We have studied acoustic anharmonic effects in lead molybdate using a 150‐MHz fundamental acoustic wave. When sampling the amplitude of the second harmonic acoustic wave with a laser probe, the light scattered by the fundamental acoustic wave may lead to severe mistakes in the determination of nonlinear coupling constants if care is not taken. This measurement can be carried out so as to get not only the true acoustic second harmonic amplitude but also to get an experimental proof of the phase lag −&pgr;/2 between second harmonic and fundamental acoustic waves.