  In the context of wireless acoustic power transfer, high intensity focused
ultrasound technology aims at the reduction of spreading losses by
concentrating the acoustic energy at a specific location. Experiments are
performed to determine the impact of nonlinear wave propagation on the
spatially resonant conditions in a focused ultrasonic power transfer system. An
in-depth analysis is performed to explain the experimental observations. The
results show that the efficiency of the energy transfer is reduced as nonlinear
effects become more prominent. Furthermore, the position of the maximum voltage
output position shifts away from the focal point and closer to the transducer
as the source strength is increased. The results and analysis are relevant to
the development of novel efficient ultrasonic power transfer devices when using
focused sources.
