A monitoring system comprising ultrasound sensors to monitor the structure of a composite material part during production is automatically calibrated using reference sensors mounted on reference blocks and placed in the production environment of the part. The automatic calibration comprises actuating a reference sensor to transmit an ultrasonic wave and measuring the amplitude of a reference echo constituted by the transmitted ultrasonic wave after it has passed through the reference block. The measured amplitude is compared to a set point value and the gain is applied to the reference sensor to obtain an amplitude value of the reference echo substantially equal to the set point value. The gain applied to the reference sensor is applied to the ultrasound sensors of the same type as the reference sensor. The operation is performed for each reference sensor, and successively for all of the stages of production of the part.