Permeability is an important physical property of wood that affects its drying properties, impregnation, and sound absorption [1,2,3,4], determining many of its applications as a porous material. Modifications for improving the gas permeability of wood have been studied since the 1930s, and remain a popular research topic . For example, Xu et al.  reported that supercritical carbon dioxide (SC-CO2)-treated Paulownia fortunei could remove tyloses in vessels to improve permeability. Tanaka et al.  reported that Douglas-fir pit aspirations, margo, and torus were destroyed after ultrasonic treatment, improving permeability. Taghiyari  reported that nanosilver-impregnated Polulus nigra and Carpinus betulus showed increased permeability. This is because the nanosilver particles not only removed extractives, but also destroyed the tissues of vessel elements by impregnation pressure.
As introduced above, various special methods for improving permeability have been studied, but the most widely used wood modification is heat treatment. Heat treatment disrupts wood cell walls and expands the intercellular space . From this change, heat-treated wood has increased pore size and porosity and improved gas permeability [8, 9]. In particular, Taghiyari and Avramidis  reported that applying the silver nano-impregnation process to heat treatment promotes microcracks in the wood cell wall, which has a positive effect on permeability. Kolay and Kang  observed cell wall structural changes in steam-exploded Cocos nucifera, a type of heat-treated wood modification. These changes caused an improvement of sound absorption performance. Wood modification that can affect permeability in solid wood species can also affect sound absorption, as these two properties are related closely to the porous structure of wood .
This study focused on wood modification by microwaves. Microwave treatment is a type of heat treatment using microwaves . Microwave heating occurs due to the polarizing effect of electromagnetic radiation at 300 MHz and 300 GHz . In 1945, the Raytheon Company in the United States, developed and popularized a commercial microwave for cooking . Since then, microwave treatment has been used in various industries .
Various studies on microwave treatment have been reported in the field of wood modification. The main purpose of applying microwave treatment in wood modification is to improve the permeability [1, 16, 17].
Wang et al.  reported that the micro-void number increased after microwave treatment in Pinus sylvestris var. mongolica wood. Microwaving damaged the pit membranes, ray cells, and intercellular layer of tracheids. These modifications led to improved permeability and improved sound absorption performance of the wood.
Weng et al.  reported that, after pretreatment of Chinese fir lumber using a microwave, pit membranes were damaged, ray parenchyma cells and tracheids were detached, and micro-cracks in the radial section were observed. This treatment contributed to shortening of the drying time due to formation of new pathways for moisture transfer.
Poonia et al.  reported that cracks increased after microwave treatment of Eucalyptus tereticornis wood. For this reason, permeability improved and contributed to the increase of preservative absorption.
Previous studies have shown that microwave treatment of wood degrades the microstructure and increases permeability. The high permeability of wood is due to the many open pore structures [9, 19, 20]. The sound absorption coefficient of wood is related to its permeability [11, 21, 22].
On the laboratory scale, the impedance tube mainly is used for measuring the sound absorption coefficient and allows calculation of the result in a very fast time of less 10 s . This permits simple estimation of the permeability and impregnation properties of wood by measuring the sound absorption performance.
The effect of microwave treatment described in previous studies is limited to a few species, and there is no known information about the efficacy of microwave treatment in improving permeability and sound absorption in various species. For this study, North American Spruce (Picea sitchensis), North American Douglas fir (Pseudotsuga menziesii), and Japanese larch (Larix kaempferi) samples were prepared as very popular species for structural timber in Korea. The sound absorption coefficients of transverse, radial, and tangential sections of the three types of microwave-treated softwoods were investigated. This study showed the importance of improvement of sound absorption capability depending on tree species and anatomical plane after microwave treatment. The results will be useful for predicting the permeability and impregnation of microwave-treated wood.
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