The disclosure relates to a method for carrying out acoustic Thevenin calibration of probes or hearing aids. The method comprising presenting a plurality of different acoustic loads, one acoustic load at a time, to the output of the probe or hearing aid, the source characteristic of which is to be determined. Each of the acoustic loads is characterized by a known acoustic input impedance and an additional frequency dependent complex correction factor [Delta]Z(f). By applying the method according to the present disclosure the relationship between the sound pressure and the volume velocity at the input of the acoustic load generated by the probe can be determined for a plurality of frequencies, thereby obtaining the substantially correct source characteristic of the probe or a hearing aid. Specifically the acoustic loads are the input impedance of respective waveguides or other suitable cavities, and the known acoustic input impedances are determined analytically. The correction factors are adjusted individually for each waveguide or cavity, for instance in an iterative process.