## 2 Materials and Methods  
Resonance spruce (_Picea abies_ L. [PERSON]) specimens (44 \(\times\) 12 \(\times\) 450 mm) with no defects (cracks, knots, etc.) were sorted into three groups, i.e., reference group, High Temperature (HT) treated at 180 \({}^{\circ}\)C, and HT treated at 200 \({}^{\circ}\)C, with 40 specimens in each group. At first, all specimens were conditioned at a temperature of 20 \({}^{\circ}\)C and an air humidity of 40% in a climatic chamber (Memmert CTC256). These conditions match with approx. EMC 7.7%. Moisture content was determined using the oven-dry method according to EN 13183-1(2002) [21] and the following equation (Equation (1)):  
\[EMC=\frac{1}{B}\times ln\frac{A}{ln\frac{1}{\varphi}} \tag{1}\]  
where, according to de Boer-Zwicker pay: \(A\) = 7.731706 - 0.014348*T; \(B\) = 0008746 + 0.000567*T.  
The hygroscopicity of the wood was evaluated via the equilibrium moisture content in both the unmodified and modified specimens using the standard gravimetric method (Equation (2)):  
\[W=\frac{m_{w}-w_{0}}{m_{0}}\times 100 \tag{2}\]  
where \(m_{w}\)--weight of saturated wood, \(m_{0}\)--weight of bone-dry wood. The main characteristic of dimension changes is volume swelling because it includes all dimensions of the specimen. In this research paper, volume swelling was used to compare unmodified specimens with modified specimens. For this comparison, the air humidity increase from 40 to 60% was selected. Volume swelling was determined using the following equation (Equation (3)):  
\[\alpha_{i}=\frac{a_{imax}-a_{i}(w)}{a_{i}(w)}\times 100\ (\%) \tag{3}\]  
where, \(\alpha_{imax}\)--dimension in any direction after swelling; \(\alpha_{i}\)_(w)_--the same dimension before swelling. Moderate thermal modifications at 180 \({}^{\circ}\)C for 8 h and 200 \({}^{\circ}\)C for 10 h were selected for the purpose of this study. The modification course is shown in Figure 1, and the thermal modification chamber scheme is shown in Figure 2. After this modification, all specimens were climatized at a temperature of 20 \({}^{\circ}\)C and an air humidity of 60%. The acoustical properties and dimension changes were measured in every humidity status. To test the dynamic properties, the sample was positioned on two flexible supports (free-free support condition). The points of support were located in the nodes (minimum amplitude) of the fundamental bending mode shape of vibration (at 0.224 and 0.776 of the sample length, which is in 100.8 and 349.2 mm length of the specimen) [22; 23]. The first and second bending frequencies, longitudinal natural frequencies, logarithmic decrement of damping, and the sound velocities were measured. The flexural vibration was induced by the impact of a soft hammer to the center of the sample perpendicularly to the length (see Figure 3). The sound propagation velocity was measured using the FAKOPP Ultrasonic Timer apparatus with US10 sensors in longitudinal and transversal directions. Figure 1: Temperature and humidity courses with the modifications (a) 180 \({}^{\circ}\)C; (b) 200 \({}^{\circ}\)C.  
The derived acoustical properties are as follows:  
Dynamic Young's modulus (Equation (4))  
\[{E_{L}}^{\prime}=\ c^{2}\ \times\ \rho \tag{4}\]  
\(c\) --sound velocity; \(\rho\)--density. _Dynamic bending modulus_ of elasticity (Equation (5))  
\[E_{b}=\frac{f_{1}^{2}\times m\times l^{2}}{3.56^{2}\times w\times\frac{h^{3}}{1 2}} \tag{5}\]  
where \(f_{1}\)--first bend frequency; \(m\)--specimen weight; \(l\)--specimen length; \(w\)--specimen width; \(h\)--specimen thickness. _Acoustic conversion efficiency_ (Equation (6))  
\[ACE_{L}=\frac{\sqrt{\frac{E_{L}^{{}^{\prime}}}{\rho^{3}}}}{\tan\delta} \tag{6}\]  
where \({E_{L}}^{{}^{\prime}}\) is the dynamic Young's modulus; \(\rho\)--density; _tan_\(\delta\)--internal friction. Figure 3: Measuring the acoustical properties: (**A**) soft hammer; (**B**) ultrasonic sensor or microphone; (**C**) specimen; (**D**) soft pads.