An evaluation of effective radiuses of bulk-wave ultrasonic transducers as circular piston sources for accurate velocity measurements

The effective radius of a bulk-wave ultrasonic transducer as a circular piston source, fabricated on one end of a synthetic silica (SiO2) glass buffer rod, was evaluated for accurate velocity measurements of dispersive specimens over a wide frequency range. The effective radius was determined by comparing measured and calculated phase variations due to diffraction in an ultrasonic transmission line of the SiO2 buffer rod/water-couplant/SiO2 standard specimen, using radio-frequency (RF) tone burst ultrasonic waves. Fourteen devices with different device parameters were evaluated. The velocities of the nondispersive standard specimen (C-7940) were found to be 5934.10 +/- 0.35 m/s at 70 to 290 MHz, after diffraction correction using the nominal radius (0.75 mm) for an ultrasonic device with an operating center frequency of about 400 MHz. Corrected velocities were more accurately found to be 5934.15 +/- 0.03 m/s by using the effective radius (0.780 mm) for the diffraction correction. Bulk-wave ultrasonic devices calibrated by this experimental procedure enable conducting extremely accurate velocity dispersion measurements.