## 4 Conclusions  
Improving and optimizing the sound absorption performance of polyurethane foam by a prepositive microperforated polymethyl methacrylate panel was conducted in this research. Through theoretical sound absorption modeling, identification of the acoustic characteristic parameters, optimization of the composite structures, finite element simulation, and standing wave tube measurement, research achievements of this study were concluded and summarized as follows. 1. Prepositive microperforated polymethyl methacrylate panel was utilized to form a composite sound-absorbing structure together with the polyurethane foam, which aimed to improve the sound absorption performances within the low-frequency range. Theoretical sound absorption model of the investigated polyurethane foam and that of the proposed composite sound-absorbing structure were constructed through the transfer matrix method, which supplied a theoretical basis for identification of the acoustic characteristic parameters and optimization of the composite structure. 2. Acoustic characteristic parameters of the polyurethane foam were identified by the cuckoo search algorithm based on its experimental data of sound absorption coefficients, and the obtained porosity and static flow resistivity were 0.958 and 13078 Pa-s/m\({}^{2}\) respectively. Comparisons of the sound absorption coefficients of the polyurethane foam with various thicknesses in theory and those in actual proved effectiveness and accuracy of the identification method. The achieved acoustic characteristic parameters provided a steady foundation for the improvement and optimization of the sound absorption performance of the polyurethane foam. 3. For the polyurethane foam with different thicknesses and the various investigated frequency ranges, structural parameter optimization of each composite sound-absorbing structure was realized through the cuckoo search algorithm, which aimed to achieve the maximum average sound absorption coefficient. Sound absorption performance of the polyurethane foam was \begin{table}
\begin{tabular}{c c c} \hline \hline
**The Investigated Sound Absorber** & \multicolumn{2}{c}{**Investigated Frequency Range (Hz)**} \\ \cline{2-3}  & **100–1000** & **100–2000** \\ \hline Proposed optimal composite structure & 0.5296 & 0.6482 \\ Utilized original polyurethane foam & 0.1885 & 0.3392 \\ Standard porous metal [27] & 0.1529 & 0.2477 \\ Uniform compressed porous metal [27] & 0.2098 & 0.4531 \\ Gradient compressed porous metal [27] & 0.3325 & 0.5412 \\ Microperforated compressed porous metal panel [29] & 0.1657 & 0.3403 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparisons of actual average sound absorption coefficient of the optimal composite sound-absorbing structure with that of other porous materials when the total thickness was 20 mm. significantly improved by a prepositive microperforated polymethyl methacrylate panel, especially for the conditions of small thickness of the sample and low frequency of the sound. The gained optimal structural parameters offered guidance for the following finite element simulation and sample preparation. 4. Sound absorption characteristics of the composite sound-absorbing structures were verified by finite element simulation in the virtual acoustic laboratory and validated through standing wave tube measurement in the AWA6128A detector. Consistencies among sound absorption coefficients of the composite structures in theory, those in simulation, and those in actual were very good, which proved the effectiveness and accuracy of the used cuckoo search identification/optimization algorithm, the founded theoretical sound absorption model, and the utilized finite element simulation method. 5.