## 3 Full-Scale Measurement in Real Urban Streets for Model Validation
### Estimation of Acoustic Parameters of Building Facades  
The scattering coefficient for each building facade is needed to validate the calculation model. However, the measurement of the scattering coefficient of large building facades is difficult. The most common approach in previous studies is to use a hypothetical parameter that varies from 0 to 1 [13]. In this case, the comparison between simulations and measurements can only prove that the measured data is in the range of the simulation result instead of analyzing the detailed accuracy. In this study, the laboratory-measured data from previous studies [39; 40] were used to estimate the scattering coefficients of the actual building facades. A series of laboratory measurements focused on the scattering characteristics of the plate partially covered by geometric diffusers have been carried out, and the detailed scattering data of the samples with different shapes, sizes, and areas were reported [39; 40; 41; 42]. It agreed with the result in reference [32] that the diffuse characteristics were mainly determined by the height (h) and the coverage density (CD) of the diffuse structures on the plate, as shown in Figure 5a. The frequency characteristics were mainly determined by the height of the structure. As shown in Figure 5a, in the laboratory experiment, the scattering coefficients increased in accordance with an increase in the frequency, and it reached a peak when h/\(\lambda\) (the height to the wavelength ratio) was approximately 0.1. When h/\(\lambda\) > 0.1, the curve was mainly constant with small fluctuations. Within the effective frequency range, the scattering coefficient was mainly determined by the coverage density, as shown in Figure 5b. With the increase in CD from 0, the scattering coefficient increased rapidly, and the growth rate gradually decreased. A decrease in the scattering coefficient was found when CD was very large (CD > 0.6). Therefore, the relationship between the coverage density and the scattering coefficient can be fitted as quadratic regression curves, as shown in Figure 5b. Three curves were established for three typical geometric diffusers respectively, including hemisphere (data from [39]), groove (data from [39]), and rectangular structures (data from [40]). As shown in Figure 5, the difference between these curves is fairly small (<0.2). Measured scattering coefficients of more complex structures [41] are also shown in Figure 5b, including a rectangular shape with flutters, a trapezoid shape, and triangular shape diffusers. As shown in Figure 5b, the scattering coefficients of these structures are close to those of the basic structures. They also agree with the result in Ref. [32] and indicates the feasibility of the estimated scattering coefficient of the actual building facade. As shown in Table 1, by analyzing the morphology of the building facades, an overall coverage density of irregularities could be easily obtained, which could be further applied to the regression equations in Figure 5b to estimate the scattering coefficient. The estimated scattering coefficient in five measured streets varies from 0.07 to 0.74. The scattering coefficient of the actual building facade has been investigated in Ref. [13] by comparison between the simulation with the measurements. The scattering coefficient of build facades defined as a periodic arrangement of windows, which is very similar to Facade A of Street 4 in this study, is suggested to be 0.4-0.7. As shown in Table 1, the scattering coefficient of Facade A in Street 4 is estimated at 0.52-0.71, which is quite similar to the result in reference [13]. It should be noted that there are large structures in the actual building facade, i.e., sizable rectangles on Street 3-FA. When the dimensions of the plane surface are significantly larger than the wavelength, it functions similarly to a specular reflector rather than a diffuser [43].