A method comprising - carrying out acoustic Thevenin calibration of a probe or hearing aid, said probe or hearing aid comprising a measuring microphone and a sound emitting source, by - presenting a plurality of different acoustic loads, one acoustic load at a time, to the output of the probe or hearing aid such that the sound emitting source and the measuring microphone of the probe or hearing aid are placed exactly flush with an input reference plane of each acoustic load when presented - while each of said acoustic loads is presented, emitting an acoustic stimulus via the sound emitting source and measuring a response via the measuring microphone, and - calculating Thevenin parameters based on the measured responses and determined input impedances for said acoustic loads, respectively, said determined input impedances for each of said acoustic loads are calculated based on a known acoustic input impedance of said acoustic load and an additional frequency dependent complex correction factor ΔZ(f) corresponding to said acoustic load, and - using said Thevenin parameters to calculate an impedance of an unknown acoustic load based on measurements obtained by said probe or hearing device from said unknown acoustic load, - wherein said acoustic Thevenin calibration is used to determine the relationship between the sound pressure and the volume velocity at the input of the acoustic loads generated by the probe for a plurality of frequencies thereby obtaining source characteristics of the probe or the hearing aid, and - wherein said frequency dependent complex correction factor ΔZ(f) for each of said acoustic loads is calculated in order to compensate for geometric mismatches between the probe or hearing aid and said acoustic load. - carrying out acoustic Thevenin calibration of a probe or hearing aid, said probe or hearing aid comprising a measuring microphone and a sound emitting source, by - presenting a plurality of different acoustic loads, one acoustic load at a time, to the output of the probe or hearing aid such that the sound emitting source and the measuring microphone of the probe or hearing aid are placed exactly flush with an input reference plane of each acoustic load when presented - while each of said acoustic loads is presented, emitting an acoustic stimulus via the sound emitting source and measuring a response via the measuring microphone, and - calculating Thevenin parameters based on the measured responses and determined input impedances for said acoustic loads, respectively, said determined input impedances for each of said acoustic loads are calculated based on a known acoustic input impedance of said acoustic load and an additional frequency dependent complex correction factor ΔZ(f) corresponding to said acoustic load, and - using said Thevenin parameters to calculate an impedance of an unknown acoustic load based on measurements obtained by said probe or hearing device from said unknown acoustic load, - wherein said acoustic Thevenin calibration is used to determine the relationship between the sound pressure and the volume velocity at the input of the acoustic loads generated by the probe for a plurality of frequencies thereby obtaining source characteristics of the probe or the hearing aid, and - wherein said frequency dependent complex correction factor ΔZ(f) for each of said acoustic loads is calculated in order to compensate for geometric mismatches between the probe or hearing aid and said acoustic load.