## 3 Results and Discussion
### Sound Absorption Performance of the Composite Sound Absorbing Structure
#### 3.2.3 Contrast with the Original Polyurethane Foam  
Actual average sound absorption coefficients of the composite sound-absorbing structures were compared with those of the polyurethane foams, and they are summarized in Table 4. It could be observed that principles of absolute value and relative percentage of improvement of the actual average sound absorption coefficients were consistent with those of theoretical data in Table 2. Moreover, it could be found that for the investigated frequency range of 100-1000 Hz, the actual average sound absorption coefficient of the composite structure consisted of the polyurethane foam with the initial thickness of 10 mm was 0.4015, and it was close to that 0.4077 of the original polyurethane foam with the thickness of 40 mm, which indicated that the optimal composite sound absorbing structure could achieve the same sound absorption performance by only utilizing 1/4 of the original material and occupancy space. Accordingly, for the investigated frequency range of 100-2000 Hz, the actual average sound absorption coefficient of the composite structure consisted of the polyurethane foam with the initial thickness of 30 mm, which was 0.7257, and there was a gap between it and that 0.7778 of the original polyurethane foam with the thickness of 40 mm, which agreed with the normal principle that further improvement of the sound absorption performance in a large frequency range was more difficult than that in a small frequency range. Furthermore, absolute value and relative percentage of improvements of the actual average sound absorption coefficient were 0.3149 and 363.63% respectively when the investigated frequency range was 100-1000 Hz and the initial thickness of the polyurethane foam was 10 mm, while those were only 0.1219 and 18.59% respectively when the investigated frequency range was 100-2000 Hz and the initial thickness of the polyurethane foam was 40 mm, which could further prove that the improvement of the sound absorption property was more effective when the total thickness of the sound absorber and the investigated frequency range were small.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Thickness of Original** & **Investigated Frequency** & \multicolumn{2}{c}{**Actual Average Sound**} & \multicolumn{2}{c}{**Improvement**} \\ \cline{3-5}
**Polyurethane Foam (mm)** & **Range (Hz)** & **Polyurethane Foam** & **Absolute Structure** & **Relative Value** & **Percentage** \\ \hline
10 & 100–1000 & 0.0866 & 0.4015 & 0.3149 & 363.63\% \\
10 & 100–2000 & 0.1451 & 0.5254 & 0.3803 & 262.10\% \\ \hline
20 & 100–1000 & 0.1885 & 0.5296 & 0.3411 & 180.95\% \\
20 & 100–2000 & 0.3392 & 0.6482 & 0.3090 & 91.10\% \\ \hline
30 & 100–1000 & 0.2690 & 0.6184 & 0.3494 & 129.89\% \\
30 & 100–2000 & 0.5006 & 0.7251 & 0.2245 & 44.85\% \\ \hline
40 & 100–1000 & 0.4077 & 0.6803 & 0.2726 & 66.86\% \\
40 & 100–2000 & 0.6559 & 0.7778 & 0.1219 & 18.59\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparisons of actual average sound absorption coefficients of the optimal composite sound-absorbing structures with those of the polyurethane foam.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Thickness of Original** & **Investigated Frequency** & \multicolumn{2}{c}{**Average Sound Absorption Coefficient**} \\ \cline{3-5}
**Polyurethane Foam (mm)** & **Range (Hz)** & **In Theory** & **In Simulation** & **In Actual** \\ \hline
10 & 100–1000 & 0.3969 & 0.3963 & 0.4015 \\
10 & 100–2000 & 0.5179 & 0.5170 & 0.5254 \\ \hline
20 & 100–1000 & 0.5409 & 0.5398 & 0.5296 \\
20 & 100–2000 & 0.6611 & 0.6585 & 0.6