The insights of diffraction theory are useful in the interpretation of P-velocity measurements of suspensions. After introducing the notion of suspension, this paper compares some experimental P-velocity measurements presented by Marion 1990 with an acoustic modeling of the same experiment on a computer. Finally, a diffraction-oriented study shows that in comparison to Biot's theory, the P-velocity measured at a low frequency is slower than the high-frequency measurement. Moreover, the critical frequency between the high-frequency and low-frequency domains is directly related to the radius of the grains.