Predicting Sound Absorption of Stacked Spheres: Combining an Analytical and Numerical Approach

Tire-road noise is a serious problem, but can be significantly reduced by the use of porous asphalt concrete. Here, the sound absorption of the porous asphalt concrete is important and can be predicted by ground impedance models. Yet, modeling porous asphalt concrete is complex, especially when nonlocal effects and scattering effects are considered. The objective of this research is to predict the sound absorption coefficient for a three-dimensional porous structure. The proposed solution is obtained using a novel modeling approach, in which the total solution of the sound field is found by combining the solutions of two subsystems: a background sound field and a scattered sound field. The background sound field contains the (analytical) solution of the sound field including the viscothermal energy dissipation inside the pores of the porous asphalt concrete. In the second subsystem, the (numerical) solution for the scattering on the rigid stone skeleton of the pavement is found. For both subsystems, we use a model containing two layers: an air layer and a viscous air layer with a certain granular structure. The main advantage of this modeling approach is the (relatively) low computation time. In this paper, the proposed modeling approach and the validation of this approach are described. The modeling approach is validated for normal incident plane waves absorbed and scattered by various structures of stacked marbles, using the impedance tube technique. This approach can be applied to predict the absorption coefficient of porous structures, like asphalt concrete roads. Moreover, it can be used as design tool to optimize the sound absorption of new road surfaces.