## 3 Results and Discussions
### Validation of the Optimal Composite Structures  
Detections of sound absorption coefficients of the composite sound absorbing structures were obtained by the AWA6128A detector (Hangzhou Aihong instruments Co., Ltd., China), as shown in Figure 7[40; 41; 42]. The diameter of cross-section of the AWA6128A detector is 96 mm, and its measurable frequency range is (1500 Hz, 6640 Hz). Length of the slideway is 1000 mm, which supports motion of the pulley to detect peak value and valley value of the reflected wave. Measurable thickness of the detected acoustic absorber (including the cavity) was 0-60 mm, which indicated that the proposed composite structures with the limited total thickness of 100 mm go beyond the measuring range of the equipment. In fact, the sound absorber with total thickness of 100 mm has few application prospects in the industrial noise reduction, because it takes too much space. It can be found that composite structure of PCMC and that of PCMCPC can obtain relative better sound absorption performances among the proposed composite structures. Thus, these two composite sound absorbing structures with limited total thickness of 30 mm and 50 mm were prepared and assembled according to the optimal structural parameters in Tables 3 and 4, and the samples are shown in Figure 8. The utilized porous copper sample in this research was purchased from YiYang Foam Metal New Material Co., Ltd., Yiyang, Hunan, China, and thicknesses of the common samples provided by the supplier were 2 mm and 5 mm. Therefore, in order to reduce fabrication cost, each used porous metal sample included some standard blocks and one special block, as shown in Figure 8. Moreover, length of the rear cavity was adjusted by the sample holder in Figure 8a. Figure 7: Schematic diagram of AWA6128A detector for measuring the sound absorption coefficient. (**a**) Structure diagram; (**b**) physical picture. Sound absorption coefficients were achieved according to the standing wave tube method [8; 9; 34; 35; 36; 37; 40; 41; 42], as shown in the Figure 9. It was found that distribution and tendency of the sound absorption coefficients in the experimental result were consistent with those in the theoretical result. The selected porous metal was porous copper, and its structural parameters were consistent with those in the theoretical modeling process. The desired microperforated panels were fabricated by laser beam drilling of the steel spring plates according to the corresponding structural parameters. Taking the machinability and optional parameters of the equipment into consideration, the diameter of the hole \(d\) and distance of the neighboring holes \(b\) of the optimal structural parameters for the microperforated panels were handled to keep two decimal places. Meanwhile, considering common dimension of the steel spring plate easy to buy, thickness of the panel \(t\) was also kept two decimal places for the approximation. Lengths of the cavities \(D_{a}\), \(D_{f}\), and \(D_{b}\) were adjusted by the additive manufacturing of blocks with the desired sizes. In addition, quantitative comparisons of average sound absorption coefficient in the theoretical result, simulation result, and experimental result are summarized in Table 6. It can be observed that the actual average sound absorption coefficient of four investigated composite structures exceeded 96%, though they are slightly smaller than those of the corresponding theoretical result. The major reason for this phenomenon is that there exists fabrication error in the preparation of porous metals and microperforated panels. Meanwhile, measuring error in the measuring process also affected the experimental result, although each sound absorber was measured by more than 10 times to reduce the accidental measuring error. By this method, feasibility and effectiveness of these optimal composite structures were validated. Figure 8: The prepared optimal composite structures.