The beneficial effects of porous surfaces on tyre/road noise are twofold:
- The open structure of the surface reduces the compression and expansion of air in
the tyre tread profiles;
- the acoustic absorption suppresses mechanical and aerodynamic noise
generated by the rolling tyre on the road.
The acoustic absorption effect is not restricted to tyre/road noise only but is also effective in reducing mechanical noise, radiated from the underside of the vehicle where the oil pan and the gearbox housing form the main sources of engine noise. This was found to be the case not only by passenger cars but also by heavy trucks.
The acoustic effect of most existing porous road surfaces, however, is limited by the high macro/mega texture level of these surfaces, that causes extensive vibrations of the mechanical system of the tyre. Therefore progress can only be made by optimizing all relevant properties of the porous layer and carefully adjusting noise suppressing mechanisms to noise generating characteristics.
In Fig 1 . a schematic overview is displayed of the acoustical processes relevant to the
generation and suppression of rolling noise on absorbing surfaces and the surface properties that influence these processes.

The grey blocks represent the major noise generating processes. Each of these processes exhibit specific frequency and speed characteristics. For instance the aerodynamical and profile resonance effects emit noise at higher frequencies and with a higher speed index than the texture induced vibration component. The surface properties, relevant for the excitation of the different processes are given at the left column. However, much more study is needed to exactly define these surface related parameters. The indications are therefore only tentative.
In this model, the acoustic impedance of the surface, primarily acts as an absorber of noise.
An important side effect is the suppression of aerodynamic noise because if the low flow
resistance in the surface. Some slight effects may be expected from the change in boundary conditions in the contact patch.
The effect of the acoustic absorption is modeled by the total noise emission multiplied by the acoustic transmission of the surface. The development of a noise reducing porous surface must therefore be focused on the following issues:
1 . optimization of the surface characteristics with respect to the noise generating mechanisms;
2. tuning the acoustic absorption characteristic to the resulting spectral distribution of acoustic energy.
Since the noise components differ with respect to vehicle type, frequency content and
speed index, an optimal result can not be obtained in all conditions but will be restricted to a certain speed range and a given mix of vehicle types.


Fundamental study of the acoustic properties of porous road surfaces of different types

was performed under simulated trafficked conditions on our test field on the airfield of Welschap. Although this study was limited to small vehicles and a small set of truck tires it already demonstrated the great potentials of porous surfaces to reduce traffic noise.
These results were corroborated by a full scale test on a motorway where the most
interesting types from the Welschap study were investigated under normal trafficked conditions consisting of cars, light and heavy trucks. The relevant parameters of the tested surfaces are given in table I. Adjacent to the test sections a reference surface was lain.

Table 1 Overview of the test sections on the Motorway A-17 and their acoustic absorbtion characteristics in terms of the maximal absorption and its frequency .

surface typeStone size gradingthickness(mm)frequency of maximum absorbance in Hzmax. absorbance %
dense 0-16---
porous 6-16 55 840 89
porous 4-8 60 810 95
double layer
4-8 (top) 25 (top)
40 (bottom

The noise emissions were measured according to the Statistical Pass-by method (ISO 11819-1). The results were interpreted in terms of a reduction relative to the adjacent dense reference surface (see Fig. 2).

This test clearly shows the great effect that porous surfaces have on the noise emission of road traffic. This effect is not limited to passenger cars but, as can be seen from the graph, also to heavy trucks. The maximal reduction of 6 dB re. DAC 0/16 is comparable to a reduction of about 10 dB re. To transversely brushed concrete.


The most interesting surface in this test is the double layer type with a fine graded top layer. (4/8-11/16). This type not only shows good reduction figures at high speed for both light and heavy vehicles, but also at low speed very satisfying reduction figures of 4 dB re. to dense surfaces were found. Even under conditions of full accelerating heavy trucks (ISO 362) significant reductions of 2 to 4 dB were observed (it was used as official test track by some manufacturers before ISO 10844 came into service).
These satisfying results at low speed were obtained by combining a low textured top layer with an absorption characteristic that was frequency tuned to the texture induced vibration effect. The importance of these two aspects is also demonstrated in fig. 3. where the reduction for two speed ranges and for two grading of the stone size is given as a function of the frequency of maximal absorption of the road surface. As can be seen the stone grading, and thus surface texture, is an important parameter. At low speed the reduction is furthermore influenced very strongly by the frequency of the absorption peak of the road surface.

With kind permission of M+P Consulting Engineers