## 1 Introduction
For the purpose of further improving sound absorption performance of the polyurethane foam, the composite sound-absorbing structure was proposed, which included the polyurethane foam and other sound-absorbing materials [10; 11; 12; 13; 14]. Barium titanate/nitrile butadiene rubber and polyurethane foam were combined to form a sound absorbing material with alternating multilayered structures by [PERSON] et al. [10], which gained excellent low-frequency sound absorption performances due to the organic combination of the airflow resistivity, resonance absorption, and interface dissipation. [PERSON] et al. [11] developed new composite materials based on sawdust as reinforcing material and polyurethane foam as a binder agent, and their sound absorption coefficients were over 0.5 in the large frequency range. The rigid polyurethane foam and panel were combined to form the rigid foam composites by [PERSON] et al. [12], and excellent sound absorption was obtained when the aluminum hydroxide was 20 wt%. [PERSON] et al. [13] fabricated the polyurethane/poly(vinylidene fluoride)/multi-walled carbon nanotube (PU/PVDF/MWCNT) composite foam, which could benefit the sound absorption property by introducing the interfacial damping and local piezoelectric damping effects. Acoustic properties of the polyurethane foam with the addition of bamboo leaves particles were investigated by [PERSON] and [PERSON] [14], the results of which obtained the best sound insulation with high transmission loss around 18.9 dB over the entire frequency range of 100-6300 Hz. Although these composite structures developed from the original polyurethane foam achieved better sound absorption performance [10; 11; 12; 13; 14], their fabrication processes were complex, which indicated that the fabrication costs were increased. Moreover, these composite structures were different materials for the original polyurethane foam, which meant that improvement of the sound absorption performance required replacement of the polyurethane foam in use, such as meeting room, cinema, theatre, and so on. The replacement not only increased the cost, but also took more time. Therefore, the prepositive microperforated polymethyl methacrylate panel was used to form the composite structure with polyurethane foam in this research, which indicated that improvement of the sound absorption performance was realized by adding an appropriate thin panel instead of replacement of the material. By this way, cost and time of the alteration could be reduced, which was favorable to promote practical application of the proposed composite structure. Therefore, improving and optimizing sound absorption performance of the polyurethane foam by prepositive microperforated polymethyl methacrylate panel was conducted in this study. The sound absorption mechanism of the polyurethane foam and that of the composite sound absorbing structure were investigated according to their structures, and their theoretical sound absorption models were constructed based on the Johnson-Champoux-Allard model [15; 16; 17] and [PERSON]'s theory [18; 19; 20] by the transfer matrix method [21; 22; 23]. Afterwards, identification of acoustic characteristic parameters of the polyurethane foam and optimization of structural parameter of the composite structure were realized through the cuckoo search algorithm [24; 25; 26; 27]. Later, the composite structure was verified by the finite element simulation method [28; 29; 30; 31] and validated through the standing wave tube measurement [32; 33; 34; 35]. Finally, the accuracy of identified acoustic characteristic parameters of the polyurethane foam and effectiveness of improved sound absorption performance of the composite structure were proved.