## 2 Methods
Thus, the acoustic absorption coefficient can be defined as the difference from 1 of the ratio of the reflected acoustic energy flux--\(\theta_{r}\) to the incident acoustic energy flux--\(\theta_{i}\):  
\[\alpha\ =\ 1-\frac{\theta_{r}}{\theta_{i}} \tag{1}\]  
Acoustic attenuation for the total noise level--\(\Delta L\)--is a measure that assesses the effectiveness of the material in terms of its absorbent properties and is obtained by considering the sound pressure level before placing the material sample (at the origin point)--\(L_{1}\)--and the sound pressure level after the tested material sample--\(L_{2}\). \[\Delta L\ =\ L_{1}-L_{2} \tag{2}\]  
The reduction in the total noise level in the vehicle cabin, through passive noise control, can be evaluated by determining the acoustic attenuation using the calculation relationship according to the area:  
\[\Delta L\ =\ 10 lg^{A}/A_{o},\text{[dB]} \tag{3}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Acoustic Parameter**} & \multirow{2}{*}{**Symbol**} & \multirow{2}{*}{**Unit**} & \multicolumn{2}{c}{**Frequency**} \\ \cline{5-6}  & & & **400–1000** & **1000–4000** \\ \hline Coefficient of acoustic absorption & \(\alpha\) & - & 0.05–0.2 & 0.2–0.5 \\ \hline Sound attenuation & \(\Delta\)L & dB & 10–20 & 20–30 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Performance requirements for composite structures. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & **Variable Name** & **The Type of the Variable** & **Measure Unit** & **Limits Provided** & **SPSS Codification** \\ \hline
3 & Thickness & Continuous & mm & 1...100 mm & - \\ \hline
4 &
\begin{tabular}{c} Sound/acoustic \\ absorption coefficient \\ (alpha) \\ \end{tabular} & Continuous & - & 0...1 & - \\ \hline
5 & Frequency & Continuous & Hz & 125...6300 Hz & - \\ \hline
6 & Acoustic comfort (k) & Continuous & - & \begin{tabular}{c} 0—does not \\ contribute and \\ 1—contributes to \\ ensuring acoustic \\ comfort \\ \end{tabular} & 0 = without acoustic comfort, 1 = with \\  & & & \
\begin{tabular}{c} 0—does not \\ contribute and \\ 1—contributes to \\ ensuring acoustic \\ comfort \\ \end{tabular} & 0 = without acoustic comfort, 1 = with \\  & & &
\begin{tabular}{c} 0—without acoustic comfort, 1 = with \\ acoustic comfort \\ \end{tabular} & acoustic comfort \\ \hline
6 & \
\begin{tabular}{c} Numerical code of \\ the material \\ \end{tabular} & Categorical & - & 1–254 &
\begin{tabular}{c} String (codes show the number of the \\ studied material sample) \\ \end{tabular} \\ \hline
7 & Material type & Categorical & - & 1—basic material, 2, —composite \\ \hline
8 & \
\begin{tabular}{c} Absorption acoustic \\ class \\ \end{tabular} & Categorical & - &
\begin{tabular}{c} Based on value from \\ Table 2 (first column) \\ \end{tabular} & 1 = A class, 2 = B class, 3 = C class, 4 = D class, 5 = E class, 6 = other class \\ \hline
9 & \
\begin{tabular}{c} Absorption class \\ performance \\ \end{tabular} & Categorical & - &
\begin{tabular}{c} Based on value from \\ Table 2 (last column) \\ \end{tabular} & 1 = Very high-performance absorbent \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._where \(A_{0}\) is the equivalent area for the interior cabin configuration of the vehicle [m\({}^{2}\)UA]; A is the equivalent area of acoustic absorption of the cabin, with absorption material applied [m\({}^{2}\)UA];  
The calculation formula for the equivalent absorption area is  
\[A\ =\ \sum_{i=1}^{n}\alpha_{i}S_{i} \tag{4}\]  
where \(S_{i}\) is the area corresponding to the \"\(i\)\" portion of the vehicle cabin, [m\({}^{2}\)]; \(\alpha_{i}\) is the acoustic absorption coefficient corresponding to the material of the \"\(i\)\" portion of the vehicle cabin [103].