## 6 Conclusions and Outlook  
Sound absorbers based on MPPs could be an attractive alternative to the conventional porous and fibrous absorbers when following requirements such as durability, recyclability, cleanliness and environmental sustainability. Since MPP absorbers are theoretically expected to return the same acoustic behavior regardless of their constitutive material, in principle they can be made of any green material, provided that it is hard enough to support micro-perforation. In particular, stainless steel guarantees a long service life, being an unalterable material when applied indoors, and a good hygiene, being resistant to mold and fungi. A possible continuation of this research could be a detailed LCA analysis of a selection of materials usable to make MPP. The present work aims to outline a method for the design and the numerical validation of specific sound absorbers where both active and reactive acoustical properties have to be considered. Thanks to a full-spectrum FDTD simulation method the effects of this material on the intelligibility criteria were explored. First, three custom MPP specimens were designed and manufactured using a 3D additive metal printing process. The analytical predictive models of MPP in single- and double-layer configurations were validated through experimental measurements with the impedance tube obtaining the acoustical properties in terms of sound absorption coefficients and normalized surface impedances. Then, wider surfaces of double-layer MPPs were simulated in a calibrated 3D room corresponding to a real lecture hall. Results showed that MPPs mainly operate in the central octave band of 500 and 1000 Hz with a further useful contribution at 250 and 2000 Hz. Since this frequency range is the one most affecting speech intelligibility and at the same time the most undermined in historical lecture halls, MPPs seem to be a high-performance solution in cases similar to the present one. The university lecture hall taken as a case study is intended to show the positive effect of MPPs as acoustic treatment to the enhancement of speech intelligibility. In terms of time-dependence, the finite-difference \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Reference** & **Range (Hz)** & **Calculation Method** & **Boundary Condition** & **Output** \\ \hline [PERSON] and [PERSON] 2010 [68] & 100–5000 & BEM & Transfer Impedance & SPL, Insertion Loss \\ [PERSON] and [PERSON] 2015 [66] & 30–6000 & FEM & Surface Impedance & – \\ [PERSON] and [PERSON] 2018 [65] & 125–1000 & FEM & Surface Impedance & T\({}_{30}\), SPL \\ [PERSON] et al. 2019 [69] & 710–1400 & FEM & Surface Impedance & Power dissipation \\ [PERSON] and [PERSON], 2019 [67] & 31.5–8000 & FDTD & Surface Impedance & SPL, Insertion Loss \\ [PERSON] et al., 2020 [56] & 100–4500 & FDTD & Surface Impedance * & Conversion method \\ \hline \hline Present work & 125–4000 & FDTD & Surface Impedance & T\({}_{30}\), C\({}_{50}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Framework of recent works focused on wave-based simulations of micro-perforated panels. The frequency range managed by the authors, the calculation method, the typology of boundary condition employed and the output parameters obtained are reported. time-domain model allowed us to better analyze both the reactive effects of the treatment and the scattering effects of the lecture hall's geometry. Finally, for all these reasons the results of the present work should be indicate a robust method to experimentally measure and test the performance of specific materials, especially considering that the demand for high-performance and sustainable materials, such as the micro-perforated panels, is expected to increasingly grow in the sector of sound absorbing treatments. Conceptualization: M.G., D.D. ; data curation: M.C. ; formal analysis: D.D., M.G. ; funding acquisition, M.G. ; investigation, M.C. and L.B. ; methodology, M.G., D.D. ; project administration, M.G. ; resources, L.B. and M.G. ; MATLAB software: M.C.