Sound absorption of porous structures: a design tool for road surfaces

The sound radiation caused by tyre/road noise can be reduced significantly by the use of porous road surfaces. A hybrid analytical/numerical modelling approach has been developed to predict the sound absorption coefficient of such road surfaces. Furthermore, the modelling approach has been used as a design tool to optimise the sound absorption of porous road surfaces in the design phase. Using this design tool, two porous surfaces have been developed and constructed at a special test area. These road surfaces have been measured extensively for both sound absorption and for noise radiation in combination with different tyres. This research is carried out within the project `Silent and Safe Road Traffic'. The goal of this project was to find methods and measures to reduce the noise from tyre/road interaction while ensuring (wet) grip. The developed hybrid analytical/numerical modelling approach is based on the combination of the solutions of two subsystems: an analytically described background sound field and a numerically solved scattered sound field describing the scattering of the sound waves on the (assumed rigid) porous structure. Furthermore, the sound absorption caused by viscothermal effects inside the air-filled pores is included analytically in the modelling approach. Also, the sound absorption coefficient for oblique incidence can be predicted using this modelling approach. This is an important property when considering tyre/road noise, since most traffic noise is received at oblique incidence. Therefore, the sound absorption for oblique incidence should be considered when predicting the noise reduction by porous road surfaces. The main advantage of the developed hybrid modelling approach compared to a full numerical model is the low computation time, because (1) no mesh refinement is needed for the mesh of the structure inside the air-filled pores, since the viscothermal effects inside the pores are included analytically, and (2) the air domain surrounding the structure can be relatively small, since the scattering problem is localised around the porous structure and the background sound field is included analytically. In addition, the developed modelling approach can be applied to predict the sound absorption for any three-dimensional porous structure. The work presented here focuses on structures of tube resonators and on granular structures. Both types of porous structures are used for the validation of the modelling approach. For normal incidence, the modelling approach is validated using the impedance tube technique. The correlation between the measured sound absorption coefficient and the predicted sound absorption coefficient was extremely good for both the tube resonators and structures of stacked glass marbles. To validate the modelling approach for oblique incidence, a large sound hard box filled with glass marbles was measured using a small cubic microphone array. This validation was more complex, since the measurement technique introduced various uncertainties. However, the model results and measurement results showed good correlation. Furthermore, the developed modelling approach was adjusted in such a way that structures made from sound absorbing materials can be modelled as well. To demonstrate this, a structure of coupled tube resonators has been designed and manufactured with the 3D printing technique, an upcoming technique suitable for manufacturing complex sound reducing panels. The measured sound absorption coefficient of this sample showed an influence of the material properties on the sound absorption coefficient, which could be predicted fairly well with the adjusted modelling approach