Unfortunately, Friture does not provide the ability to calibrate a microphone to a specific sensitivity of the microphone in use, therefore the measurements that are presented in Friture are not referenced to any absolute value. Typically in an acoustic laboratory the microphones will all be calibrated using a calibrator that is presenting a 1000Hz tone at exactly 94dB to the microphone. That calibration provides the measurement software with a sensitivity reference (with units like mV/Pa or similar) so that what the microphone is 'hearing' can be referenced absolutely to the threshold of human hearing which can be expressed in pascals, which is a unit of pressure and is equal to 0dB. Friture does allow you to adjust the scales of the graphs, but those adjustments only move the view and not the reference. Somewhere in the code there is some kind of reference value, but I could not tell you where that is, or what that is, in any case, that setting is not exposed for the user to change in any way.