Abstract: Application of a hearing aid alters the distribution of sound pressure at the tympanic membrane in the ear. Measurements of sound pressure of a free ear and with a hearing aid are performed to evaluate the alteration produced and the in situ frequency response of the hearing aid itself. The following steps are performed: calculating the REUG; calculating a magnitude; calculating one of two components and designing of the filter for the compensation for such component; calculating the REAG; calculating one of two components. The sum of two components is the Insertion Gain; designing the compensating filter of the POST component of the Insertion Gain; testing and verifying, on the subject wearing the hearing aid, the compensating filter of REIG, and possible refinement procedure to correct the compensation of the Insertion Gain obtained in the previous step; tuning the hearing aid for deafness taking into account the compensating filter.