2017 年 83 巻 845 号 p. 16-00344
In this paper, we report the use of sound-absorbing materials, specifically sound-absorbing structures of bundled rice straws, that are oblique arranged incident to the sound waves. In addition, we investigated the normal and oblique incident sound-absorption coefficients of sound-absorbing structures made of bundled rice straws in free space. In this study, we placed sound-absorbing structures made of bundled rice straws at an angle against the direction of the thickness in order to elongate the rice-straw element without increasing the apparent thickness. We then experimentally investigated the increase in sound absorption in the low-frequency range. The following conclusions were drawn. For test samples in which rice straws were arranged both in a normal direction and in an oblique manner, the value of the normal incident sound-absorption coefficient in free space was close to the results obtained using acoustic tubes. For test samples with rice straws arranged both in the normal direction and in the oblique manner, the sound-absorption peak of the oblique incident sound-absorption coefficient in free space was shifted to the high-frequency side compared with that of the normal incident sound-absorption coefficient and the sound-absorption curve became flat in the high-frequency range. These observations suggest that a sound-absorbing material made of rice straws functions in the same way as a sound-absorbing material that has pores on its incident face. We demonstrated that the use of an oblique arrangement of rice straws inside acoustic tubes and in free space can improve the sound absorption in low-frequency range without increasing the apparent thickness of the material.