Comparison of the measured high-frequency and estimated low-frequency stiffness coefficient |$C_{33}$| change for 5 MPa change in axial stress. The open symbols represent |$\Delta C_{33}$| computed with the average values of the third-order elastic coefficients and the whiskers indicate the range given by the third-order elastic coefficients within the standard deviation distance from the average |$c_{\mathrm{ijk}}$| values. Closed symbols represent direct measurements at ultrasonic frequency.