# Sound Wave Absorption Coefficient and Sound Velocity in Thermally Modified Wood
## 3 Results and Discussion
In the radial direction, there was an increase in the tested speed for specimens with 29 mm oak and red oak by 7.5% and 5.3%, respectively, and for 100 mm oak and red oak by 10.5 and 10.1%, respectively (Table 3). In the case of the remaining samples, the value of the sound propagation velocity in this direction decreased. The reason for this phenomenon may be the presence of specific wood rays in the oaks. Both species have very wide tree rays. Their structure can help to increase the sound velocity in wood. In addition, oak wood was characterised by a relatively small loss of density. Table 4 shows the material's dynamic modulus of elasticity values before (MOE\({}_{\text{D}}\)) and after thermal modification (MOE\({}_{\text{DHT}}\)). These are the values calculated for the average value of the sound speed propagation only in the longitudinal direction. Generally, the modulus of elasticity determined using ultrasonic testing is much higher than that using destructive testing. The obtained MOE values, therefore, exceed [PERSON]'s modulus values found in the literature ([PERSON], 1998). As mentioned above, the density of all the analysed samples decreased. In the case of the sonic modulus of elasticity, a similar decrease was recorded in pine and beech wood, which was approximately 7.5%. However, the change in the MOE for oak wood was surprising. In pedunculate oak (_Quercus robur_ L.), a reduction in MOE by more than 18% was noted, whereas, in red oak, it increased by more than 20%. This increase was mainly due to an increase in the speed of sound propagation in this species. For red oak, it was greater than 10%. By analysing the relationship from which the MOE was calculated, it can be seen that the sound velocity was squared. In addition, considering that the density decrease was the lowest of all the analysed species (Table 1), such a high increase in MOE was not surprising. The sound absorption coefficient was interpreted for each species, and then the materials used before and after the modification process were compared. In order to better illustrate the variability of the wave absorption depending on the frequency, a relationship is presented in Figure 4. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Wood Species**} & \multicolumn{3}{c}{**Oven Dry**} \\ \cline{2-4}  & \(\mathbf{MOE_{D}}\) & \(\mathbf{MOE_{DHT}}\) & \(\mathbf{\Delta MOE_{D}}\) \\ \hline  & **(MPa)** & **(\%)** \\ \hline Beech & 19,182 \(\pm\) 1922 & 17,745 \(\pm\) 2089 & \(-\)7.5 \(\pm\) 0.42 \\ Oak & 19,401 \(\pm\) 1935 & 15,757 \(\pm\) 2416 & \(-\)18.8 \(\pm\) 0.68 \\ Red oak & 15,333 \(\pm\) 1757 & 18,459 \(\pm\) 2332 & +20.4 \(\pm\) 1.72 \\ Scots pine & 25,135 \(\pm\) 2134 & 23,318 \(\pm\) 2688 & \(-\)7.2 \(\pm\) 0.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Dynamic modulus of elasticity before and after heat treatment. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Wood Species**} & \multicolumn{3}{c}{**29 mm**} & \multicolumn{3}{c}{**100 mm**} & \multicolumn{3}{c}{**Average**} \\ \cline{2-10}  & \(\mathbf{V_{RR}}\) & \(\mathbf{V_{LL}}\) & \(\mathbf{V_{TT}}\) & \(\mathbf{V_{RR}}\) & \(\mathbf{V_{LL}}\) & \(\mathbf{V_{TT}}\) & \(\mathbf{V_{RR}}\) & \(\mathbf{V_{LL}}\) & \(\mathbf{V_{TT}}\) \\ \cline{2-10}  & \multicolumn{3}{c}{**[m/s]**} \\ \hline Beech & 1951 & 5365 & 1066 & 1522 & 5708 & 1302 & 1737 & 5536 & 1184 \\ \hline Oak & 2368 & 5283 & 1149 & 2345 & 5206 & 1321 & 2230 & 5244 & 1235 \\ \hline Red oak & 2650 & 5455 & 1044 & 2230 & 5629 & 1266 & 2440 & 5542 & 1155 \\ \hline Scots pine & 1820 & 6364 & 885 & 2062 & 6495 & 811 & 1991 & 6430 & 848 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of sound propagation velocity values after thermal treatment (V—sound velocity; R—radial direction; L—direction along the fibres; T—tangential direction). The human hearing range is from 16 Hz to 21,000 Hz, with the human ear being the most sensitive in the 2000-3000 Hz range [34].