## 4 Results and Discussion
### Attenuation Behavior of Organic Materials  
This section reports the attenuation characteristics of engineering polymers and wood. The results are tabulated in Table 6. An extra column is added as in Table 5 for the values of the damping factor, \(\eta\), since this is often used in the polymer field. The apparent damping factor, [\(\eta\)], is used for quadratic spectra found in some wood results (with a linearity assumption at frequencies below 1 MHz). Most organic materials showed linearly dependent frequency spectra, although several exceptions were found for wood samples. Attenuation data and representative spectra for PMMA, PC, and PVC are given in Table 1 and Figures 9 and 10. For these three samples and eight other polymers, \(\alpha\) values at 1 MHz ranged from 91 (PMMA) to 1471 dB/m (poly-tetra-fluorinated ethylene, PTFE). The range of observed \(\eta\) values was from 0.008 (PMMA) to 0.077 (neoprene). These results agree with values reported elsewhere within a factor of two or better [11; 24; 95]. Among the \(\eta\) values of nine materials listed in [95], five matched with the present results, while the rest were within a factor of two (either higher or lower). Among six \(\alpha\) values in [24], three matched within 20%, while the rest were within a factor of two. The Kaye-Laby table [11] from varied sources gave poorer agreements, but one of four was well-matched, while the other three were again within a factor of two. A part of the discrepancies certainly arose from different material origins used in various works, but the present results were always within the bounds of literature values. This fact offers additional confidence in the methodology and data of this work. For epoxy resins, reliable attenuation measurements are available. [PERSON] et al. [50] found C\({}_{\text{d}}\) values of 373 to 391 dB/m/MHz for TRA-CASR3012 epoxy using an immersion method. [PERSON] et al. [96] reported 338 dB/m/MHz for JER epoxy (matrix resin for Mitsubishi 340 prepreg). These values are 25% to 45% higher than in test O9, but are within the expected range for epoxy resins. Note that the epoxy used for test O9 was a high-strength resin designed for wear resistance, as it was used as an AE sensor face plate. Tests O12 to O41 were performed on 14 wood samples from 12 wood species. Nine samples were old stock, which were kept indoors for many years. The estimated length of storage is listed in the notes column in Table 7. As these samples were of natural origin, the data obtained need to be used with caution, as many factors can alter the microstructural conditions. The attenuation values of most wood samples showed linearly dependent frequency spectra, with \(\alpha\) values at 1 MHz ranging from 174 (pine) to 5455 dB/m (pine plywood, surface normal direction). Note that \(\alpha\) values at 1 MHz numerically equal C\({}_{\text{d}}\) + C\({}_{\text{n}}\) values in Table 7. For example, white oak has values of 415, 2667, and 760 dB/m for the three directions. Here, four of the wood samples can best be fitted to quadratic attenuation spectra, while two cases had the Rayleigh scattering term. The scattering term was small at 1 MHz, but increased rapidly with frequency. Hardwoods seemed to have lower \(\alpha\) values. Some softwood samples showed high attenuation, but pine gave the lowest \(\alpha\) value among the wood samples. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Test** & **Material** & **C\({}_{\textbf{d}}\)** & **C\({}_{\textbf{a}}\)** & **\(\alpha\) at 1 MHz** & **Damping** & **Velocity** & **Tickness** & **Density** & **Notes** \\ \hline No.