In this contribution, we study the transmission of a one dimensional acoustic wave between two fluid media with the classical lattice Boltzmann scheme. The two media have the same hydrodynamic equations but different equilibrium distributions. We take here the case where the incident wave is normal to the interface. The theoretical modal study of this problem shows the presence of a reflected wave and Knudsen modes localised at the interface. This analysis leads to results in good agreement with numerical simulations.