Traffic noise is still considered as one of the most important sources of discomfort by neighbouring populations of urban and high-trafficked roads. Before the introduction of low noise pavements, acoustical reinforcements of building façades and noise barriers were the only possibilities to control and abate traffic noise. Low noise pavements currently developed or in study should permit to reduce traffic noise by a few decibels. Results have been already found for some pavement families, but for near-field configurations only. According to the results of recent research projects on quiet road traffic in France, Germany and other European countries, the reduction potential of tyre road noise amounts to about 10 dB compared to the present situation. The acoustical properties of road surfaces play the most important role in this context. To actually realise these 10 dB on real roads it is mandatory that a change of
paradigm in road construction will take place. The physical requirements for low
noise road surfaces are well-defined. Therefore, road construction has to
migrate from a trial-and error course of action to controlled, reproducible and
well-defined laying methods. Computational models for the prediction for the
prediction of rolling noise depending on road surface properties can help to
achieve this goal.
The aims of the project are twofold and therefore the structure of the project mainly consists of two work-packages (WP):
- Application of models (SPERoN and HyRoNE) for the prediction of noise emissions due to the interaction of the surface texture with an average car tyre including the laying and characterisation of three experimental surfaces (Part II of this study).
- Simulation of sound propagation of those emissions - which are usually calculated or measured close to the source - over large distances in order to prove the efficiency of the optimisation at a remote receiver and presentation of an internet database with results of those calculations (Part III of the study).
Those two WPs were embraced by the supporting WPs “Consortium management” and “Dissemination”.
WP Validation and application of models for the prediction of rolling noise:
- This WP was intended to demonstrate the change of paradigm in road construction. Low noise road design was realised on real road pavements. The SPERoN and the HyRoNEmodels were applied as adapted tools to design new textures for low noise road surfaces. After a theoretical description of the optimised surface texture issued from the fitting calculations, some samples were built and tested in a first step in laboratory and finally in situ following the current updated experimental techniques.
WP Simulation of sound propagation:
- The main goal of this WP was to examine whether the acoustical qualities of the new texture measured in the near field of the vehicle will persist at large distances, in front of façades, or not. In order to identify this relationship, a method developed in France was used. This method requires to use accurate propagation models which can take into account both ground effects (for heterogeneous ground) and atmospheric effects which can be very important at large distances.
This project aimed at the optimisation of new road pavement concepts with respect to tyre road noise in a wider context of the abatement of road traffic noise. French and German institutes headed by the "Laboratoire Central des Ponts et Chaussées" (LCPC) in Nantes and the "Bundesanstalt für Straßenwesen" (BASt) in Bergisch Gladbach cooperated throughout this project, which finished in March 2009. The theoretical approach is based on the SPERoN ("Statistical Physical Explanation of Rolling Noise") model. In a first stage, the model was validated for different kinds of French road surfaces and compared to the French model “HyRoNE”. The validation showed differences of the computations and the measurements on the test tracks of the LCPC in Nantes of mostly less than 2 dB(A) which shows that SPERoN is a valuable prediction tool.
In a second step, the model was applied as an adapted tool to design and build new textures for low noise road surfaces. Two new approaches were developped together with a third more conventional for comparison. The first approach were glass beads closely packed in a matrix of resin. The second new approach was a very smooth surface with diagonal grooves of randomly varying distance. This structure cannot be built with conventional means. Therefore, a moulding tool was produced to be imprinted into a specially designed asphalt mix surface. A comparison of the sound pressure levels computed with SPERoN due to the real, measured texture data and those ones measured on the test tracks of the INRETS at Satolas shows a nice agreement. However, the textures produced in this project are not yet reproducing the originally intended texture and accordingly the noise levels are still higher than those of the ideal computed texture. Therefore, the development of new production processes and materials which reproduce the ideal texture and their noise level should be the aim of potential follow-up projects.
In parallel to these hybrid approaches which need a physical model of tyre-road contact as input, this project has also permitted to develop a tyre-road contact model which takes into account the viscoelastic behaviour of the tyre tread. In this study, it has been focussed on the contact between a single indenter and a viscoelastic half space with rubber-like properties. This represents a first attempt before modelling the contact between a real road surface and a tyre in the time domain. Such a model could then be used in hybrid as well as in fully physical approaches for a more realistic tyre-road noise prediction. Finally, in a third step, the results issued from SPERoN were used as an input for outdoor sound propagation models developed in France and Germany. To extend the classification from the near to the far field, one of the aims of WP3, 10 typical road configurations were defined to analyse the most common influences on the sound propagation. After computation of all the test cases, we noted that the three different approaches used for calculation (Analytical model, Parabolic Equation and Boundary Element Method) gave, when they could be compared, almost
identical results in terms of excess attenuation relative to free field. Concerning the acoustic characteristics of road pavements, they were only known in terms of pass-by LAmax in the near field of the road (7.5 m) or in terms of CPX index in the tyre near field. That was very interesting but needed to be completed to characterise the pavement acoustic impact with respect to Lden, in the neighbours’ vicinity. For this purpose, a special tool has been built to help the user in identifying the noise impact of road pavements whatever the geometrical configuration of the road and the receiver location. After calculation of a very large number of possible combinations, the common French and German database (DEUFRABASE) was born.
It permits by a few “mouse clicks” to obtain basic information which can be helpful to select the right pavement for a specific situation. Consequently, the whole data obtained after calculation and gathered in a common German- French database (DEUFRABASE) will be used similarly in both countries in the ranking procedure of our road pavements with respect to Lden close to the housing façades located several hundred meters from the road side and generally to predict levels for typical geometrical configurations, road traffic characteristics and meteorological conditions.
Findings of the study are published in detail by a final report (German only) which is available online via https://www.tib.eu/de/suchen/id/TIBKAT%3A612053385/DEUFRAKO-Prediction-and-propagation-of-rolling/?tx_tibsearch_search%5Bsearchspace%5D=tn