When a tyre is rolling on an irregularity in a road surface and this irregularity has a horizontal dimension between about 1 cm and several tens of centimetres, vibrations are induced in the tread and the side walls of the tyre. This causes the tread and the side walls to radiate noise, like a drumhead. The greater the vertical dimension of the irregularity, the stronger the phenomenon. Tyre vibrations are generated most effectively when the road surface irregularity has the same horizontal dimension as the tyre-road contact area (which is about 8 cm for passenger cars). One of the most striking examples is the drumming noise of a car travelling on a sett-paved road.

It follows from this that a texture-optimized road surface must exhibit as few coarse irregularities (“megatexture”) as possible if noise emission from tyres is to be minimized.

When a tyre is rolling on a very smooth road surface, air is compressed at the front of the tyre-road contact area and subsequently escapes noisily. At the rear of the contact area air is aspirated noisily. With cars travelling at a reasonable speed on an even road surface, this phenomenon can be observed as a hissing sound. This noise-generating mechanism can be suppressed by providing the road surface with a fine texture (irregularities with horizontal dimensions smaller than 1 cm, i.e., “fine macrotexture”) and/or by making the pavement porous. In the former case air is allowed to escape horizontally between irregularities before it is compressed. In the latter it can find its way out vertically through voids in the pavement.

A road pavement can be given sound-absorbing properties by making it porous. This can be achieved by an appropriate choice of grading for the aggregates in the asphalt mixture. Conventional dense asphalt has a voids ratio of about 3 % and its voids are not interconnected or in contact with air. Porous asphalt (PA) has a voids ratio of about 25 %. The voids are interconnected and in contact with air. They are winding channels originating at the road surface and continuing down to the dense course under the porous course. Such a road pavement will absorb part of the incident sound energy (while converting it to heat) and reflect the other part. A dense pavement will reflect virtually 100 %.

To achieve effective sound absorption, the porous course of the pavement must be at least 4 cm thick and its voids ratio must be at least 20 %.

Porous road pavements abate noise in several ways, among other things by suppressing the horn effect.

A problem with porous pavements is that the voids tend to become clogged with dirt, especially on roads carrying calm (< about 90 km/h) and/or little traffic. This clogging rapidly deteriorates the acoustical quality of the pavement (leading to a loss in noise reduction of 1 dB(A)/year). Roads carrying faster traffic do not have this problem, as they can rely on a phenomenon called “self-cleaning effect”: in rainy weather car tyres force rain water through the voids, which are cleaned in the process. As a result, loss in noise reduction on motorways is only 0.3 – 0.4 dB(A)/year.

Another problem with porous pavements has to do with winter maintenance: when icy or covered with snow, they are less easy to clear – although this is not insurmountable, as demonstrated e.g. in the Netherlands, where porous asphalt (PA) is used systematically on trunk roads.

A pavement with elastic properties in the same order of magnitude as the tyres themselves is extremely effective in preventing tyre vibrations. However, both asphalt and concrete are several orders of magnitude stiffer than car tyres, so that both types must be considered as “hard”. The difference in noise production between the two types of pavement cannot, therefore, be explained by their difference in stiffness. Experiments with rubber particles admixed to asphalt showed only small effects on tyre noise (typically 1 dBA). On the other hand, Japanese and Swedish experiments with poroelastic pavements composed of about 80 % of rubber granules from scrap tyres and about 20 % of polyurethane have shown that elasticity can play a major part in reducing tyre noise. Such pavements have not been used so far in Europe on account of their limited durability, but they are the subject of research (see e.g. the EU funded and BRRC lead European PERSUADE project).

A low-noise pavement can be defined as a pavement with at least one of the following three properties:

• texture;
• absorption;
• elasticity,

optimized so as to reduce tyre noise.

Another definition sometimes used is: a type of pavement which reduces road traffic noise by at least 3 dB(A) with respect to a reference pavement (most often in dense asphalt concrete).

Examples of low-noise pavements (first definition) include:

• porous asphalt (PA);
• two-layer porous asphalt;
• thin surface courses;
• resin-bound (fine-graded) surface dressing;
• fine-graded concrete;
• porous concrete;
• poroelastic pavements (still at the testing stage).