does GLM show different SPL readings compared to another microphone I have?
The GLM program uses a microphone-specific correction file in Autocal for room correction calculation and in the drawing of frequency response curves. That being said, there is no concern that the Autocal calibration would not work properly.
The displayed SPL reading is calculated by the GLM adapter based only on the signal from the microphone, without the use of a correction file. Due to the use of the uncorrected response of the microphone, there can be a difference between the SPL reading reported by the GLM and a separate SPL meter.
The GLM microphone also uses Z-weighting. Usually, dedicated SPL meters have A/C-weighting, which is why they can show different values compared to the GLM microphone. Therefore, it might be useful to use a precise SPL meter with the corresponding weightings if you want to ensure the highest precision for the SPL level of your system.