## 4 Comparison between Calculation Models and Full-Scale Measurements
### Reverberation Time along the Street  
The mean absolute differences (MAD) between the measurements and simulations are calculated to analyze the model accuracy, as:  
\[MAD=\frac{\sum_{1}^{n}\lvert RT_{simulated,n}-RT_{mussured,n}\rvert}{n}. \tag{9}\]  
Figure 6: Predicted and measured sound pressure level (SPL) attenuation of the five actual street canyons. S is the scattering coefficient of the building facade. (**a**)–(**e**) are results in Streets 1–5. Different from SPL, RT is mainly determined by the late reflected sound. Therefore, the reflection modeling scheme shows significant influences on RT simulations, which lead to different accuracies with these three calculation models. Figure 7 shows the RT comparison between calculation models and measurements, where IMCRI shows superior performance compared to the other two models. Good agreement between IMCRI and measured data are found in all five streets. With the estimated scattering coefficient, \(\mathrm{S_{r}}\), the MAD of IMCRI for all 50 receiver positions in five streets are 0.22 and 0.26 s for the 500 and 1000 Hz octave bands, respectively. Figure 7: Predicted and measured reverberation time (RT) (\(\mathrm{T_{30}}\)) of the five actual street canyons. \(\mathrm{S}\) is the scattering coefficient of the building facade. MAD is the mean absolute differences between the measurements and IMCRI. As shown in Figure 7, the accuracy of IMCRI varies in the five streets. In Street 1, Street 2, Street 4 and Street 5, the MADs of IMCRI with S\({}_{\text{r}}\) are smaller than 0.2 s in the 500 and 1000 Hz octave bands. A relatively poor performance of IMCRI is found for Street 3, with a MAD larger than 0.6 s. It is probably caused by the overestimation of the scattering coefficient. As discussed in Section 3.3, all the laboratory-measured data in Figure 5b used to estimate the scattering coefficient in the actual streets were measured using limited implementations: 0.1 \(<\) h/\(\lambda\) \(<\) 0.4. However, in Street 3, diffuse structures with depths greater than 1 m can be found, which results in a significantly larger h/\(\lambda\) in the considered frequency range. As reported by previous studies, the scattering coefficient decreases with the increase of h/\(\lambda\) when h/\(\lambda\) is large [41]. With the scattering coefficient of 0.3 S\({}_{\text{r}}\), good agreement between IMCRI and measurements can be achieved (MAD \(<\) 0.2). Supplementary measurements of the scattering coefficient with a larger range of h/\(\lambda\) can be carried out to solve the problem. The depth of measured structures in Figure 5 varies from 1.5 to 4.7 cm. In reference [41], the scattering coefficients of structures with a depth of 8 cm was measured within the frequency range of 100 to 5000 Hz. With such implementations, the scattering coefficient when h/\(\lambda\) \(>\) 1 can be investigated. Although the performances of IMCRI with S\({}_{\text{g}}\) and S\({}_{\text{r}}\) are similar, IMCRI with S\({}_{\text{r}}\) is more accurate than IMCRI with S\({}_{\text{g}}\) in most measured streets. In Street 1, Street 2 and Street 3, the MADs of IMCRI with S\({}_{\text{g}}\) are approximately 0.2 to 0.4 s larger than that of IMCRI with Sr. The possible reason for this phenomenon is that S\({}_{\text{r}}\) is more similar to the scattering characteristic of the actual building facades, because the shape of the structures attached to the building facades is relatively similar to that of rectangular structures, as shown in Figure 4. As reported by previous studies, the influence of the scattering coefficient is relatively small when the scattering coefficient is large [17; 19]. In Street 4 and Street 5, the estimated scattering coefficients are between 0.5 and 0.7, while the difference between S\({}_{\text{g}}\) and S\({}_{\text{r}}\) is approximately 0.15. Therefore, the IMCRI with different scattering coefficients shows very similar results in these streets.