Sizing of small voids less than focused beam diameter with C-scan acoustic microscope

The small objects which are far less than the focused beam diameter were imaged with a C-scan acoustic microscope, and the acoustic image sizes were compared with the actual sizes. The sizes of flat strips of width over 0.01 mm and steel wires of diameter over 0.02 mm were estimated within 30% error by using the reflection ratio, which is the ratio of the intensity of reflection signal from an object to that from a reference plane. The validity of this method is also shown for small spherical voids within ceramics.