The effect of surface cleanness of thin defects on the reflection of ultrasound

In a previous paper, the authors described experiments in which simulated defects of thickness 20–200 nm were detected ultrasonically. The defects were produced by creating thin gaps between the surfaces of optical flats. A relation between reflectivity and cleanness of the surfaces was noted. This paper describes new experiments and some theoretical considerations which further explore the relation between cleanness and reflectivity. It is concluded that the surface properties of the thin defects tested affect the reflectivity. In particular, it appears that the surface energy of the defect attracts additional material, which provides a mechanism for transmission of sound across the defect.