The application relates to an in-situ method to measure and correct or adjust the sound signal presented to the eardrum by means of a hearing aid and a hearing aid employing such a method. The hearing aid comprises at least one microphone (1), at least one digital signal processor (2) for transforming the microphone signal into a transformed signal according to a desired transformation function, a receiver (3), a sensing means (4) for sensing the sound signal appearing in front of the eardrum and at least one comparison means (5). A model of the electroacoustic system of the ear and the hearing aid is established and stored in the hearing aid, which model simulates the sound signal in the earcanal in front of the eardrum. This model is adapted in response of an error signal generated in case the difference between the representation of the sensed signal and the simulated sound signal is above a predetermined threshold.