This work investigates the ultrasound propagation within a liquid-solid fluidized bed. The acoustic mapping of the reactor is achieved by means of a hydrophone. A spectral analysis is carried out on the measured signals to quantify the cavitation activity. The effects of several parameters on the spectral power distribution is appraised – including emitted ultrasound power, liquid superficial velocity and solid hold-up. Results show that increasing US power promotes a higher energy transfer from the driving frequency toward the broad-band noise – which is the signature of transient cavitation – and yields a stronger acoustic shielding. The presence of a flow opposite to the acoustic streaming may affect the sonoreactor behavior by sweeping the cavitation bubbles away from the ultrasonic horn. Finally the presence of millimeter sized particles significantly increases wave attenuation, presumably due to viscous losses on the one hand, and through the contribution of their surface defects to bubble nucleation on the other hand. Moreover, the influence of the solid hold-up appears to depend upon the particle material (glass or polyamide).