# Comparative Study of Sound Transmission Losses of Sandwich Composite Double Panel Walls
## 3 Double Panel Air Cavity, Sandwich and Composite Structures
### Sandwich Composite Structures
#### 3.3.2 Composite Cylindrical and Curved Shells  
These types of sandwich double wall structures have also attracted attention in the aerospace and marine industry. Their practical application has been seen in the construction of sandwich composite fuselage [59, 77]. In the previous subsection, the flat sandwich composite structures were assumed by researchers to have finite structures. However, in the calculation of the STL for the cylindrical composite shells, they are mostly assumed by researchers to be infinitely long. A good number of contemporary authors have investigated the effects of various parameters on the STL of cylindrical sandwich composite shells. [PERSON] et al. [68, 78] obtained analytical solutions of STL of composite cylindrical shells and the effect of angle of wrap, shear and rotation of the shells on the STL were in investigated. In their findings, the STL of the composite shells were influenced by the angle of wrap of the layers. However, in the high frequency region, a decreased STL was observed due to the effect of shear wave transmitted but this effect was insignificant in the low frequency region. [PERSON] et al. [69] also analytically obtained the STL of composite cylindrical shell using third order shear deformation theory. The schematic representation of the structure is seen in Figure 6. The authors investigated various parameters such as stacking sequence, Mach number, shell radius, shell thickness, material types and Young's modulus. In their investigation, there was significant effect of the STL when different composite materials were used. The STL evidently increased with shell thickness at low, medium and high frequency zones. Moreover, with increased Mach number and shell radius the STL was seen to decrease especially in the low frequency region. Furthermore, the stacking sequence that involved more zero degree plies were noticed to give better STL. In addition, they also compared the STL results between the composite cylindrical shell and aluminum cylindrical shell. It was seen that the latter gave higher STL than the former in all the controlled regions, i.e., stiffness, mass and coincidence regions. The reason for the higher STL of aluminum cylindrical shell is because it has higher density than their composite cylindrical shell counterpart. The STL of composite curved shells was also studied by [PERSON] et al. [79]. One of the advantages of metallic curved or cylindrical shells over composite curved or cylindrical shells is their higher ring frequency which in turn produces higher STL. Further, another limitation of the composite shells which has affected the lowering of their STL is that their coincidence zone has the proclivity to extend over a wide frequency range than their metallic counterpart. However, insulators, metamaterials and poroelastic materials have been introduced in the cores of composite cylindrical or curved shells to raise their ring frequencies and further improve their STL characteristics. [PERSON] et al. [80] introduced  
Figure 5: Schematic representation of sandwich composite flat panels with different core and cavity types. poroelastic material between doubly curved sandwich composite thin-walled panels. Different design variables such as material type and stacking sequence on their STL were investigated. In their result, it was found that the STL of the poroelastic composite curved shell increased at the medium and high frequency regions. More so, it was observed that at the coincidence dip frequency, the STL of the poroelastic structures were higher than structures without poroelastic cores. Both numerical and experimental investigation were undertaken by [PERSON] et al. [81] to study the effect of foam attachment on the STL of thick sandwich composite curved shell. Two configurations were tested--a curved sandwich panel with attached foam and another without foam attachment. The result of their findings indicated that at frequency above 400 Hz, the STL of the curved panel with foam were higher than those without foam.