# Traffic Noise Reduction Strategy in a Large City and an Analysis of Its Effect
## 2 Materials and Methods
### Geometry of the Noise Barrier  
A literature review on the geometry of noise barriers is shown in Table 2. It was found that, in recent years, L-shaped has been the most common geometry to be used for road noise barriers, while curved cantilever and straight-edge noise barriers were also used by a small number of researchers. To investigate the effectiveness of different noise barriers, five types of noise barriers (see Figure 3) were modeled on both sides of the roads in a small area (0.785 km\({}^{2}\)) of Guangzhou University in Panyu District. The simulation was repeated six times (included the case without a noise barrier), and only the geometry of the noise barrier was changed during each simulation. Figure 4 shows the noise maps of the small  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Frequency (Hz)** & **Rock Wool** & **Slag Wool** & **Polyurethane Foam** & **Superfine Glass Wool** \\ \hline
250 & 0.32 & 0.39 & 0.31 & 0.35 \\
315 & 0.46 & 0.52 & 0.32 & 0.60 \\
400 & 0.74 & 0.71 & 0.46 & 0.80 \\
500 & 0.82 & 0.78 & 0.63 & 0.88 \\
630 & 0.87 & 0.85 & 0.75 & 0.86 \\
800 & 0.92 & 0.88 & 0.92 & 0.85 \\
1000 & 0.93 & 0.91 & 0.96 & 0.86 \\
1250 & 0.93 & 0.92 & 0.93 & 0.65 \\
1600 & 0.92 & 0.90 & 0.87 & 0.88 \\
2000 & 0.94 & 0.92 & 0.70 & 0.91 \\ \hline
**NRC** & **0.75** & **0.75** & **0.65** & **0.75** \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sound absorption coefficients and noise reduction coefficients of different sound absorption materials [38].  
area with and without L-shaped noise barrier during daytime and nighttime. It can be seen that the noise level of this area was significantly reduced after the construction of the L-shaped noise barrier in both daytime and nighttime.  
In addition, areas with sound pressure levels (SPLs) lower than 60 and 50 dBA in the daytime and nighttime, respectively, were analyzed in order to compare the noise attenuation performance of different noise barriers, as shown in Table 3. It was noted that the noise reduction effects of T-shaped, cylindrical, and roof-edge noise barriers were slightly better than those of L-shaped and curved cantilever noise barriers in daytime. However, during the nighttime, the noise reduction effects of L-shaped and curved cantilever noise barriers were slightly better than those of T-shaped, cylindrical, and roof-edge noise barriers. This study focuses more on noise reduction in the nighttime because the noise compliance rate in the nighttime is much lower than that in the daytime (see the previous work of the authors [36] for more details). Thus, L-shaped noise barriers were selected in the present study to mitigate urban traffic noise.  
\begin{table}
\begin{tabular}{c c} \hline
**Literatures** & **Geometry** \\ \hline “Analysis on noise reduction effect of expressway noise barrier and study on its applicability” [39] & L-shape \\ “Design of noise barrier for expressway near urban area” [40] & L-shape \\ “Effect analysis of noise barrier design on traffic noise in residential road” [41] & Curved cantilever \\ “Analysis of the influence of expressway traffic noise on the acoustic environment of residential area” [42] & L-shape \\ “Study on the characteristics of noise barriers with different structures” [43] & L-shape \\ “Study on influence of noise from main road on campus and design of noise barrier” [44] & Straight edge \\ \hline \end{tabular}
\end{table}
Table 2: Literature review of the geometry of noise barriers.  
Figure 3: Geometries of different noise barriers. (**a**) L-shape (left); (**b**) L-shape (right); (**c**) T-shape; (**d**) cylindrical; (**e**) roof edge; (**f**) curved cantilever (left); (**g**) curved cantilever (right).