## 4 The Impact of Spatial Shape on the \(T_{20}\) Change Rate  
Figure 5 presents the calculation results, with the abscissa indicating the surface scattering coefficient and the ordinate indicating the \(T_{20}\) change rate. The curve in Figure 5 shows how the \(T_{20}\) change rate varied as the surface scattering coefficient increased. The ordinate value was consistent with the just noticeable difference (_JND_) value of \(T_{20}\) at a regular interval of 5% [23; 24; 25; 30]. As shown in Figure 5, \(T_{20}\) tended to decrease as the scattering coefficient increased in differently shaped spaces. This variation trend is consistent with existing study results [8; 11; 14; 16; 21]. However, the decreasing trend was different across different spatial shapes.  
Figure 4: Decay curves related to the reverberation chamber under surface scattering coefficients of different values.  
Figure 5: The impact of the scattering coefficient on the \(T_{20}\) change rate under the same volume and different shapes Note: A—rectangular space, B—single sidewall tilted, C—both sidewall tilted, D—backward tilted, E—both sidewalls and backwall are tilted at the same time, F—ceiling tilted, G—single sidewall tilted, H—both sidewalls and ceiling are tilted at the same time, I—all sidewalls and ceiling are tilted at the same time, Re Chamber—reverberation chamber.  
The \(T_{20}\) decay curves in Figure 5 can be roughly classified into four groups. Within each group, the decay curves shared a similar variation trend; the variation trend of the decay curves was obviously different across groups. The more surfaces were spatially changed, the lower the RT change rate from increased surface scattering became.  
In accordance with the space adjustment described above, the rectangular space was adjusted in three ways. First, the rectangular space remained unchanged, and was marked Space A. Second, the walls or ceiling were tilted separately; specifically, a single side wall was tilted, both side walls were tilted, the rear wall was tilted, and the ceiling was tilted. Spaces B, C, D, E, and F were obtained in this way. Third, both walls and the ceiling were adjusted at same time. Specifically, a single side wall and the ceiling were adjusted; both side walls and the ceiling were adjusted; and rear walls and a ceiling were added to both side walls. Spaces G, H, and I were obtained in this way. In each group of spaces, the curves of the \(T_{20}\) change rate shared a similar trend; the maximum difference in the change rate was less than 3% and the difference in the change rate between groups was approximately 5%. Evidently, each group of spaces shared a consistent change-rate trend, while the change-rate trend was obviously different across groups.  
\(T_{20}\) varied most significantly in relation to the scattering coefficient in Space A. \(T_{20}\) change rate tended to decrease evenly; it decreased most significantly (by 25%) when the scattering coefficient was 0.9. In Spaces B, C, E, and F, the \(T_{20}\) change rate was approximately the same under four states, and the maximum decrease in the \(T_{20}\) change rate was approximately 20%. In Spaces D, G, H, and I, the \(T_{20}\) change rate was approximately the same under three states, and the maximum decrease in the \(T_{20}\) change rate was approximately 20%. In the reverberation chamber, the \(T_{20}\) change rate with the scattering coefficient was less than 3%.  
The curves of the \(T_{20}\) change rate shown in Figure 5 presented a dual-slope trend, with 0.3 as the demarcation point. When the scattering coefficient ranged between 0.01 and 0.3, \(T_{20}\) presented a stage of rapid decrease; the maximum change to \(T_{20}\) (17%) occurred in the rectangular space. The minimum change to \(T_{20}\) was 8%. Both were greater than 5%. When the scattering coefficient ranged between 0.3 and 0.99, the \(T_{20}\) change tended to be smooth, while the \(T_{20}\) change rate was always lower than 5%.