## 1 Introduction  
Polyurethane foam is considered a promising and prospective sound-absorbing material for its high porosity, low density, fine transparency, mature fabrication technology, low manufacturing cost, excellent machinability, and so on [1, 2], which meets some special requirements in the fields of sound absorption and noise reduction [3, 4]. [PERSON] et al. [1] studied sound absorption behavior of the flexible polyurethane foam with distinct cellular structure, the research result of which indicated that the strong gelling catalyst could generate a high number of small cells for a better sound absorption property. A new tung oil-based polyurethane composite foam (TOPUF) was prepared by [PERSON] et al. [2]through a one-step method, and the average sound absorption coefficient of 0.518 and transmission loss of 19.05 dB in the frequency range of 100-6300 Hz were gained with the filler content of 0.3 wt%. [PERSON] et al. [3] proposed and fabricated multifunctional fabric-reinforced composites (MFRCs), which could obtain the average sound absorption coefficient of 0.7 in the 1000-4000 Hz range. The nitrile butadiene rubber-polyurethane foam composites (NBRPFCs) with stratified structure were designed and optimized by [PERSON] et al. [4], and the sandwich structure with thickness ratio 1:8:1 could achieve the optimal average sound absorption coefficient of 0.56 in the 500-1600 Hz range. These sound absorbers with fine sound absorption performance [1; 2; 3; 4] prove that the polyurethane foam has huge potential in the noise reduction, which makes it a new research hotspot in acoustics and material science. In order to promote practical application of the polyurethane foam in the sound absorption and noise reduction, it is essential to identify its acoustic characteristic parameters, especially the porosity and static flow resistivity, because they are the most important factors to determine the sound absorption performance [5; 6; 7; 8; 9]. [PERSON] et al. [5] studied sound absorption performance of the polyurethane foam with porosity in the range from of 16.0% to 88.6%, which achieved the optimal sound absorption coefficient of 0.66 in the low frequency range of 250-600 Hz. Magnesium hydroxide filler was used by [PERSON] et al. [6] to fabricate the polyurethane composite foam for improving its acoustic property, the results of which proved that the sound absorption coefficient was improved by 70% at the optimal open porosity of 0.63 compared with the non-filler case. Acoustic characteristic parameters of the polyurethane/rice husk (PU-RH) composite were investigated by [PERSON] et al. [7], which consisted of the porosity, static flow resistance, tortuosity, and characteristic constants, the research results of which proved that sound absorption coefficients of PU-RH composite increased at low frequency along with a small decrease at high frequency with the addition of RH. The gradient-compressed porous material was proposed by [PERSON] et al. [8] to develop acoustic absorber with high-efficiency and thin thickness, the results of which indicated that sound absorption properties could be adjusted through changing permutations of the compressed layers with different porosity and static flow resistivity. [PERSON] et al. [9] investigated influences of the acoustic characteristic parameters to sound absorption properties of the multilayer composite made of glass, wool, glue, and polyurethane foam, which indicated that influences of the outer material were more obvious than that of the inner material. These achievements on influences of the acoustic characteristic parameters to sound absorption performance of the polyurethane foam proved that identification and optimization were critical procedures to develop a novel sound absorber from the polyurethane foam for practical application in the noise reduction.