In a 4th generation Sodium Fast nuclear Reactor (SFR), liquid sodium - the coolant - is topped by an argon cover gas. Due to this presence of argon, various phenomena can lead to gaseous microbubbles presence within sodium. This study deals with the determination of the void fraction of the bubble cloud. The first experiments are conducted with an air-water experimental set-up. A Helmholtz resonator is used as container of the microbubble cloud. First, a linear acoustic method based on the Wood model is presented. It allows a quantitative link between low frequency speed of sound and void fraction. The obtained results with and without bubbles are in accordance with expected void fractions. Then we applied Nonlinear Resonant Ultrasound Spectroscopy (NRUS). Note that the Helmholtz resonance is far below the bubble's resonant frequency. We present a model and associated numerical simulation of NRUS experiments. The objectives were to observe nonlinear behaviour and to link it with bubble's characteristics: void fraction and size. The nonlinear behaviour of the cloud is clearly observed but it is difficult to analyze due to the presence of a cavitation phenomenon. This presence makes the second objective nearly impossible to reach...at present time.