## 5 Experimental Validation of the Optimal Combination Structure  
Although the average sound absorption coefficient of the sound absorber increases along with an increase in the limited total thickness, the increase in the dimensional size of the sound absorber would require more installation space, which might limit its practical application. Moreover, the larger the sound absorber, the higher its fabrication cost. Therefore, experimental validations of the optimal sound absorbers when the limited total thickness was 30 mm and 50 mm were conducted with an AWA6128A detector (Hangzhou Aihua Instruments Co., Ltd., Hangzhou, China) according to the standing wave method [13; 27; 28], and a schematic diagram of the experimental system is shown in Figure 5. The porous metal used was porous copper, which was purchased from YiYang Foammetal New Materials Co., Ltd., Yiyang, China. The microperforated panel was fabricated by the laser beam drilling of spring steel according to the optimal structural parameters in Tables 1 and 2[16; 33]. After the required samples were prepared, they were assembled to form the desired sound absorbers investigated in this study. These sound absorbers were measured by an AWA6128A detector to obtain their sound absorption coefficients in the frequency range of 100-1800 Hz. According to the operating instruction of the AWA6128A detector, the cross-sectional shapes of the detected sound absorbers were round, and their diameters were 96 mm. Meanwhile, the measured sound frequencies were 100 Hz, 200 Hz,  
Figure 4: Comparisons of the theoretical results and simulation results of sound absorption coefficients of the four investigated sound absorbers with different limited total thicknesses. (**a**) 30 mm; (**b**) 50 mm; (**c**) 100 mm. 300 Hz, 400 Hz, 500 Hz, 600 Hz, 700 Hz, 800 Hz, 950 Hz, 1100 Hz, 1300 Hz, 1500 Hz, and 1800 Hz, which consisted of 13 frequency points. Comparisons of the theoretical data, simulation data, and experimental data for the four investigated sound absorbers with limited total thicknesses of 30 mm and 50 mm are shown in Figures 6 and 7, respectively. It was found that the experimental data were basically coincident with the theoretical data and the simulation data, which indicated that the constructed theoretical sound absorption models and finite element simulation models for these four sound absorbers were effective and accurate. The actual average sound absorption coefficient for the sound absorber consisting of microperforated panel + cavity + porous metal + cavity was 62.9615% and 73.5923% when the limited total thickness was 30 mm and 50 mm, respectively, exhibiting a satisfactory sound absorption performance in the low frequency range. Figure 5: Schematic diagram of the AWA6128A detector for sound absorption coefficient measurement. Figure 6: Comparisons of theoretical data, simulation data, and experimental data with the limited total thickness of 30 mm. (**a**) Porous metal + cavity + microperforated panel + cavity; (**b**) Microperforated panel + cavity + porous metal + cavity; (**c**) Porous metal + cavity; (**d**) Microperforated panel + cavity. The average sound absorption coefficients achieved in the theoretical analysis, finite element simulation, and experimental measurement of sound absorbers with a limited total thickness of 30 mm and 50 mm are shown in Tables 4 and 5, respectively. It was found that the actual average sound absorption coefficients for the combination structure of microperforated panel + cavity + porous metal + cavity were clearly better than those of the porous metal + cavity + microperforated panel + cavity sound absorber with the same constraint conditions, which indicated that the microperforated panel + cavity + porous metal + cavity structure was an efficient sound absorber for low frequency noise reduction. Meanwhile, it was observed that there were obvious differences between the theoretical average sound absorption coefficients of the sound absorber consisting of microperforated panel + cavity and its actual value.