Effect of Microwave Treatment on the Sound Absorption Performance of Radiata Pine

【Objective】Because of the poor sound absorption performance of radiata pine(Pinus radiata), microwave modification was used to improve the sound absorption performance of radiata pine. The influences of different microwave power, treated time and wood grain directions on the sound absorption performance of radiata pine were explored so as to improve the added value of wood and to expand the application field of wood. 【Method】First of all, the microwave treated wood was obtained by drying the moisture content of radiata pine green wood to 40%-60% in different microwave powers(100 kW, 120 kW and 140 kW)and different treated times(20 s and 30 s). Transfer function method was used to measure the sound absorption coefficient(SAC) of microwave treated wood on their radial section and tangential section, in order to discuss the changing rules of the sound absorption performance of radiata pine in varying treated parameters. Optical microscope and scanning electron microscope(SEM)were used to observe the anatomical structure and microwave treated wood microstructure morphology, respectively. 【Result】Compared with the control samples, the density and moisture content of radiata pine decreased significantly after microwave treatment. The sound absorption performance of radiata pine was improved to different degrees in different microwave treated parameters. The SAC increased about 4.79%-201.9%, and the best average SAC was up to 0.320. The SAC of microwave treated wood under different parameters did not change significantly in the low frequency range below 1 000 Hz. The sound absorption curve showed a trend of first rising and then falling, and the sound absorption trough appeared around 1 000 Hz. The difference of SAC was significant above 1 000 Hz, and the curve of SAC showed an upward trend. The higher the microwave power and the longer the treated time, the better the sound absorption performance of the microwave treated wood, and the higher the trend of the sound absorption coefficient curves. Compared with the radial section sample, the SAC of tangential section of the treated wood was 9.6% to 29.6% higher. When the SAC of the radial section of the control wood was above 1 000 Hz, it was lower than that of the control wood tangential section; when it was below 1 000 Hz, it was higher than tangential section of the control sample. It can be seen that there were a large number of uniseriate rays and a small amount of fusiform rays on the tangential section of radiata pine compared with the radial section by the optical microscope. The microstructure of the microwave treated samples and the control samples was observed by SEM, the results showed that the true middle lamella and the tracheid wall of radiata pine treated by microwave were cracked and pit membrane disappeared, these changes increased the wood porosity and connectivity between adjacent pores, made a greater probable friction between pore wall and sound waves transmitted inside the wood, increased sound energy loss so as to achieve a better sound absorption performance. Variance analysis and partial correlation analysis showed that the correlation among microwave power, treated time and wood grain directions on SAC were 0.519, 0.637 and 0.705, respectively. The three variables had a moderate influence on the sound absorption coefficient, among which grain directions had the greatest influence on the sound absorption performance. 【Conclusion】 The SAC of the tangential section and the radial section of the untreated wood was 0.167 and 0.106, respectively, and the sound absorption performance was not very well. Microwave modification could be an effective method to improve the sound-absorbing performance of wood, but it might cause the destruction of wood microstructure. The optimal microwave treatment parameters were 140 kW microwave power, 30 s treated time, and the direction of wood grain was tangential section. The SAC of microwave treated wood was up to 0.320, which is known as sound absorption material. © 2021, Editorial Department of Scientia Silvae Sinicae. All right reserved.