# An Experimental Study of the Performance of a Crossed Rib Diffuser in Room Acoustic Control
## 4 Conclusions  
This paper described a crossed ribs diffuser (CRD) as an effective tool for room acoustic control and explained an experimental investigation of its effectiveness. The CRD was constructed by overlapping two 1D periodic rib diffusers with different specifications. We first evaluated the random-incidence scattering coefficients of CRD and 1D periodic diffusers using a 1/5 scaled model and numerical method. The CRD overcame an important weak point of 1D periodic diffusers by which high scattering performance was obtained in a narrow frequency band, achieving a better scattering coefficient than 1D rib diffusers composing the CRD in a wide band. Then, the absorption performances of manufactured specimens were compared via sound absorption measurements using a reverberation room. The CRD showed a diffuse field absorption coefficient of around 0.2 above 1 kHz without absorptive materials, which was slightly higher than those of 1D periodic diffusers. Here, we also measured the absorption coefficient of the CRD-A, inserting an absorbing layer between upper and lower ribs of the CRD, resulting in a porous-type absorption mechanism and broadband high absorption coefficient. We also presented that the diffuse field absorption coefficient of CRD-A was predicted accurately using a theoretical model for the single permeable membrane absorber. Subsequently, we conducted an experiment as the demonstration of CRD performance. The experimental results demonstrated that the CRD and the CRD-A improved the reverberation parameters and speech clarity in the small meeting room more than JND values. In particular, the installation of CRD-A onto two walls reduced \(EDT\) more than 4.8 JND compared with that of a vacant room at 500 Hz and higher. Furthermore, the CRD was able to improve the reverberation parameters and speech clarity more than the 1D rib diffuser with same installation area as broadband. Finally, we visualized structural differences in reflected sounds among those of a flat wall, the CRD and the CRD-A. The results clarified that CRD controlled acoustics by preventing multiple reflections between parallel walls with scattering reflection, whereas the CRD-A improved acoustics by removing later reflected sounds with their highly absorptive properties. To explore the further applicability of the crossed ribs structures, we are now  
Figure 13: Visualized virtual source locations on the \(x\)–\(y\) plane for the source to receiver distance of S2 to R2 in the 2 kHz octave band in case 0 (**Left**), case 5 (**Center**) and case 6 (**Right**).  
attempting the next experiment with a larger meeting room. In addition, the presented CRD design was a trial. Thus, the optimum geometrical specification for the CRD is unclear in this stage. Development of a prediction method for the scattering property of CRD is also a subject for examination in future studies.  
Conceptualization, T.Y., Y.U., N.M. and Y.M.; investigation, T.Y.; methodology, T.Y.; supervision, T.Y.; project administration, Y.U.; resources T.Y., Y.U., N.M. and Y.M.; validation, T.Y.; visualization, T.Y.; writing--original draft, T.Y.; writing--review and editing, T.Y., Y.U., N.M. and Y.M. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.