## 8 Conclusions  
This study investigated the impact of surface scattering on \(T_{20}\) in differently shaped spaces. A rectangular space was used as the reference space model. On this basis, spatial shapes were adjusted.  
The \(T_{20}\) was affected by both the inherent morphology diffusivity and surface scattering coefficient. When the morphology diffusivity was high, reflections were reflected in different ways, and the surface scattering coefficient only slightly affected the \(T_{20}\). For example, the \(T_{20}\) in the reverberation chamber did not vary with the surface scattering coefficient due to the complex shape. However, the surface scattering coefficient significantly affected the \(T_{20}\) when the morphology diffusivity was low. For example, in the rectangular space, the \(T_{20}\) change rate was 25% when the surface scattering coefficient changed. The morphology diffusivity could be re-graded by adjusting the quantity and positions of the tilted walls and ceilings. Therefore, the morphology diffusivity grades accurately described the aforementioned analysis.  
For imperfect diffusion spaces, such as a rectangular space, there is a turning point in each curve of the change rate of the \(T_{20}\). The scattering coefficient at the inflection point is the critical scattering coefficient. For spaces such as the rectangular space, the critical scattering coefficients were all 0.3; in the reverberation chamber, the critical scattering coefficient was 0.01. Evidently, no critical scattering coefficients exist in perfect diffusion spaces, although they do exist in imperfect diffusion spaces. The results of this study show that the critical scattering coefficients of imperfect diffusion spaces are approximately 0.3.  
The morphology diffusivity of space is proposed. The morphology diffusivity of space can be assigned according to the combination of different surfaces in space. Each morphology diffusivity value corresponds to a diffusion degree level, and different levels correspond to different maximum change rates of the \(T_{20}\). For example, when the wall or ceiling is inclined separately, it corresponds to Grade II; when the wall and ceiling are inclined at the same time, it corresponds to Grade III. The \(RT\)'s accuracy can be improved according to the morphology diffusivity and the scattering coefficient in the \(RT\) calculations of performance spaces.  
In spaces with different volumes, the impact of the surface scattering coefficient on \(T_{20}\) is unaffected by the size of spatial volume; specifically, inherent morphology diffusivity is not affected when spatial volume ranges between 3000 and 30,000 m\({}^{3}\).  
In this study, only the uniform arrangement of sound absorption coefficients was assessed; the uneven distribution of sound absorption coefficients will be researched in the future.  
The individual contributions of authors are as follows: calculation model, data curation and writing, X.Z.; supervision and review, J.K. and H.M. All authors have read and agreed to the published version of the manuscript.  
The support from the UKRI (EP/X123456/1) is acknowledged.  
The authors declare no conflict of interest.