# Sound Wave Absorption Coefficient and Sound Velocity in Thermally Modified Wood
## 3 Results and Discussion  
Table 1 shows the wood density of the material before and after thermal modification. The results are mean values calculated for at least six pairs of samples from each wood species. Figure 3: Scheme of the impedance tube (Bruel & [PERSON], type 4206). Based on the above information, it can be seen that after the thermal modification process, the wood density value decreased. This decrease is caused by a loss of mass, which is related to the decomposition of some chemical components of the wood structure. The decrease in density depends on the temperature of the modification process; the higher the temperature, the higher the density loss. The decomposition and evaporation of some chemical components of wood decrease its hygroscopicity [18]. The most significant percentage density changes were recorded for the sample of beech wood, which was 23%, followed by oak at 16%, pine at 13%, and red oak at 12%. The analysis of the sound propagation speed was divided according to the species used in the study and their dimensions. In Table 2, a summary of the average sound propagation speed values in the direction along the fibres and in the radial and tangential direction in the material before thermal modification is presented. From the data presented in Table 2, it can be confirmed that the highest sound propagation speed occurred in the longitudinal direction, followed by the radial and tangential directions, respectively. The relationship VL \(>\) VR \(>\) VT, as mentioned by [PERSON] [32], was also confirmed in this case. Due to the larger diameter of the 100 mm diameter samples, a slightly lower wave velocity was observed. The highest results were recorded for the pine samples in the direction along the fibres. The longest anatomical elements characterise this species. The average tracheids length in pine was approximately 3.1 mm, whereas the average length of the fibres was approximately 1 mm [33]. Then, considering the obtained average values, oak, beech, and red oak can be distinguished. After the process of thermal modification of wood, the values of the sound propagation velocity changed. The results are presented in Table 3. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Wood Species**} & \multicolumn{3}{c}{**Oven Dry**} \\ \cline{2-4}  & \(\rho\) & \(\rho_{HT}\) & \(\Delta\rho\) \\ \hline \multicolumn{4}{c}{**(kg\(\cdot\)m\({}^{-3}\))**} & **(\%)** \\ \hline Beech & \(743\pm 32.6\) & \(579\pm 38.0\) & \(-22.1\pm 1.02\) \\ Oak & \(686\pm 41.5\) & \(573\pm 46.1\) & \(-16.5\pm 0.94\) \\ Red oak & \(679\pm 49.1\) & \(601\pm 50.7\) & \(-11.5\pm 0.92\) \\ Scotts pine & \(641\pm 43.5\) & \(564\pm 38.3\) & \(-12.0\pm 0.98\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Wood density before and after heat treatment. \begin{table}
\begin{tabular}{c 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}  & & & & & **[m/s]** & & & & \\ \hline Beech & 2439 & 5331 & 1457 & 2274 & 4831 & 1354 & 2356 & 5081 & 1405 \\ \hline Oak & 2203 & 5458 & 1219 & 2115 & 5178 & 1355 & 2159 & 5318 & 1287 \\ \hline Red oak & 2516 & 5247 & 1228 & 2025 & 4257 & 1268 & 2271 & 4752 & 1248 \\ \hline Scots pine & 1865 & 6094 & 901 & 2172 & 6430 & 1060 & 2019 & 6262 & 981 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of sound propagation velocity values before modification (V\({}_{\text{RR}}\)—sound velocity in the radial direction; V\({}_{\text{LL}}\)—sound velocity along the fibres; V\({}_{\text{TT}}\)—sound velocity in the tangential direction). The tested values decreased for the 29 mm and 100 mm samples for all species, only in the tangential direction.