# Study on Noise Reduction with Paving Different Low Noise Pavement Materials
## 1 Introduction  
The road traffic noise is significantly influenced by the tire/road noise generated by the vehicle driving on the pavement, especially when the vehicle is operating in good condition. This is because the tire/road noise becomes the dominant noise source of the road traffic noise [1, 2]. When studying the amount of noise reduction using a particular low noise pavement material, we often need to measure the road traffic noise level. When different lanes of the highway set up different structures of low noise pavement materials, it is necessary to study the impact of each pavement on the road traffic noise, and thus some guidelines can be given on choosing pavement materials. Generally, we can measure the roadside sound pressure level and/or the tire/road noise to study the effectiveness of pavement materials on reducing the road traffic noise [3]. It is the most popular way to measure the noise pressure level on the side of the road when studying the noise reduction performance of low noise pavements. This is often measured at 7.5 m or 15 m away from the roadside. In general, a measurement microphone is required to quantitatively measure the sound pressure level, while the speed, the model, and the weather conditions are also needed to be written down and recorded. There are three main methods to measure roadside noise. With a low traffic flow, statistical pass-by method (SPB) [4, 5] is often used to measure the maximum sound levels of hundreds of individual vehicles when they are passing. In this case, the measurement microphone is placed at a fixed position on the side of the road. Note that hundreds of measurements are necessary so that we can average these sound pressure levels of light vehicles, heavy vehicles, and other vehicle types to get a reliable measurement value atsome standard speed. Control pass-by method (CPB) is similar to SPB, where the only difference is that a specific combination of tires and vehicles is used for CPB. With SPB and CPB, we can compare pavements at different road sections. Because the result is only an \"average\" value, some errors may be inevitable in evaluating different road surfaces due to that vehicles may be different each time when measuring with SPB and the coupling between specific tires and pavement materials may also be different each time when using CPB [6; 7; 8]. Different from SPB and CPB, the third method averages the sound pressure level of a specific vehicle, which is known as the time-average method and uses the continuous flow traffic time-integrated model (CTIM) [9]. In this method, the measurement microphone is set to measure all traffic noise within a fixed period of time, which usually ranges from 5 to 30 min. Note that the recording traffic flow, the vehicle type, and its speed are also needed to be recorded. All the above mentioned methods have to consider the surrounding reflectors and/or other interference factors. As an example, the weather conditions should be written down and recorded, especially when the wind exists that may have a significant influence on the measurement results. Moreover, we also need to eliminate some other interferences, such as aircraft noise, dog barking, and so on. By doing so, the impact of these interferences can be reduced dramatically. Compared with measuring traffic noise on the side of the road, if one can measure tire/road noise close to the interaction between the tire and the pavement, then the noise reduction performance of low noise pavement can be studied in a better way [7]. Nowadays, there are two often-used methods to measure this tire/road noise, where one is close-proximity (CPX) method [10] and the other is on board sound intensity (OBSI) method [11; 12]. CPX is more popular in Europe and has been released as ISO standard 11819-2, while OBSI is developed by General Motors, and it is popular in the United States. Note that both CPX and OBSI place microphones close to the interaction of the tire and pavement to record the measurements of transducers as the vehicle running [13].