This study aims to highlight the limitations of the classical ultrasonic testing methods in assessing the porosity level in CFRP composites. From the manufacturing perspectives, samples of thermosetting composite materials with unidirectional fiber orientation, featuring diverse distributions and volume fractions of porosity, are prepared in autoclave. Two kind of porosity distribution are studied, the first one with residual porosities all over the sample thickness and the second one with porosities clustered in a single interface between two plies. These samples undergo testing thanks to an immersion ultrasonic scanning method, employing two planar transducers to measure the transmission through the sample. The attenuation of elastic waves is then calculated comparing the amplitude of the Fourier-transformed acquired signals. It is well known that ultrasonic attenuation can be directly link to void rate but the results of the scans reveal the incapacity of this classical method to asses clustered porosities. Hence, a complementary parameter made to emphasize the potential localized nature of the porosities is proposed.