## 3 Experimental Procedures
When the A\({}_{\text{o}}\) mode was well suppressed, the slow-moving S\({}_{\text{o}}\) mode was analyzed as well, but this mode was weak and typically not analyzed. For the asymmetric wave analysis, the initial part of signals (S\({}_{\text{o}}\)-mode segment) was truncated, although adequate suppression was usually achieved using signal differences. With the signal difference processing, faster A\({}_{\text{1}}\)-mode signals emerged, but signal levels were low and were usually discarded in later wavelet transform analysis. These features are displayed in Figure 3 for the case of a PS sheet (Sample Number: G1b). Figure 3a shows symmetric waves with front side (blue curve), back side (green curve), and summed (halved, red curve). The propagation distance was 200 mm, and the signal arrived at 97 \(\upmu\)s (with the velocity of 2.06 mm/\(\upmu\)s). Although lower amplitude signals were hidden, the summed red curve represented the averaged signal. Note that the amplitude scale was converted to displacement, as the KRN sensor has 1-pm/mV sensitivity for these signals. The A\({}_{\text{o}}\) mode arrived at 170 \(\upmu\)s, as indicated by blue and green curves, showing peaks of 85 to 90 pm. The summed signal (red curve) suppressed strong peaks of A\({}_{\text{o}}\) signals, and it reached only 20 pm at 176 \(\upmu\)s. The former A\({}_{\text{o}}\) segment of 170 to 190 \(\upmu\)s followed the trend of the S\({}_{\text{o}}\) signal well. In most of the tests reported, the signal summation step was utilized in attenuation determination, as signal modulation is averaged out, as shown in Figure 3a. This step may be omitted in the case of symmetric modes when A\({}_{\text{o}}\) signals are truncated. Figure 3b is the case of asymmetric waves with the same color codes as in Figure 3a, except the red curve represents the differences of the front and back signals (and the amplitude is halved). The amplitude of the difference signal (in the pm unit), mostly A\({}_{\text{o}}\) mode from 170 \(\upmu\)s, follows those of the front (blue) signals (and the reverse of back (green) signals). The propagation distance was also 200 mm, and the signal arrived with the velocity of 1.18 mm/\(\upmu\)s. The initial S\({}_{\text{o}}\) component (to 170 \(\upmu\)s, not shown) was substantially reduced and omitted in the subsequent wavelet transform (WT) analysis. In order to obtain an attenuation spectrum, a set of six to eight detected signals at different propagation distances were analyzed using AGU-Vallen Wavelet software (ver. R2019.0926.2, Vallen Systeme, Wolfrathausen, Germany, 2019) [18]. A Choi-Williams transform was selected. Spectrograms of wavelet transform of signals shown in Figure 3a,b are plotted in Figure 3c,d, respectively. Corresponding velocity dispersion curves are superposed as well. For the symmetric wave WT (Figure 3c), the S\({}_{\text{o}}\) mode was the strongest at 400 \(\pm\) 20 kHz, and it followed the S\({}_{\text{o}}\)-mode dispersion curve (drawn in red) from 100 to 600 kHz. The propagation distance was 200 mm. The superposed dispersion curves matched the initial part of the WT intensity. At 400 kHz, the S\({}_{\text{o}}\) curve was 3.8 \(\upmu\)s before the peak intensity. This time delay is due to the build-up of Lamb wavelet from a fast-rising excitation pulse on the transmitter. The inserted WT spectrogram is that for 125-mm propagation distance. The time delay at 400 kHz was again 3.8 \(\upmu\)s, indicating the delay, or signal risetime, was from Lamb wavelet formation process. Other modes were absent, except for the A\({}_{\text{o}}\) mode below 250 kHz, which was easily excluded during subsequent WT analysis. Figure 3d gives an example of the asymmetric wave WT spectrogram. A high-intensity A\({}_{\text{o}}\) zone was located at 420 \(\pm\) 60 kHz and extended down to 40 kHz and up to 700 kHz.