Experimental validation of a modelling approach to predict sound absorption for porous asphalt roads

Tyre-road noise is a serious problem. One of the parameters to reduce the noise radiation is the sound absorption coefficient of the road, which is (partly) determined by properties such as the porosity of the road, the size and shape of the stones. In this study, a novel `hybrid' (analytical/numerical) model is used to predict the three-dimensional sound field in and above porous structures -- modelled as rigid granular structures -- and to predict the absorption coefficient for oblique incident sound waves. In this modelling approach, the air inside the pores of the structure is considered, including viscothermal effects in the pores and scattering of the sound waves due to the, assumed, rigid granular structure, representing the pavement. In this paper, the validation of the hybrid (analytical/numerical) modelling approach with impedance tube measurements for normal incident sound waves is described for two three-dimensional configurations. It is concluded that this novel hybrid modelling approach has been validated with good results for normal incident plane waves.