Measurements of tyre/road noise and of acoustical properties of porous road surfaces

Abstract

The three main noise sources in modern cars and trucks are tyre/road interaction, the power plant, and the wind turbulence. Tyre/road interaction noise is receiving increasing attention. The main tyre/road interaction mechanisms consist of: 1) impacts between the tyre tread and the road, which cause radial, tangential, and sidewall tyre tread and carcass vibration and consequent noise radiation, and 2) air displacement mechanisms caused by the tyre, the major one probably being the so-called "air pumping" in, or between, the tyre tread and the road surface. In this paper, the results of tyre/road interaction noise measured on different types of road pavement surfaces are presented. The results were obtained by means of the close proximity method. The sound absorption coefficients of dense and porous road surfaces were measured in the Sound and Vibration Research Laboratory at Auburn University 1) using core samples with 102 and 153 mm diameter impedance tubes and 2) with the same two impedance tubes mounted vertically on pavement slabs with surfaces similar to those of the core samples. In addition, the sound absorption coefficient of road surfaces of similar type to those of the cores and slabs was measured in situ, also with the tubes mounted vertically. The peak sound absorption coefficient measured of the fine and coarse mix aggregate porous surfaces suggests that the first peak frequency and peak absorption coefficient magnitude is only slightly different for the two types of porous surfaces. Since the fine mix aggregate porous surface is smoother, it is preferred since it should result in less tyre tread impact noise and thus lower overall tyre noise than the coarse aggregate surface. A porous surface of between 38 and 50 mm thickness is recommended for the type of porous surface examined, if a peak absorption frequency of about 1000 Hz is desired. Such a surface would be most effective at reducing interstate highway noise of automobiles. In addition, porous road pavement surfaces have the further advantage that they drain water well and reduce the splash up behind vehicles during heavy rainfalls.