## 4 Results and Discussion
### Attenuation Behavior of Organic Materials
& & dB/m/MHz & dB/m/MHz\({}^{\mu}\) & _fac_tor & mm/ias & mm & Mg/m\({}^{3}\) & \\ R1 & GRRC rod & 74.6 & & 0.014 & 4.96 & 117/B3/35 & 1.97 & TDM-1 \\ R2 & GRRC rod & 72.6 & & 0.013 & 4.96 & 117/0 & 1.57 & \\ R3 & GRRC rod & 84.2 & & 0.015 & 4.95 & 117/0 & 1.57 & \\ R4 & GRRC T & 173 & 5.946 (n = 3) & 0.019 & 3.07 & 55.5 & 1.97 & \\ R5 & GRRC pulndron & 121 & & 0.022 & 5.06 & 7.0488 & 2.07 & TDM-1 \\ R6 & GRRC pulndron & 119 & & 0.022 & 5.06 & 13.2488.8 & 2.07 & TDM-1 \\ R7 & GRRC pulndron & 99.1 & & 0.018 & 5.06 & 25.4488.8 & 2.07 & TDM-1 \\ R8 & GRRC pulndron & 106 & & 0.020 & 5.06 & 48.5 & 2.07 & \\ R9 & GRRC pulndron & 118 & & 0.022 & 5.06 & 25.4 & 2.07 & \\ R10 & GRRC plate L & 231 & & 0.042 & 4.95 & 48.3 & 1.82 & \\ R11 & GRRC plate T & 396 & 27.083 (n = 3) & 0.043 & 2.96 & 20.0 & 1.82 & \\ R12 & GRRC plate S & 189 & 45.946 (n = 3) & 0.021 & 2.99 & 18.5 & 1.82 & \\ R13 & GRRC plate S-PL & 202 & & 0.031 & 4.15 & 24.8 & 1.81 & \\ R14 & GRRC plate S-PL & 441 & & 0.046 & 2.84 & 27.2 & 1.81 & \\ R15 & GRRC AS4 UD L & 91.0 & & 0.031 & 9.32 & 36.6 & 1.53 & \\ R16 & GRRC AS4 UD T & 247 & 4.938 (n = 3) & 0.029 & 3.16 & 24.3 & 1.53 & \\ R17 & GRRC AS4 UD S & 196 & & 0.023 & 3.15 & 20.0 & 1.53 & \\ R18 & GRRC G50 UD L & 110 & & 0.046 & 11.30 & 13.3287.7 & 1.58 & TDM-1 \\ R19 & GRRC G50 UD L & 99.7 & & 0.041 & 11.30 & 28.7 & 1.58 & \\ R20 & GRRC G50 UD L & 100 & & 0.041 & 11.30 & 13.3 & 1.58 & \\ R21 & GRRC G50 UD T & 129 & & 0.013 & 2.83 & 20.0 & 1.58 & \\ R22 & GRRC G50 UD UD & 148 & & 0.015 & 2.79 & 28.9 & 1.58 & \\ R23 & GRRC AS4/MR15 L & 155 & & 0.046 & 11.33 & 14.5 & 1.45 & \\ R24 & GRRC AS4/MR15 T & 1052 & 279.00 (n = 3) & 0.084 & 2.17 & 19.0 & 1.45 & \\ R25 & GRRC AS4/MR15 S & 865 & 151.35 (n = 3) & 0.073 & 2.30 & 18.5 & 1.45 & \\ R26 & GRRC TCCThe wood samples are all anisotropic because of the nature of tree growth. The vertical tree growth direction will be referred to as longitudinal (L), the center to surface direction as radial (R), and the normal direction relative to these two directions as circumferential (C). These terms are the same as those used for rods or piping [97]. Eight samples were tested in two or three directions. The longitudinal direction always coincided with the length direction, but the surface normal for transverse directions of a sample were usually rotated away from the R or C direction, caused by the tree growth. For intermediate cases, the more dominant direction was given first. The L direction always showed the lowest attenuation among the three (or two) directions. The attenuation usually increased by between two and five times (15-fold in pine; tests O31 and O32) in R or C directions. For large anisotropic effects, as seen in wood samples, it is likely that the usual isotropic elasticity analyses are inapplicable. By calculating transmission coefficients specific to the wave propagation direction, these effects may be minimized for wood, as attenuation was high. Indeed, for red oak (tests O15 and O16) and pine plywood (tests O40 and O41), both TDM-1 and TDM-2 gave comparable results. Diffraction correction values were also small in most regions of examination, so these can be ignored. These two effects will also be considered for fiber-reinforced composites in the following section. The quadratic attenuation spectra were observed only in samples of the R/C orientation; that is, waves impinged on latewood (annual growth ring) layers at shallow angles. Since the latewood parts have much higher elastic modulus (and hardness) values than earlywood parts [98], scattering can be expected. However, it is difficult to find any mechanisms that predict the observed quadratic spectrum. In this section, the attenuation behaviors of engineering polymers and wood are described. Engineering polymers exhibited linearly dependent frequency spectra. The observed attenuation levels agreed with the literature values reasonably well.