Abstract
The sound absorption coefficients of powder beds comprised of fine powders (white carbon and vermiculite) were determined using a reverberation room. The characteristic curves of the powder beds 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 the powder layers and porous sound-absorbing materials (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, which are considered to be due to the combined effects of powder vibration and viscous dissipation in the pores of the polyurethane foam. The combination of two materials possessing different sound absorption characteristics results in an expansion of the sound absorption frequency range and indicates the possibility of a new sound absorbing material 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. It was found that a simulation using the propagation constant and characteristic impedance of the constituent materials is useful in designing the frequency characteristics of these constructions.
