Measurement of acoustic reflection coefficients by an ultrasonic microspectrometer

An ultrasonic microspectrometer (UMSM) was developed in order to evaluate the elastic properties of a solid specimen at a small spot on its surface. In this system, spherical-planar-pair (SPP) lenses were used, by which the acoustic reflection coefficient of a liquid/solid interface was measured as a function of the incident angle in the frequency range from 20 to 140 MHz. Using a specimen of fused quartz whose material constants were well known, the measurement accuracy was examined. The phase velocity of a leaky Rayleigh wave was obtained from the phase change of the reflection coefficient with 0.4% accuracy in this frequency range. For a specimen of steel with a large acoustic attenuation, bulk attenuation factors and their frequency dependence were successfully estimated by computer-fitting of the reflection coefficient. As an example of anisotropic materials, the reflection coefficient of X-cut quartz was also measured. Measured phase of the reflection coefficient was in good agreement with numerical calculation.<>