Sound absorbing characteristics of granular materials with elastic modulus varying along gravitational direction for vertical and horizontal sound propagations

Abstract

Elastic modulus of granular materials consisting of non-adhered solid grains depends on its compressive stress. As vertical compressive stress increases with depth due to gravity without any additional force, the elastic modulus of granular materials also increases along the gravitational direction. This study investigates the sound absorbing characteristics of granular materials with such gradient elasticity for both vertical and horizontal sound propagations, considering applications to vertical walls, horizontal ceilings, and floors. The normal incidence sound absorption coefficients of hollow glass beads, chosen as a representative granular material, were measured using an impedance tube. This measurement was conducted under conditions where the sound wave propagated perpendicular and parallel to the gradient direction of elasticity. The frequency and magnitude of the first peak in the sound absorption coefficient varied with the direction of sound propagation. A method for predicting this variation was proposed, showing that the frequency at which the first peak occurred could be predicted within an error of 10%. Additionally, the effects of the thickness and shape of the absorbing surface on the sound absorption coefficients are elucidated through calculations.