Prediction of Sound Absorption of Stacked Granular Materials for Normal and Oblique Incident Sound Waves

Abstract

Tire-road noise is a problem in many (densely) populated areas. It can be significantly reduced by using porous asphalt concrete. A challenge is to develop porous asphalt concrete, such that the most dominant frequencies in tire-road noise will be absorbed by the road surface. It is especially important to also reduce and absorb oblique incident sound waves, since tires radiate noise normal to the tire surface, which means oblique incident waves on the road surface. Predicting the behavior of porous asphalt concrete using models is complex, especially when non-local effects and scattering effects are included. The objective of this paper is to show a modeling approach to predict sound absorption for oblique incident waves in three-dimensional porous materials. Using this method, one is able to predict the sound absorption of porous road surfaces in the design phase. This modeling approach includes a two-step approach in which first the viscothermal energy dissipation inside the pores between the rigid materials (stones) are estimated and then, secondly, the non-local effects such as scattering on the st ones within the porous road surface are computed using a finite element model. The combination of both sound fields gives the total sound field in and above the three-dimensional porous material, which is used to determine the sound absorption coefficient. The analytical viscothermal and scattering solution are discussed in this paper and the modeling approach is validated with experiments using a box with stacked marbles for several angles of incidence.