# Sound Wave Absorption Coefficient and Sound Velocity in Thermally Modified Wood
###### Abstract  
The present work analyses the absorption coefficient of sound waves and the speed of sound propagation in thermally modified wood. The high resistance to weathering, fungi, and better dimensional stability, and therefore the broad physical properties of this material, are well known. However, the literature lacks numerous analyses of its acoustic characteristics. During the study, high-density species, such as oak, red oak, and beech were used, in contrast to pine. Pine wood during this test was characterised by a most rapid increase in the sound absorption coefficient value, in the range of 1000-6300 Hz, and reached the highest value from all wood species. Among all species, the highest value of the examined parameter was obtained for beech wood and pine wood, which were 0.213 (at frequency 3 kHz) and 0.183 (at 6.3 kHz), respectively. The sound velocity decreased for all species only in the tangential direction.  
wood; sound absorption coefficient; sound velocity; dynamic modulus of elasticity; heat treatment +
Footnote †: _Applied Sciences_
conducted by [PERSON] and [PERSON] [17], there is a visible relationship between the absorption coefficient and wood density. The species used in the study were oak, ash, sapele, pine, balsa, birch, poplar, meranti, alder, and elm. An increase in the value of the sound absorption coefficient was noted with increasing density in the frequency ranges of 2 kHz, 1 kHz, and 4 kHz. Thermal modification of wood also leads to changes in its density [18]. According to the research, a more significant weight loss of spruce wood was recorded with an increase in the temperature of the modification process. At 180 \({}^{\circ}\)C, the decrease in density was 4.8%. The decomposition and evaporation of some chemical components of wood also decrease its hygroscopicity [19]. By examining the acoustic parameter of thermally modified wood, its potential as a sound-absorbing material can be determined. In the case of thermally modified wood, the study by [PERSON]'s [9] proves that an increase in temperature and the duration of the modification process increases the sound absorption coefficient. The growth rate of the coefficient was greater for the high-frequency bands. When the process was carried out at 220 \({}^{\circ}\)C for 18 h, the sound absorption coefficient for the frequency of 250 Hz was 0.028, whereas for 4 kHz, it was 0.037. Japanese larch wood was tested as well. Other scientists have also dealt with the sound absorption coefficient, e.g., in pellets [20], musical instruments [21], or other materials [20; 22; 23; 24], and even with some modifications [25; 26; 27]. The sound absorption properties and the potential of wood and wood-based materials as sound absorbers were also characterised [28; 29]. There are no reports on the effect of thermal modification on the sound absorption coefficient for other types of wood. The aim of this study was to determine the sound wave absorption coefficient of thermally modified wood. Research in this area was carried out on the wood of Scots pine, European beech, pedunculate oak, and red oak. In addition, how the sound velocity changed in the analysed wood after thermal modification was checked.