Scattering of laser-generated ultrasonic waves in steel

Laser-generated ultrasonic waves were produced in the ablation regime at the surface of three steel samples, each hating a different grain size. We show that the "noise" following the first longitudinal pulse behaves, with respect to frequency, as if it was the result of the scattering of the incident waves by the inhomogeneities in the material. We show that the grain size is related to the level of the relative "noise" spectrum, i.e. the "noise" spectrum compared to the first longitudinal pulse spectrum.