# Sound Wave Absorption Coefficient and Sound Velocity in Thermally Modified Wood
## 2 Materials and Methods  
In order to study the sound wave absorption coefficient of thermally modified wood, four species were used as follows: scots pine (_Pinus sylvestris_ L.), beech (_Fagus sylvalica_ L.), oak (_Quercus robur_ L.), and red oak (_Quercus rubra_ L.). The tests were performed both before and after the thermal treatment. The study of the sound wave absorption coefficient was carried out in two stages. The same material was compared before and after thermal modification to obtain the values of the tested features. The analysed wood species were tested using an impedance tube during the first stage. In the second stage, the twin material was thermally modified. Samples were subjected to thermal modification by the most often used method, in a steam atmosphere, according to the procedure of ThermoWood [2; 3; 18; 30]. The wood modification was carried out under industrial conditions at 190-213 \({}^{\circ}\)C. After the thermal treatment, the samples were conditioned under laboratory conditions (T = 20 \({}^{\circ}\)C and RH = 65%). When the samples reached the same equilibrium moisture content as before the modification, they were subjected to testing for the sound wave absorption coefficient. Due to using an impedance tube, in particular two tubes, the samples were in the form of discs with diameters of 29 mm and 100 mm. The thickness of each sample was 23 mm. Figure 1 shows an exemplary pair of samples made of beech wood before and after the thermal modification. The samples of unmodified and modified wood of a given species were obtained from a single board (Figure 2). The board from which the samples were cut out was 50 mm thick. After marking the places where the samples were to be cut, the board was cut lengthwise (along the dotted line). Then, one part was subjected to thermal modification, and the control samples were cut out from the other part. After modification, the modified wood samples were cut out from the previously marked places. Elements with different diameters were cut in pairs to obtain samples corresponding to each other in density. At least six pairs of samples were prepared for each species. The sound wave absorption coefficient was determined using an impedance tube (Bruel & Kjaer, type 4206) with a PULSE measurement card. This apparatus consisted of  
Figure 1: A pair of beech wood samples before (**a**) and after thermal modification (**b**). Figure 2: Sample cutting scheme. an impedance tube, a loudspeaker, a microphone probe, and a generator (Figure 3). The measuring range was 50-1600 Hz for samples with a diameter of 29 mm and 500-6300 Hz for samples with a diameter of 100 mm. The test consisted of placing a sample in an impedance tube, onto which sound waves were generated perpendicular to its surface. The sound pressure, which determines the field interference distribution, was measured using two microphones. As a result, information on the reflection and absorption coefficient of the signal passing through the tested material was obtained. The tests were performed using the ISO 10534-2 standard [31]. The speed of sound propagation in the three-dimensional part was measured by measuring the time it took for the wave to travel through the samples. The material sampler type 543E (UNIPAN, Poznan, Poland) transmitted and received transducers with a frequency of 1 MHz. The speed of the sound wave on each sample was calculated using the following formula:  
\[V=\frac{s^{L,R,T}}{t}\Big{(}\mathrm{m}\cdot\mathrm{s}^{-1}\Big{)} \tag{1}\]  
L, T, and R refer to the sample size in the longitudinal, tangential, and radial directions (m), respectively, with the propagation time (s). The sound absorption coefficient of the samples was determined. The sound velocity and the sound absorption coefficient were determined on the same samples.