## 3 Parameter Optimization for Better Sound Absorption Performance
The major reason for this phenomenon is thought to be that the low frequency sound absorption capacity of the porous metal was lower than that of the microperforated panel with the same total thickness, and absorption of the low frequency sound by the microperforated panel required a large cavity. Thus, it would be better to put the microperforated panel in front of the porous metal in the combination structure. It was observed that the optimal length of the cavity for the sound absorber consisting of porous metal + cavity was 0 in all three conditions, and the available space was filled with the porous metal, which indicated that sound absorption performance of the pure porous metal would be better than that of the porous metal + cavity, with the same limited total thickness [7]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters** & \begin{tabular}{c} **Porous Metal + Cavity +** \\ **Microperforated Panel + Cavity +** \\ **Microperforated Panel + Cavity** \\ \end{tabular} & \
\begin{tabular}{c} **Microperforated Panel + Cavity +** \\ **Cavity** \\ **Cavity** \\ \end{tabular} &
\begin{tabular}{c} **Porous Metal + Cavity** \\ **Panel + Cavity** \\ \end{tabular} \\ \hline Thickness of the porous metal \(D\) (mm) & 0.0000 & 29.6258 & 30.0000 & 0.0000 \\ Length of cavity backing the porous metal \(D_{1}\) (mm) & 0.0000 & 0.0000 & 0.0000 & 0.0000 \\ Diameter of the microhole \(d\) (mm) & 0.2535 & 0.2938 & 0.0000 & 0.2535 \\ Thickness of the panel \(t\) (mm) & 0.4347 & 0.3742 & 0.0000 & 0.4347 \\ Distance of the neighboring microblocks \(b\) (mm) & 1.6035 & 1.7233 & 0.0000 & 1.6035 \\ Length of cavity backing the microperforated panel \(D_{2}\) (mm) & 29.5663 & 0.0000 & 0.0000 & 29.5653 \\ Optimal average sound absorption coefficient (\%) & 65.0575 & 71.4354 & 44.7572 & 65.0575 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The obtained optimal structural parameters when the limited total thickness was 30 mm. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters** & \begin{tabular}{c} **Porous Metal + Cavity +** \\ **Microperforated Panel + Cavity** \\ **Microperforated Panel + Cavity** \\ \end{tabular} & \begin{tabular}{c} **Microperforated Panel + Cavity +** \\ **Porous Metal + Cavity** \\ \end{tabular} & \
\begin{tabular}{c} **Porous Metal +** \\ **Cavity** \\ \end{tabular} &
\begin{tabular}{c} **Microperforated Panel + Cavity** \\ **Panel + Cavity** \\ \end{tabular} \\ \hline Thickness of the porous metal \(D\) (mm) & 44.9469 & 49.8194 & 50.0000 & 0.0000 \\ Length of cavity backing the porous metal \(D_{1}\) (mm) & 0.0000 & 0.0000 & 0.0000 & 0.0000 \\ Diameter of the microhole \(d\) (mm) & 0.4790 & 0.2042 & 0.0000 & 0.2701 \\ Thickness of the panel \(t\) (mm) & 0.1000 & 0.1806 & 0.0000 & 1.5878 \\ Distance of the neighboring microblocks \(b\) (mm) & 8.3711 & 1.0000 & 0.0000 & 1.0000 \\ Length of cavity backing the microperforated panel \(D_{2}\) (mm) & 4.9531 & 0.0000 & 0.0000 & 48.4122 \\ Optimal average sound absorption coefficient (\%) & 74.6653 & 80.2518 & 70.7277 & 73.4605 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The obtained optimal structural parameters when the limited total thickness was 50 mm. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters** & \begin{tabular}{c} **Porous Metal + Cavity +** \\ **Microperforated Panel + Cavity** \\ **Microperforated Panel + Cavity** \\ \end{tabular}  & \begin{tabular}{c} **Microperforated Panel + Cavity +** \\ **Cavity** \\ \end{tabular}  & \ \begin{tabular}{c} **Porous Metal +** \\ **Cavity** \\ \end{tabular}  & \begin{tabular}{c} **Microperforated \\ **Panel + Cavity** \\ \end{tabular}