## 5 Results and Discussion
### Parametric Analysis
#### 5.3.2 Parametric Analysis of Acoustic Properties  
Figure 8 illustrates the parametric studies on the effects of the related design parameters on the STL of the CFCPs. Based on the numerical results, it is interesting to see that when the density of foam increased, the f\({}_{\rm r}\) (the first dip frequency) moved to a lower frequency; nevertheless, the STL at f\({}_{\rm r}\) increased, as shown in Figure 8a. In contrast, when increasing the Young's modulus of foam, the dip will move to a higher frequency, while the STL at f\({}_{\rm r}\) will remain at the same level (see Figure 8b). Combining Figure 8a,b, it can be seen that before the first resonance dip, the STL property of the sandwiched panels was proportional to the Young's modulus, whereas it was inversely proportional to the foam density. After the first dip, however, the trend was converse. Regarding the Mass Control Zone, when the high-density foam is used in the corrugated sandwich panel, the nonlinearity of the STL caused by the structural complexity is obviously weakeded (see the blue curve in Figure 8a, in the frequency range of 1000-4000 Hz). On the contrary, this nonlinearity became increasingly obvious with the increase of the Young's modulus (see Figure 8b), which will provide difficulties in the design of STL in the high-frequency range. The effects of the resin density and its Young's modulus on the STL are shown in Figure 8c,d), respectively. Similar to the effects of the foam density, the first resonance dip (f\({}_{\rm r}\)) moved to a lower frequency while increasing the resin density; nevertheless, the STL at f\({}_{\rm r}\) increased. In addition, it can be seen that in the Damping Control Zone (about 300-1000 Hz), the STL fluctuated sharply with the frequency increase, which is likely owing to the change of the resin density, which significantly affects the mode vibration properties of the sandwiched panels. A similar effect of the resin density was also found in the Mass Control Zone; nevertheless, this effect gradually decreased with the increasing frequency (see Figure 8c). In Figure 8d, it can be seen that the Young's modulus of resin greatly affected the STL in the Stiffness Control Zone and the Damping Control Zone, which is consistent with the observations obtained in the mode analysis. Different to the effect of foam modulus, the first dip (f\({}_{\rm r}\)) will move to a higher frequency, while the STL at f\({}_{\rm r}\) will also increase. When E = 3.4 \(\times\) 10\({}^{2}\) MPa, the effect of the mode changing blurs the boundary between the Stiffness Control Zone and the Damping Control Zone. Compared to the basis modulus value (E = 3.4 \(\times\) 10\({}^{3}\) MPa), both increasing and decreasing the resin modulus can enhance the STL in the medium- and high-frequency range (except for E = 3.4 \(\times\) 10\({}^{4}\) MPa in the middle-frequency band). Regarding the effects of the fiber volume fraction, as depicted in Figure 8e, a lower fiber fraction provided a higher STL in the initial stage, i.e., the Stiffness Control Zone. However, a lower fiber fraction tended to shift the bottom point of the curve to the high-frequency range, which resulted in a relatively lower STL during the following stages. The reason is that the increase in the fiber volume fraction caused a significant increase in the weight of the corrugated panel, while the stiffness of the corrugated panel did not change significantly; hence, the mode frequencies of the corrugated panel decreased, which made the first natural dip shift to the left. In addition, increasing the volume fraction of the fiber cloth can obviously increase the STL of the corrugated panel in the Mass Control Zone, which eventually improves the STL at the first dip. Based on the above analysis, it can be found that increasing the fiber volume fraction or the fiber layer thickness is an effective approach to enhance the STL of the corrugated panels at the middle- and high-frequency bands.