Low noise road surfaces. A state-of-the-art review.

Abstract

This article summarizes present knowledge concerning the design and performance of low noise road surfaces. As a background, noise generation mechanisms and mechanisms causing noise reduction by surface design are explained, as well as the currently best measurement procedures. Guidelines on how to design a surface for low noise characteristics are given, for example texture optimization. Low noise surfaces include types like surface dressings with very small chippings, exposed aggregate cement concrete and thin bituminous surfaces ; also machine-ground cement concrete is mentioned. However, the type with the greatest potential for noise reduction is porous surfaces, both with bituminous and cement binders. It is shown that for these, the product of voids content and thickness is a good descriptor of noise reduction. Thicknesses of 40-50 mm and voids contents of 20 % are the minimum required in order to obtain a reasonable noise reduction, but there is no point in going to extreme thicknesses, say beyond 100 mm. It seems that the best traffic noise reduction attainable with such surfaces is around 8 dB (A) in new condition, using small chippings in the top layer. A surface named Twinlay has been especially optimized to have a long acoustical lifetime for urban applications at speeds around 50 km/h, otherwise clogging of porosities by dirt is a serious problem in urban areas. In most cases, regular cleaning operations are required. In applications where speeds are 90-130 km/h, there is a significant self-cleaning of porous surfaces and the acoustic lifetime can be acceptable even without cleaning. The article reviews the experience with low noise surfaces in various countries and also presents some futuristic designs, like an acoustical multi-resonator pavement and pavements made mainly of rubber particles, the latter having a potential of 10 dB (A) of noise reduction. There is no doubt that road surfaces will be used frequently to reduce traffic noise and can give substantial effects. One shall, however, observe that this may mean serious economic trade-offs and some other problems, as well as the long-term noise reduction efficiency still is poor for most designs.