A measurement of the harmonic content of a bounded ultrasonic beam

The nonlinearity of acoustic waves manifests itself via the generation of harmonics as an acoustic wave travels through a medium. This phenomenon evolving under the influence of competing mechanisms such as absorption, diffraction, and dispersion can be described for a finite source by examining changes in wave shape and beam profile. Measurements of the acoustic amplitude of the fundamental and several harmonic components across the sound beam produced by a finite source in water are presented. The sound field was produced by a plane circular transducer 2 cm in diameter and driven at a frequency of 3 MHz. Beam profiles were mapped separately for the fundamental and the first two harmonics at various distances ranging from 15 to 25 cm by use of a miniature hydrophone. The frequency response of the hydrophone was calibrated for each involved frequency by comparing the probe output to the acoustic amplitude measured simultaneously by the light diffraction technique. The significant features of the resulting profiles are found to be consistent with available theoretical results.