The process of adapting a hearing aid to its wearer is to be simplified. To this end the acoustic conditions in the auditory canal, especially the acoustic impedance, are estimated by measuring the input impedance of the earpiece on the hearing aid. For adjusting the hearing aid it is worthwhile using the mechanical resonance of the system of hearing aid and auditory canal which can be detected with the aid of the input impedance. This is produced by a simplified equivalent circuit diagram from which the corresponding acoustic variables can be obtained.