Topographical Mapping of Surface and Interface Profiles by Using Acoustic Interferometry

The model is presented to explain formation of interference fringes appearing in acoustic images obtained by the acoustic microscopy. In the model, the residual vibration in the vibrator is overlapped by the vibration due to the ultrasonic wave reflected from the surface of the solid. The model was confirmed by the periodical change in the output of the vibrator with the water path. Then acoustic images of an inclined glass plate were observed to examine the influence of the inclination angle, the focal position and the wave frequency on the fringe spacing. A height difference corresponding to the fringe spacing was about a half wavelength of the ultrasonic wave in water. The interference fringes were also observed with a specimen with a deep notch under bending test and showed a good agreement with moire fringes. The presented model suggests that the interference fringes are also formed corresponding to profiles of a reverse surface and a boundary between dissimilar substances. This was confirmed by acoustic images of copper plates with a tapered edge and a shallow groove on the reverse surface, and also those of a copper plate with a dint on the reverse surface, covered by tin alloy solder.