Traffic noise is becoming increasingly important. In fact, road traffic remains the biggest source of noise pollution in Europe, and according to the World Health Organization (WHO), noise from road traffic alone is the second most harmful environmental stressor in Europe, behind air pollution. Not only that, but more and more people are now living in densely populated areas and commuting to their place of work by car. The main sound sources are engine noise, tyre/road interaction and wind noise caused by the shape of the vehicle.

For cars driving at a constant cruising speed, tyre/road contact is the most important sound source for velocities above 25km/h. An example of the sound radiation caused by tyre/road noise is shown in Figure 1. Here, the sound level measured at positions close to the tyre for a car driving at a constant cruising velocity of 80km/h is shown. The maximum sound level is seen around 900Hz. Typically, the highest sound levels for tyre/road noise are seen in the frequency range between 800 and 2,000Hz.

Extensive research has been carried out to find the influence of tyre properties on the radiated noise, using simple and more complex models. For example, there are different models focusing on the tyre tread design, on the stiffness properties of the tyre, or on the tyre resonance frequencies.

The radiated noise also depends on the road surface and can be minimised by designing a silent road; for example, one with a smooth or elastic road surface, such that excitations are minimised. Secondly, the radiated noise can be (partly) absorbed, for example, by a porous asphalt road surface. Furthermore, the combination of elastic and porous road surfaces, the so-called poroelastic road surfaces, is also possible – though many practical problems remain.

To be able to predict the radiated noise due to tyre/road contact already in the design phase, different tyre models and sound absorption models of the road are developed. A limitation in the sound absorption models is that it is assumed that the sound waves are directed perpendicular to the road surface, while in reality, oblique incident sound waves occur more often (see fig. 2). Therefore, the sound absorption for oblique incidence should be considered when predicting the noise reduction by porous road surfaces. Further, the latter models cannot really be applied in the design phase of a road surface to predict the sound absorption because of their complexity and long computation times.

New sound absorption model

At the University of Twente a hybrid analytical/numerical modelling approach has been developed to predict the sound absorption coefficients of (porous) road surfaces for normal and oblique incident sound waves.

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 (see fig. 2).

The main advantage of the developed hybrid modelling approach compared to a full numerical model is the low computation time:

  1. No mesh refinement is needed for the calculation grid of the structure inside the air-filled pores, since the viscothermal effects inside the pores are included analytically.
  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.

Validation

The developed modelling approach can be applied to predict the sound absorption for any three-dimensional porous structure, e.g. structures of tube resonators and 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.

“the modelling approach has been used as a design tool to optimise the sound absorption of porous road surfaces in the design phase”

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 (see fig. 3). This validation was more complex, since the measurement technique introduced various uncertainties. However, the model results and measurement results showed good correlation.

Application in design environment

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 model was used as a design tool to find road parameters which should optimise the sound absorption coefficient of porous road surfaces for the reduction of tyre/road noise for oblique incidence.

The optimisation is focused on the 1,000 Hz octave band, since this octave band includes the most important frequencies in tyre/road noise. As a design tool, the model results only give guidelines, since no model has been made with a structure exactly resembling a potential asphalt mixture. The model results are combined with design criteria based on measurements and criteria found in the literature.

This resulted in the following road design criteria:

  • A high porosity
  • Small stone sizes
  • A substantial layer thickness
  • A smooth road surface

Furthermore, some construction constraints had to be taken into account firstly the porosity of the road surface is limited to 25% and secondly the layer thickness of a single layer of asphalt concrete is limited, depending on the porosity and stone size.

Therefore, the choice was made to develop and manufacture two prototype roads:

  • A single layered road with the maximum layer thickness
  • A double layered road to achieve a larger layer thickness

Finally, the stone sizes (and material) used for these prototype roads depend on the availability of the material and the requirements for (wet) grip.

The first prototype road surface is a single layered porous asphalt concrete, called OPA 6 (track 8), with stone sizes ranging from 2mm to 6mm and porosity of about 25%. The other road surface is a double layered porous asphalt concrete, called Twinlay (track 7), with stone sizes between 2mm and 4mm and a similar high porosity to that of track 8.

Results

The sound absorption coefficient of the test tracks for normal incidence is measured in-situ with the impedance tube technique.

The tube was placed perpendicular to the road surface using a flange, as shown in Figure 4. For each road surface, the sound absorption coefficient was measured in ten equally spaced locations in a longitudinal direction of the test track. The results for the two prototype tracks and track 6, a reference road surface, are given in Figure 5. Both prototype tracks have a high sound absorption coefficient. The results for track 7 show that the sound absorption coefficient in the 500Hz octave band is slightly larger, which can be explained by the greater layer thickness of this track.

The results in figure 6 and figure 7 clearly show that the sound absorption coefficient depends both on the frequency and the angle of incidence. The measurement results show that the peak in the sound absorption coefficient shifts to higher frequencies for a larger angle of incidence. For Ɵ = 0 and Ɵ = 30, the maximum sound absorption coefficient is in the 1000 Hz octave band and for Ɵ = 60 the maximum value is shifted to the 2000 Hz octave band.

Conclusion

While the developed modelling approach is validated for only for simple packings of stacked spheres, the predicted trends for certain road parameters are confirmed by the measurements. Therefore, the approach can be used as a tool to optimise the sound absorption coefficient of porous road surfaces based on trends, but cannot predict the absolute value of the sound absorption coefficient for new road surfaces. Moreover, with some additional research, the developed approach can be combined with existing Tyre Road Noise models with which the sound radiation of a rolling tyre can be predicted. Combining this model with the hybrid modelling approach is expected to improve the predictions of the sound radiation of a rolling tyre, since the sound absorption coefficient predicted with this modelling approach depends on both the frequency and the angle of incidence.