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.

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The research on low noise pavement in Japan started at the end of 80's. It was about ten years later than that of European countries. This paper historically and categorically included the researches concerning low noise pavement. When considering dense asphalt pavement as control pavement, the drainage asphalt pavement and porous elastic road surface are identified as low noise pavement. The author summarized the results of all the papers and acquired the following conclusions:(1) The result of the rough estimation through all the papers concerning initial noise reduction effect and the effect degradation indicates the amount of the performance of drainage asphalt pavement might be 3-5 dB (A) and the annual degradation is about 1 dB (A)/year.(2) The summary of the paper concerning the performance recovery gave us the reasonable explanation that the cleaning machine is only available to recovering the water drain effect but not to restore the noise reduction performance.(3) Porous elastic road surface showed the great improvement in the noise reduction performance. It also lets us expect that porous elastic road surface will give the fundamental solution to the highway noise problem in urban areas of Japan.

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The aim of this study is to clarify the sound absorption mechanism of porous asphalt pavement by comparing it with those of the glass wool and urethane foam etc., which are well-known porous sound absorbing materials. In order to elucidate the sound absorption mechanism, we measured the propagation constant and characteristic impedance of a sound wave traveling inside the material under the plane wave incident condition using an acoustic tube, and calculated the behavior of the sound waves in the material based on the measurement results. We concluded that the sound absorption of the porous asphalt pavement is caused by the following mechanism. Because the sound waves in the porous asphalt pavement material generally used in Japan exhibit less attenuation than those in glass wool or flexible urethane foam, the multi-reflected waves remain inside the material, and interfere with the wave reflected from the front surface of the material. In particular, in the frequency range below 1 kHz where the sound waves exhibit less attenuation inside the material, the sound absorption coefficient peaks at a frequency at which the antiphase condition is satisfied between the multi-reflective waves in the material and the sound wave reflected from the front surface of the material. Furthermore, the frequency range above 1 kHz is characterized in that since the attenuation gradually increases while the interference decreases, the sound absorption coefficient of the porous asphalt pavement is determined by its surface reflective wave.

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The drainage asphalt pavement contains consecutive pores and has the noise reduction effect. We have analyzed the factors of the effect carrying out the site measurements and the experiments related to the drainage pavement. As a consequence, we have derived the sound power level equations of vehicles, which show the noise reduction effect of the drainage pavement at a sound source. In fact, they give us smaller values of sound power level compared to those of the dense graded asphalt pavement. On the other hand we have obtained the flow resistivity of 1, 250 kPa s/m² for the drainage pavement as the noise reduction effect during propagation. This is smaller than that of the dense graded pavement, 20, 000 kPa s/m², and generates the excess attenuation in the course of sound propagation. In view of the aforementioned, we have proposed the calculation method of the sound pressure level in the roadside area of drainage pavement on a basis of the A method of ASJ Model-1993, taking into consideration these factors at the sound source and during the sound propagation related to the noise reduction effect of the drainage pavement. The sound pressure levels measured along the drainage pavement have verified this method.

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Acoustic properties of drainage asphalt are investigated to simulate the noise propagation of road traffic noise. Normalized acoustic impedance, porosity and flow resistivity are measured by an ordinary method with a small sample of maximum size of aggregate 13 mm and thickness of 45 mm. By referring to Allard's impedance model for rigid frame porous material and curve fitting technique, tortuosity of the sample is deduced. It is shown that this model gives a good fitting to normalized acoustic impedance measured and applicable to simulate the noise propagation on the drainage asphalt of road surface by a boundary element method. Next, effective flow resistivity is deduced approximately by a short range sound propagation over a drainage asphalt surface of a highway in operation. It is shown an empirical one-parameter impedance model is applicable to the prediction of noise propagation on the drainage asphalt surface by using an effective flow resistivity. Normal and random incident absorption coefficients are measured and presented for data supplements.

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This paper describes measurements of basic acoustical properties of porous pavement such as sound propagation constant, acoustic characteristic impedance and acoustic impedance. As their application, it also describes estimation of their effect on road traffic noise reduction. In porous asphalt, propagation attenuation is less than typical fibrous porous material of glass wool or rock wool, and propagation speed in it is 1/2-1/3 as slow as that in the air. From the point of acoustical characteristic impedance, porous asphalt is about ten times acoustically harder than that in the air for frequencies below about 1.5 kHz, and it has high absorption peak at a frequency below 1 kHz. Noise reduction effects of porous asphalt can be estimated by using these measured properties. It was found in this study that porous pavement causes some excess attenuation of noise. It was also found that amount of A-weighted noise reduction due to tire-road interaction noise is about 2-3 dB. Total noise reduction effect of well-constructed porous pavement is probably larger than 4 dB even at a short distance.

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Regarding the reduction of road vehicle noise by drainage asphalt pavement, two kinds of experimental investigations were performed. Firstly, sound propagation was observed on two types of drainage asphalt pavements and a dense asphalt pavement. In this experiment, an omnidirectional sound source was located under the body of a passenger car and a heavy truck. As a result, it was found that the excess noise reduction on drainage asphalt pavement can be attributed to the sound absorption during multiple reflection uncle': the vehicle body and this effect depends on the vehicle type. Secondly, pass-by tests were performed on the three kinds of pavements using the same test vehicles. In this experiment, tire/road noise was measured on each pavement and by comparing the results, the reduction of tire/road noise due to the porosity of drainage asphalt pavement was estimated. Based on the results of these investigations, a calculation model for the prediction of vehicle noise propagation on drainage asphalt pavement has been proposed.

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