Harmonic Generation Due to Non-Linear Propagation in a Focused Ultrasonic Field

A series of experimental observations of the propagation of ultrasound in a focused wavefield are presented.The wavefield studied was formed by focusing 2.08 MHz ultrasound from a planar transducer with a Perspex (Polymethylmethacrylate) lens to give a linear focusing gain of 12.5.The build up and decay of the second harmonic on the acoustic axis was observed in detail and the ' relative phase of the second harmonic measured.These results are compared with theoretical calculations based on the theory of Lucas and Muir (l983) .The overall results are in very good agreement with the theoretical predictions in the focal region although departures are observed outside this region.These are tentatively ascribed to the influence of higher harmonics not taken into account by-the theory and the non-ideal nature of the fundamental field.A linear relationship between phase and harmonic order is also noted for the harmonics in the focal region.