Analysis of influencing factors on sound absorption capacity in microwave-treated Pinus radiata wood

Microwave (MW) treatment can be utilized to improve the sound absorption of wood; however, it alters the physical and mechanical properties of the material. Various parameters such as porosity, surface roughness, air cavity depth (ACD), and check after MW treatment were measured in this study. To determine the relationship between wood intrinsic properties and sound absorption capacity, the parameters of MW power density were set to 46.30 kW h/m3, 55.56 kW h/m3, 64.82 kW h/m3, 69.44 kW h/m3, 83.33 kW h/m3, and 97.22 kW h/m3. Many macro-checks and micro-voids occurred, and pit membranes were destroyed after MW treatment as verified by scanning electron microscopy (SEM). The optimal MW-treated parameter for Pinus radiata D. Don was 97.22 kW h/m3. MW treatment increased the sound absorption capacity, decreased the density, and did not affect the surface roughness of the material. The maximum sound absorption coefficient (SAC) increased, and the SAC peak shifted to low frequency range gradually as ACD increased. The average SAC of appropriate ACD of 50 mm was 0.32. Total intrusion volume, total pore area, and permeability improved after MW treatment. The total intrusion volumes of the MW-treated sample and control sample were 2.07 mL/g and 1.81 mL/g, respectively. The pore distribution also widened after MW treatment. ImageJ software was used to determine that percentage of checks to the total sample area is moderately related to sound absorption capacity. MW-modified wood is a favorable potential acoustic absorption material for indoor and outdoor applications.