Abstract: Resulted from a multiply scattered ultrasonic pulse in polycrystalline metals, the structure noise propagates as a diffuse field. The highest velocity of the diffuse process seems to be V_l / \sqrt3.Attempt to measure directly this velocity experimentally meets the obstacle resulted from the structure noise excited by the creeping wave under the surface. Analysis shows the earliest wave arrived shouldbe the creeping wave-structure noise. Good agreement has been achieved by theoretical-experimental analyses.