## 2 Materials and Methods  
In the experiment, boards of Norway spruce wood (_Picea abies_ (L.) [PERSON]) and sycamore maple wood (_Acer pseudoplatanus_ L.) were used. The boards prepared of spruce wood (marked SW) had from 6 to 8 annual rings per centimeter and were free of cracks, knots, and biological and other defects. Almost all boards of maple wood (marked MW) had a unique wavy pattern in the grain. The boards were sawn radially into the shape of a thin right-angle board (thickness \(h=7\) mm, width \(b=30\) mm and length (along the wood grain) \(a=293\) mm). The dimensions of the boards were chosen in such a way that they met the requirements of the measurement method, and they could be used in the second step of the experiment without dimensional adjustments. The prepared boards were stored for four weeks in the laboratory (where the relative air humidity was \(\varphi=55\%\) and the air temperature was \(t=20\)\({}^{\circ}\)C). The final moisture content of the wood, which was determined by the gravimetric method, reached \(w\approx 10\%\). The experiment consisted of two steps. In the first step of our experiment, the physical and acoustic characteristics of boards were determined, i.e., density \(\rho\), resonant frequency \(f_{r}\), modulus of elasticity along the grain \(E_{L}\), specific modulus (\(E_{sp}\)), acoustic constant \(A\), speed of sound along the grain \(c_{L}\), logarithmic decrement \(\vartheta\), loss coefficient \(\eta\), acoustic conversion efficiency (\(ACE\)) and sound quality factor \(Q\). The density of each board was determined by the direct method, i.e., dimensions and mass were measured, and then the density was calculated as the ratio of mass to volume. The method of Chladni patterns was used to obtain the resonant frequency of the 4 th vibration mode (2.0) of a rectangular board with free edges [20]. The VIBROVIZER device was used for the measurements (Figure 1). The measuring device consists of a tone generator producing a sinusoidal audio signal, which is amplified by a power amplifier. A loudspeaker is connected to the amplifier, which vibrates the tested board. The board is supported on four small pieces of soft foam in a horizontal position above the loudspeaker mounted under a hole in a box. The board is sprinkled with tea leaves, which will begin to collect along the nodal lines of a particular vibration mode upon resonance. Knowing the resonant frequency of vibration mode (2.0), it is possible to determine the modulus of elasticity \(E_{L}\) (Pa). The formulas for calculating the modulus of elasticity \(E_{L}\) (Pa) are  
\[E_{L}=D_{1}\cdot 12\cdot\eta;\;\eta=1-\mu_{LR}\cdot\mu_{RL};D_{1}\approx 0.0789 \cdot\frac{f_{4}^{2}\cdot\rho\cdot a^{4}}{h^{2}} \tag{1}\]  
where \(D_{1}\) is the elastic constant, \(f_{4}\) (Hz) is the resonant frequency of the 4 th mode (2.0), \(\rho\) (kg\(\cdot\)m\({}^{-3}\)) is the density, \(a\) (m) is the length of wooden plate, \(h\) (m) is the thickness and \(\mu_{LR}\), \(\mu_{RL}\) are Poisson's ratios. The calculation of other PACHs (\(E_{sp}\), \(A\), \(c_{L}\), \(\vartheta\), \(\eta\), \(ACE\) and \(Q\)) was performed according to the following Equations (2)-(9) [8; 21; 22; 23]. The specific modulus of elasticity \(E_{sp}\) (m\({}^{2}\cdot\)s\({}^{-2}\)):  
\[E_{sp}=\frac{E_{L}}{\rho} \tag{2}\]  
The acoustic constant \(A\) (m\({}^{4}\cdot\)kg\({}^{-1}\cdot\)s\({}^{-1}\)), also well known as sound radiation coefficient--R:  
\[A=\sqrt{\frac{E_{L}}{\rho^{3}}} \tag{3}\]  
Figure 1: VIBROVIZER measuring device. The pipe was made of Norway spruce (_Picea abies_ ([PERSON]), while the core of the pipe was made of sessile oak (_Quercus petraea_), and the overlay was made of wild pear (_Pyrus pyruster_ ([PERSON]). The wood used to make the experimental wooden organ pipe was chosen so that the pipe could be used over a wide range of air pressures. The height of the experimental pipe cutout was also adjusted to the range of air pressures. The boards of maple and spruce wood were used for the back wall of the experimental wooden organ pipe.