## 5 Results and Discussion
### Polymethyl Methacrylate (PMMA)  
Two PMMA samples of 1.8- and 6.3-mm thickness were used as listed in Table 1. Although the bulk-wave attenuation coefficients, \(\alpha_{\mathrm{L}}\) = 91 dB/m/MHz and \(\alpha_{\mathrm{T}}\) = 253 dB/m/MHz, were previously reported [44; 45], these were for thick plates of 18 to 47 mm thickness, and the values of \(\alpha_{\mathrm{L}}\) and \(\alpha_{\mathrm{T}}\) were obtained for the samples used (cf. Table 1). Figure 14a,b show observed \(\alpha\)-S\({}_{\mathrm{o}}\) and \(\alpha\)-A\({}_{\mathrm{o}}\) values for the 1.8-mm sheet along with the calculated attenuation and dashed lines for \(\alpha_{\mathrm{L}}\) and \(\alpha_{\mathrm{T}}\). For the symmetric case (Figure 14a), \(\alpha\)-S\({}_{\mathrm{o}}\) values follow the calculated \(\alpha\)-S\({}_{\mathrm{o}}\) curve (in solid blue) up to 450 kHz, with the observed values always being higher by 10-20% than the calculated curve. Here, results of three test series are  
Figure 13: (**a**) \(\alpha\)-S values for soda lime glass sheet of 4.7-mm thickness. All with Olympus V101 as Tx and RRN-BB-PC as Rx. Calculated \(\alpha\)-S\({}_{\mathrm{o}}\) (solid blue curve), \(\alpha\)-S\({}_{\mathrm{1}}\) (dotted blue curve), \(\alpha_{\mathrm{L}}\) and \(\alpha_{\mathrm{T}}\) (blue and red dashed lines) are also plotted. Vertical bar shows 2 \(\times\) dev of data below 600 kHz. (**b**) Two sets of \(\alpha\)–A\({}_{\mathrm{o}}\) values for soda lime glass sheet of 4.7-mm thickness plotted in red symbols. Both sets used short-pulse excited NDT C16 Tx at 60\({}^{\circ}\) incident angle and KRN-BB-PC Rx. Calculated \(\alpha\)–A\({}_{\mathrm{o}}\) (solid red curve), \(\alpha\)–A\({}_{\mathrm{1}}\) (dotted red curve), \(\alpha\)–A\({}_{\mathrm{1}}\) and \(\alpha_{\mathrm{T}}\) (blue and red dashed lines) are also plotted. Vertical bar shows 2 \(\times\) dev. plotted using different symbols. For the asymmetric case (Figure 14b), \(\alpha\)-A\({}_{\rm o}\) values of three tests to 500 kHz are plotted together with the predicted \(\alpha\)-A\({}_{\rm o}\) curve (in solid red). Again, the observed data points are always above the predicted curve, by approximately 25%. As the calculations of \(\alpha\)-S\({}_{\rm o}\) and \(\alpha\)-A\({}_{\rm o}\) used the measured values of \(\alpha_{\rm L}\) and \(\alpha_{\rm T}\), one cannot attribute the differences to these values. However, an increase of 10-20% in \(\alpha_{\rm T}\) can bring theory and experiment into a good match, so this possibility needs further examination. Calculated values of deviation and percent deviation were averaged and are listed below. Figure 15a,b show observed \(\alpha\)-S\({}_{\rm o}\) and \(\alpha\)-A\({}_{\rm o}\) values for the 6.3-mm sheet along with the calculated attenuation and dashed lines for \(\alpha_{\rm L}\) and \(\alpha_{\rm T}\). One variation was added for \(\alpha\)-A\({}_{\rm o}\) testing; the Olympus V221 shear transducer was used as a transmitter with the shear  
Figure 14: (**a**) Observed \(\alpha\)-S\({}_{\rm o}\) values for 1.8-mm PMMA sheet along with the calculated attenuation and dashed lines for \(\alpha_{\rm L}\) and \(\alpha_{\rm T}\). Three test series are plotted using blue symbols. Dot data used Olympus V101 Tx and PAC Pico Rx, + symbols used Olympus V101 Tx and KRN-BB-PC Rx and triangles used NDT C16 Tx and KRN-BB-PC Rx. All with short-pulse excitation. Vertical bar shows 2 \(\times\) dev. (**b**) Observed \(\alpha\)-A\({}_{\rm o}\) values for 1.8-mm PMMA sheet along with the calculated attenuation and dashed lines for \(\alpha_{\rm L}\) and \(\alpha_{\rm T}\). Three test series are plotted using red symbols. Dots used KRN-BB-PCP Tx on the sheet 5 mm from an edge and KRN-BB-PC Rx, + symbols used NDT C16 Tx at 60\({}^{\circ}\) incident angle and PAC Pico Rx, and triangles also used NDT C16, but with KRN-BB-PC Rx. All with short-pulse excitation. Vertical bar shows 2 \(\times\) dev. direction matching the plate normal, producing nearly identical attenuation data as other transmitters with wedge.