We present a new type of experiment on the nonlinear interaction between coherent collinear longitudinal acoustic waves: Two waves with different frequencies propagate together successively in a liquid and in a solid. They interact because of the nonlinear elastic properties of both media. We have measured the magnitude of the signal resulting from the interaction, as a function of the liquid thickness. The variation law is sensitive to the phase difference between the waves generated in both media (in phase or out of phase). From this dependence the ratio and relative sign of two parameters characterizing the non-linear properties of both media can be derived without any measurement of the primary and harmonic waves amplitude. ©1997 American Institute of Physics.