Hideyuki Ando

Sound absorption by multi-layered constructions composed of powder layers and porous sheets

Sound absorption coefficients for powder beds comprised of fine powders (white carbon and vermiculite) were determined using a reverberation room. The characteristic curves showed high peaks in the low-frequency range, due to the excitation of longitudinal vibration modes in the powder beds by random incident sound. Multi-layered constructions composed of powder layers and porous sound-absorbing material (polyurethane foam sheets) were designed, and their sound absorption properties were investigated. Measurements showed that the multi-layered constructions have high sound absorption peaks in the low-frequency range, believed to be due to the combined effects of powder vibration and viscous dissipation in the pores of the polyurethane foam. The combination of two material possessing different sound absorption characteristics results in an expansion of the sound absorption frequency range and indicates the possibility of new sound absorbing materials for low-frequency sound. Sound absorption simulation based on electric transmission line theory was applied to the multi-layered constructions. The calculated results agreed well with the measured results. A simulation using the propagation constant and characteristic impedance of the constituent materials was found to be useful in designing the frequency characteristics of these constructions.

Sound Absorption Characteristics of Powder Beds Comprised of Binary Powder Mixtures

Sound absorption coefficients of various powder beds comprised of binary powder mixtures were measured using an acoustic tube, and the longitudinal elastic coefficients of the powder beds were obtained from the absorption peak frequencies. It was found that the longitudinal elastic coefficient of the powder mixture bed in an almost naturally packed state can be estimated from the respective longitudinal elastic coefficient and volume ratios of the constituent powder materials. This indicates the possibility of designing new sound-absorbing materials to suit the characteristics of the noise source.