In 4th generation nuclear reactors cooled with liquid sodium,argonmicrobubbles are present in the primary sodium. Due to the opacity of liquid sodium, acoustic control methods are chosen for operating inspections but this bubble presence greatly affects the acoustical properties of the medium. It is therefore required to characterize the microbubble cloud, i.e., to provide the bubble’s volume fraction and the size distribution. Safety demands the proposed method to be robust and applicable with few assumptions (about the bubble populations) as possible. The objective of this study is to evaluate the performance of spectroscopic methods (based on celerity and attenuation) in the presence of bubbles whose sizes and surface (or volume) contributions are very different. Two methods of evaluating the histogram and the void fraction are compared. The first is based on the inversion of the integral equation of the complex wave number derived by Commander and Prosperetti. The second, which assumes the populations to have log-normal or sums of Gaussians distributions, performs an adjustment of the distribution’s parameters to fit attenuation and celerity curves measurements. These methods are compared with experimental data obtained using ACWABUL facilities at CEA Cadarache.