## 3 Results and Discussion
### Acoustical Properties  
The first and second resonant frequencies and longitudinal resonant frequencies were captured using the free-free resonant method. Internal friction was also determined at this time. The rubber head stick was used for this task, and the specimen was supported by polyurethane segments using the formula 0.224 \(\times\) L (L = length of the specimen). These vibrations were captured using a microphone and external sound card (EDIROL FA-101) via the FireWire interface. The sound velocity was determined using the above-mentioned ultrasonic method. This measured the ultrasonic wave velocity in longitudinal and transversal directions.  
Sound velocity in the longitudinal direction is affected by high moisture content. Sound velocity is significantly lower in wood with higher moisture content due to the lower Young's modulus. The higher moisture content wood contains the lower velocity sound it achieves [1; 18; 27].  
A Scheffe test was performed. A total of seven groups were selected for this task, depending on the air humidity. These groups are described in Figure 6. The observation focused on groups 2, 4, 6 and 3, 5, 7. The only statistically significant differences were between groups 2 and 6; it means that the sound velocity was not largely affected by the modification. However, the sound velocity in the transverse direction was affected differently. In dry wood, the velocity in this direction is approximately 1200 m\(\cdot\)s\({}^{-1}\). In water, it is 1485 m\(\cdot\)s\({}^{-1}\)[1]. By comparing these two velocities, it is clear how increasing the  
Figure 5: Volume swelling in the following three groups of specimens: Ref. (unmodified), modified at 180 \({}^{\circ}\)C, and modified at 200 \({}^{\circ}\)C, \({}^{\circ}\)—outlier values; *—extreme values.  
water content in the wood increases the sound velocity increases. However, in this study, we measured values with a high variance. Therefore, it is not statistically significant. The results are shown in Figure 6.  
The derived acoustical properties that were mentioned above are dependent on measured frequencies and velocities. Therefore, there is a strong correlation with these quantities. Figure 7 shows the results.  
Generally, increasing the air humidity decreases the values of these properties. Otherwise, _tan_\(\delta\) increases with higher moisture content and it is reduced by thermal treatment. _Tan_\(\delta\) represents damping which correlates with moisture content [16; 19; 28; 29]. Specific Young's modulus is related to sound velocity and density ratio and it was affected especially by the modification at 200 \({}^{\circ}\)C. The same results were reported by many other authors [16; 18; 19; 27; 29].  
_ACE_ and _RACE_ values also depend on the moisture content in the wood. The moisture content influences density, _tan_\(\delta\) and Young modulus--all input parameters for _ACE_ and _RACE_ definition (Equations (6) and (8)). The improvement of sonic efficiency especially with the modification at 200 \({}^{\circ}\)C was reported in our study. This level of thermal treatment decreases density, _tan_\(\delta\) and increases Young modulus. The result is in agreement with other studies [4; 19; 25; 27; 29; 30]. Treatment at temperature 180 \({}^{\circ}\)C did not showed significant change comparing to untreated specimens. Bending modulus of elasticity was affected by moisture content as well (the higher moisture content the lower modulus). Only modification at 200 \({}^{\circ}\)C causes significant increase of this parameter.  
In general, based on outputs of analysis of variance the thermal modification at 180 \({}^{\circ}\)C did not cause any noticeable changes to the acoustical properties; however, the modification at 200 \({}^{\circ}\)C caused the properties to increase.