Nonlinearity parameter measurement for polymer plates using focused ultrasound

The measurement of the acoustic nonlinearity parameter B/A based on the finite amplitude method for a 1-mm-thick polymer plate which is put, together with a sound reflector set at the back, within the focal region of 18.6 MHz focused ultrasound yields extraordinary B/A values as large gas 26.6, for example, in polystyrene. The experiment also demonstrates the velocity dispersion in the sample as well as the multiple reflections within the gap between the sample and sound reflector. They both make the second harmonic component advance relative to the fundamental, and then enhance the second harmonic component by suppressing the phase delay due to diffraction. Recalculating B/A by taking these effects into consideration, reasonable values such as 11.4 in polystyrene are obtained. The B/A values in other polymer plates with stronger dispersion are estimated in a similar manner to reveal reasonable values.