# Sound Wave Absorption Coefficient and Sound Velocity in Thermally Modified Wood
## 3 Results and Discussion
The graph presented in Figure 2 shows that the sound absorption coefficient increases with increasing frequency. In the initial range of 60-500 Hz, scots pine wood achieves the highest results, whereas red oak wood achieves the lowest. This situation changes in the 500-2500 Hz range, whereas oak wood shows the highest sound absorption coefficient. At frequencies of 1600 Hz, it reaches a value of 0.12. In this range, an increase in the examined parameter for red oak wood is also visible. In the 500-3500 Hz frequency range, beech wood is characterised by the lowest value of the analysed parameter. Beechwood also shows the highest density at the beginning of the process. During this test, pine wood is characterised by a rapid increase in the sound absorption coefficient value, in the range of 1000-6300 Hz, and reaches the highest value of all wood species. The mean values for oak and red oak wood are very similar. The sound absorption coefficients of porous materials such as wood are proportional to the porosity and roughness of the inner pore wall. The differences in the sound absorption coefficients of both species are statistically insignificant. Statistically significant differences are observed between the remaining species. Similar results for the sound absorption coefficient were obtained by [PERSON] [16]. By analysing different wood species, he described that the parameter did not exceed 0.2 for a frequency of 2 kHz. The thermally modified wood was subjected to the same test. The average sound absorption coefficient is shown in Figure 5. The figure shows a rapid increase in the tested parameter above 800 Hz for all the species. Among all species, the highest values of the examined parameter were obtained for beech wood and pine wood, which were 0.213 (at frequency 3 kHz) and 0.183 (at 6.3 kHz). The increase in the sound absorption coefficient due to heat treatment in the high-frequency band is due to the increased porosity. During heat treatment, wood density was reduced, and the intercellular layers were exposed due to the shrinkage of the cell wall. This increased the porosity of the wood. If we calculate the porosity (C) of wood using Formula (3)  
\[\mathrm{C}=\left(1-\frac{\rho_{D}}{1.5}\right)\cdot 100\ (\%), \tag{3}\]  
Figure 4: Sound absorption coefficient in beech, oak, red oak, and pine wood before thermal modification. where 1.5 is the density of the wood substance, and \(\rho_{\mathbf{D}}\) is the density of the wood, we obtain 50.5% for beech, 54.3% for oak, 54.7% for red oak, and 57.3% for pine wood. The thermal modification increased the porosity to 61.4%, 61.8%, 59.9%, and 62.4%, respectively. The increase in porosity was statistically significant for all the analysed wood species. The highest increase was obtained for beech wood, which was approximately 22%, and the lowest increase was for pine wood, which was 8.95%. Based on Figure 6, it is possible to determine the impact of thermal modification on the value of the sound absorption coefficient in the wood of individual species. In the case of oak wood, the tested parameter obtained lower values in the frequency range of 500-2000 Hz after the thermal modification process. The most significant difference was noted at 1000 Hz, where the decrease was 43% compared to that of the control wood. In the case of red oak wood, the observed differences were minor. Beechwood, after modification, was characterised by a higher wave absorption coefficient than wood before thermal modification. The observed increase is visible for the frequency range 1500-5500 Hz, with the wave absorption coefficient value increasing by more than 100% for a frequency of 3000 Hz. Visible differentiation can be seen in the case of pine wood, where the sound absorption coefficient values also increased after the thermal modification of the wood. In the 800-4000 Hz frequency range, the increase was up to 25% compared to the results of the control wood.