The application relates to an in-situ method to measure and 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 microphone (1), signal processing system comprising digital signal processor (2) for transforming the microphone signal into a transformed signal according to a desired transformation function, sensor (4) sensing the sound signal appearing in front of the eardrum and comparator (5). Reference signal processor (6) generates a reference signal based on the output of microphone (1) and representative of the desired sound signal in front of the eardrum. A transfer function between receiver (3) and the output of sensor (4) is established to correct the process in reference signal processor (6). The sound signal in front of the eardrum is sensed, fed back and compared in said comparator (5) with said reference signal. In the case that the difference between the sensed signal and the reference signal is above a predetermined threshold the transformed signal is corrected to adjust said signal in front of the eardrum to the desired sound signal.