Origin of second harmonic component from high frequency ultrasound

A series of field experiments has been conducted in order to characterize the ultrasonic field produced by a high frequency PVDF transducer. Field measurements using a small spot size hydrophone shows a linear amplitude increase of the second harmonic pressure from a planar transducer due to nonlinear propagation of acoustic waves in water. Strong evidence is given, that the second harmonic component is produced almost entirely by wave propagation in the medium and not by nonlinearities of the transducer. Furthermore, field measurements using a focused transducer and 20 MHz transmit frequency shows the generation of pressure peaks of the second harmonic at places where there are zeros of the fundamental component