Experimental study on spatial distribution law of traffic noise on urban viaduct

Traffic noise is the main environmental pollution of the elevated line. In order to prevent and control this noise, it is necessary to understand the spatial distribution of the noise. In this paper, the noise monitoring and data fitting of representative points near the Ningbo viaduct are carried out by means of the field test. By setting measuring points at different distances and heights perpendicular to and parallel to the elevated line, the spatial distribution law of noise near the elevated line is studied. The results show that: (1) The noise in the transverse direction of the bridge decreases in a logarithmic curve with the increase of the vertical distance between the measuring point and the viaduct; (2) The noise along the bridge decreases in a logarithmic curve with the increase of the vertical distance between the measuring point and the intersection; (3) The noise in the elevated direction first increases with the rise of the floor, reaching the maximum on floors 10-12, and the sound pressure on floors 12-20 is basically stable.


Introduction
Urban viaducts, a crucial component of urban infrastructure, improve the effectiveness of urban medium-and long-distance transportation.However, a significant amount of monitoring information reveals that traffic noise pollution is more prominent as a result of the large capacity of the viaduct and the fast-moving velocity of vehicles [1][2][3] .In some cities with dense viaducts, even more than half of the population is exposed to road noise that exceeds environmental standards [4] , making noise pollution the fourth most dangerous kind of pollution in cities, after solid waste, water, and air pollution.The detriment caused by noise pollution is extensive and diverse [5][6][7] .Therefore, the topic of considerable curiosity in the field of environmental acoustics is how to properly quantify the traffic noise impact of urban viaducts and provide appropriate pollution control methods [8] .
In order to investigate the noise impact aspects, many researchers are currently employing the Experiment Approach to test and record representative points of urban viaducts at various times while simultaneously monitoring the traffic flow, vehicle type, and other data [9][10] .However, there are few studies on the spatial distribution law of traffic noise on viaducts.
This paper selects the typical viaducts in Ningbo -Airport Viaduct and Beihuan Viaduct as the research objects.and "Sound Environment Quality Standards" (GB3096-2008) [12] , the AWA6228 multifunctional sound level meter was used to test the daytime equivalent continuous sound pressure level Ld, nighttime equivalent continuous sound pressure level Ln, and hourly equivalent continuous sound level Leq.

Measurement point program
In order to study the noise distribution pattern in the transverse bridge direction, underneath the bridge, and in the height directions of the viaduct, we proposed the following measurement point scheme.
(1) To explore the relationship between traffic noise and its vertical distance to the elevated road line, four measurement points with different distances were selected in the vertical direction of the viaduct, and the measurement points were 1 m, 11 m, 21 m and 31 m from the viaduct ground road boundary line, as shown in Figure 1.(2) To analyze the distribution pattern of traffic noise in the horizontal direction of the viaduct, four measurement points were selected in the horizontal direction of the viaduct, and the measurement points were 10 meters, 50 meters, 100 meters, and 150 meters from the left side of the road boundary line intersecting the ground, as shown in Figure 2. (3) To assess the distribution pattern of traffic noise in the height direction of the viaduct, three neighborhoods, A, B, and C, were selected adjacent to the airport road viaduct for actual measurements on different floors.

Noise distribution in the vertical direction of the viaduct
In the vertical direction of the viaduct, the noise distribution is related to the vertical distance from the measurement point to the viaduct line.Previous researchers considered the viaduct as a line sound source and derived the sound pressure attenuation equation as follows: where L(r) is the sound pressure at the measurement point at a distance r to the line sound source, Lw is the line sound source sound pressure, and r is the distance of the measurement point from the sound source.
In this paper, a logarithmic curve is used to fit the test results, and the relationship between the measured sound pressure and the fitted curve is shown in Figure 3.According to the fitting curve in Figure 3, the measuring site's sound pressure level distribution regulation is found to be most consistent with the equation for sound pressure level attenuation when the outer 1.5 meters of the highway is selected as the line source location.The fitting point is illustrated in Figure 4. Finally, the sound source sound pressure attenuation Equation ( 1) is corrected and the noise distribution equation in the vertical direction of the viaduct is obtained.
where 1 r is the distance from the sound pressure measurement point to the outer edge of the motorway under the viaduct in m; w1 L is the sound pressure level at 1.5 m from the edge of the motorway under the viaduct, and 1 () Lr is the sound pressure level at the 1 r point in dB.

Noise distribution in the horizontal direction of the viaduct
In the horizontal direction of the viaduct, the change of sound pressure is related to the intersection of the viaduct and the ordinary road (or other viaducts), so it can be assumed that the sound pressure adjacent to the viaduct is superimposed by the noise of the viaduct and the noise of the ordinary road with which it intersects.Viaduct noise can be represented by (2), and the ordinary roads intersecting with it can be similarly assumed that it will diminish in the form of a logarithmic curve, but when the distance to the intersection is far enough (e.g., 100 m), the sound pressure can be considered to remain essentially constant.Therefore, the equation for the change in sound pressure caused by ordinary roads is as follows: where 2 r is the distance from the sound pressure measurement point to the edge of the ordinary road lane intersecting with the viaduct, in m; w2 L is the sound pressure level at 1.5 m from the edge of the ordinary road motorway; 2 () Lr is the sound pressure level at 2 r , in dB.The total sound pressure at the measurement point is the superposition of the sound pressure caused by ordinary roads and elevated lines, and the superposition diagram is shown in Figure 5.
where r1 is the distance from the measurement point to the edge of the motorway under the viaduct; r2 is the distance from the measurement point to the edge of the motorway of the ordinary road intersecting with the viaduct, the unit is m; Lw1 is the sound pressure level at 1.5 m from the edge of the motorway under the viaduct; Lw2 is the sound pressure level at 1.5 m from the edge of the motorway on the edge of the ordinary road; L is the sound pressure level at the measurement point after superposition, the unit is dB.The relationship between the measured sound pressure and the fitted curve (obtained from Equation ( 4)) is shown in Figure 6.

Figure 1 .
Figure 1.Noise measurement point arrangement in the Vertical direction of the viaduct.

Figure 2 .
Figure 2. Noise measurement point arrangement in the horizontal direction of the viaduct.

Figure 3 .
Figure 3. Noise distribution curve in the vertical direction of the viaduct.

2 Figure 4 .
Figure 4. Sound pressure fitting points in the vertical direction of the viaduct.

Figure 5 .
Figure 5. Sound pressure fitting points in the horizontal direction of the viaduct.

Figure 6 .
Figure 6.Noise distribution curve in the horizontal direction of the viaduct.

3. 3 .
Noise distribution in the height direction of the viaduct Figure7illustrates the results of the noise testing that was done on different floors of the A, B, and C