# Comparative Study of Sound Transmission Losses of Sandwich Composite Double Panel Walls
## 5 Parameter Effects and Optimization Strategy on the Sound Transmission Loss of Double Panel Walls
### Optimization of Sound Transmission Loss of Double Panel Structures
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Group**} & \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**Reference**} & \multicolumn{6}{c}{**Influence on STL**} \\ \cline{3-8}  & & & \multicolumn{2}{c}{**Frequency Region**} & \multirow{2}{*}{**Remark**} \\ \cline{5-8}  & & & **Low** & **Mid** & **High** & & \\ \hline \multirow{5}{*}{Geometrical} & Core heights & [51; 64; 71] & \(\uparrow\) & \(\Longleftrightarrow\) & \(\Uparrow{-2}\) & - & Increased STL \\ \cline{2-8}  & Core radii & [64; 69] & \(\uparrow\) & \(\Longleftrightarrow\) & \(\Uparrow{-2}\) & - & Increased STL \\ \cline{2-8}  & Cylindrical Shell radii & [68; 69; 86] & \(\downarrow\) & \(\downarrow\) & - & Decreased STL with higher radius. \\ \cline{2-8}  & Plate/shell thicknesses & [51; 69; 144; 145] & \(\uparrow\) & \(\Longleftrightarrow\) & \(\Uparrow{-2}\) & - & Increased STL \\ \cline{2-8}  & Cylindrical shell lengths & [145] & \(\downarrow\) & \(\downarrow\) & & - & Decreased STL \\ \cline{2-8}  & Finite dimension of panels/shell & [51; 146] & \(\cdots\) & \(\cdots\) & \(\Uparrow{-2}\) & - & Lower STL than infinite dimension. \\ \cline{2-8}  & Infinite dimension of panels & [51; 146] & \(\cdots\) & \(\Longleftrightarrow\) & \(\Uparrow{-2}\) & - & Higher STL than finite dimension. \\ \cline{2-8}  & Different boundary conditions & [13; 145] & \(\uparrow\) & \(\Longleftrightarrow\) & \(\cdots\) & - & STL most significant at low frequency regime. \\ \cline{2-8}  & Different core topologies & [70; 147] & \(\uparrow\) & & - & Significant difference in STL. \\ \hline \multirow{5}{*}{Material} & Mach numbers & [11; 64; 69; 100] & \(\downarrow\) & \(\Longleftrightarrow\) & \(\downarrow\) & - Poor STL. \\ \cline{2-8}  & Damping loss factors & [64] & \(\cdots\) & \(\cdots\) & - & STL decreased; however, it increased at dip frequencies only. \\ \cline{2-8}  & Young’s modulus & [31; 63; 69] & \(\uparrow\) & \(\downarrow\) & \(\Uparrow{-2}\) & - & Improved STL \\ \cline{2-8}  & Shear modulus & [29; 69] & \(\cdots\) & \(\cdots\) & \(\cdots\) & - & No significant difference in STL. \\ \cline{2-8}  & Poisson ratios & [69] & \(\cdots\) & \(\cdots\) & \(\cdots\) & - & No significant difference in STL. \\ \cline{2-8}  & Stiffness & [69] & \(\uparrow\) & \(\downarrow\) & \(\Uparrow{-2}\) & - & Improved STL \\ \hline \multirow{5}{*}{Structural} & Cavity gas types & [46; 86] & \(\uparrow\) & \(\Longleftrightarrow\) & & - & STL largely influenced. \\ \cline{2-8}  & Core types & [31; 70] & \(\uparrow\) & & & - & STL largely influenced. \\ \cline{2-8}  & Porous and fibrous material in the core & [118; 148] & \(\uparrow\) & \(\Longleftrightarrow\) & & - & Higher STL than cores without porous material. \\ \cline{2-8}  & Different metallic plate/shell combination & & & & & - & Magnitude of STL varies with different materials and combinations. \\ \cline{2-8}  & Different Composite plate/shell types & [69; 71] & \(\uparrow\) & \(\Uparrow{-2}\) & \(\Uparrow{-2}\) & - & Magnitude of STL varies with different composite materials. \\ \cline{2-8}  & Different FGM plate/shell types & [150] & \(\uparrow\) & \(\Longleftrightarrow\) & \(\Uparrow{-2}\) & - & Magnitude of STL varies with different functionally graded materials (FAMs). \\ \hline \multirow{5}{*}{Orientedated} & Incident/elevation angles & [60; 69; 145] & \(\downarrow\) & \(\cdots\) & \(\downarrow\) & - & Poor STL performance. \\ \cline{2-8}  & Azimuthal angles & [72; 100] & \(\downarrow\) & \(\Longleftrightarrow\) & \(\downarrow\) & - & STL Increased slowly at mid-high frequency zones. \\ \cline{2-8}  & Core inclination angle & [51] & \(\uparrow\) & \(\downarrow\) & \(\downarrow\) & - & Poor STL performance. \\ \cline{2-8}  & Temperatures & [72] & \(\downarrow\) & \(\downarrow\) & & - & Decreased STL. \\ \cline{2-8}  & Stacking sequences & [49; 68; 69; 71] & \(\cdots\) & & & - & STL improves with more zero piles. \\ \cline{2-8}  & Cone angles & [145] & \(\downarrow\) & \(\Longleftrightarrow\) & \(\cdots\) & - & STL poorly increase at mid frequency region. \\ \hline \hline \end{tabular}  
* \(\uparrow\): Increased STL at low frequency zone; \(\Longleftrightarrow\): Increased STL at mid frequency zone; \(\Uparrow{-2}\): Increased STL at high frequency zone.  
\end{table}