## 3 Experimental Validation and Discussions
### Results and Discussions  
Figure 7 shows the sound absorption measurement and prediction curves of the composite sound absorption structure (\(L_{g}\) = 0.6 mm). The sound absorption coefficient diagram obtained by the semi-empirical model is in good agreement with the measured results. The maximum value of the sound absorption coefficient moves to the low frequency direction, where the foamed copper material is thickened. This is because the right-end of the PP hole is not open, but the porosity of the OFC is very high, reaching 0.95, which can be considered as both ends of the hole being open. When the air in the hole is used as the sound mass and the fluid in the air gap between the PP and the OFC is used as the sound spring, there will be mass and spring resonance. When the stiffness of the air gap offsets the mass, the peak value of the sound absorption coefficient will appear. The increase in the OFC thickness will effectively reduce the stiffness of the air gap, so that the absorption peak moves to a lower frequency. As a result of the damping characteristics of the OFC itself, it will increase the dissipation of sound energy and increase the bandwidth of the peak sound absorption coefficient. It is worth noting that the absorption spectrum of OFC will show resonance corresponding to the frequency, the thickness of which is equal to one-fourth of the wavelength of the wave propagating in the layer. Due to the increase in the effective thickness, the frequency will be reduced by adding a perforated cover layer and a small air space above the hard back foam layer. In this study, the perforated covering layer and small air space are controlled, and only the thickness is changed, which further explains the reason for the low frequency shift to the peak frequency. The maximum sound absorption coefficient appears near 2500 Hz for the OFC1 + PP1. The peak absorption coefficient predicted by the semi-empirical model is in good agreement with the measured data of the corresponding frequency. The calculated value of the sound absorption band is slightly  
Figure 6: The prepared test specimens and the impedance tube: (**a**) 3D printed PP sample; (**b**) PP1 + OFC2; (**c**) test specimen mounted in the impedance tube; (**d**) the placement order of PP1 + OFC2; (**e**) the B and K impedance tube.  
wider than the measured value of the sound absorption band. Moreover, the measurement shows another absorption peak (1900 Hz) of the composite structure, depicted in Figure 7a. One reason is that OFC is very thin, similar to the PP, so it can resonate. Another reason is the resonance of the porous foam layer frame. The maximum sound absorption coefficient appears near 2000 Hz and 1800 Hz for the OFC2 + PP1 and OFC3 + PP1, respectively, as shown in Figure 7b,c. In short, the semi-empirical model is validated for the prediction of the sound absorption of composite structure.  
Figure 7: Comparison of the calculated and measured sound absorption coefficients: (**a**) composite structure composed of OFC1 and PP1; (**b**) composite structure composed of OFC2 and PP1; (**c**) composite structure composed of OFC3 and PP1.