MULTIFREQUENCY TRANSDUCER ASSEMBLY FOR NONLINEAR ULTRASONIC MEASUREMENTS

This letter provides more detail on a multi-frequency transducer assem bly used in our nonlinear image enhancement experiments [J. Acoust. So c. Am. 88, 2852 (1990)]. In these experiments, high-intensity ultrason ic waves were launched into the medium and second harmonic signals or the sum-frequency signals were detected. Therefore, a transducer assem bly was needed to operate at two different frequencies separated appro ximately by an octave. The signals in the lower frequency band were mu ch stronger than the nonlinear signals in the higher band which were t he signals of interest. The signals in both bands are coaxial and the much weaker signals in the higher band had to be extracted from the st ronger background. The heart of the transducer assembly was an acousti c beam separator. The principle of the beam separator was based on the total transmission of the acoustic waves through a thin isotropic sol id layer in a fluid medium. The beam separator served both functions o f beam relocation and acoustic filtering. The filtering was accomplish ed by using both the total transmission phenomenon and the Fabry-Perot interference phenomenon.