The relationship between the erosion rate and PRF for a fixed PD. In panels A, B, C, and D, the erosion rate was plotted versus PRF by median and range values (min. and max.) for N = 8, corresponding to PDs of 3, 6, 12, and 24 cycles, respectively. In panel A, B, and C, for PDs of 3, 6, and 12 cycles, there is a tendency for the erosion to be faster at PRFs of 19.6 kHz, 6.90 kHz, and 4.95 kHz, respectively. In panel D, for a PD of 24 cycles, the erosion rate increases with the increasing PRF.