# Comparative Study of Sound Transmission Losses of Sandwich Composite Double Panel Walls
## 3 Double Panel Air Cavity, Sandwich and Composite Structures
### Sandwich Core Support Structures  
The core of sandwich structure not only ensures transmission of sounds to the panel but also acts as a mechanical support for the system. Sandwich DPS can be made with different core supports which may include honeycomb [53; 54], foam [55; 56], corrugated core [57] or pyramidal truss core [58]. Metallic honeycomb core have been widely used in aerospace application [59]. The honeycomb sandwich core provides a bi-directional support for the double panels which in turn gives the entire DPS a high performance to weight advantage. Few studies have revealed the advantage of using honeycomb cores to improve the STL of sandwich structures. [PERSON] et al. [60] carried out a numerical study on the STL of sandwich structures with honeycomb core. In their study, honeycomb cores made from epoxy/graphite fiber reinforced polymer (FRP) were compared with those made from aluminum. Their results indicate that the STL of the former was greater than the STL of the latter below the first resonance frequency. In another study conducted by [PERSON] and [PERSON] [47], the STL of double arrowed-head honeycomb cores were investigated and compared with hexagonal honeycomb core by using an increasing and decreasing negative Poisson ratio. Their result showed that the double arrowed head honeycomb DPS with increasing negative Poisson ratio produced better sound insulation performance than the hexagonal honeycomb DPS. Different honeycomb core structures were experimentally investigated by [PERSON] et al. [61] at low frequency region. The honeycomb core can also be filled with glass fiber or foams such as polyurethane or polypropylene which are commonly used. This combination of honeycomb and foam give the structure better mechanical property. [PERSON] et al. [62] investigated both numerically and experimentally the modal characteristics of cylindrical sandwich structure with foam-filled corrugated core. The curved and flat sandwich corrugated cores are illustrated in Figure 4. [PERSON] et al. [62] studied the STL of honeycomb sandwich DPS filled with glass fiber materials. The fiber materials were filled randomly and in a ball-like structure. The authors investigated their STL with honeycomb sandwich structures that were not filled with fibers. Their results showed a rise in the STL of the glass fiber-filled honeycomb structure as compared to those not filled with fibers. The sandwich core can be stiffened or structured as a lattice truss core. This 3-D lattice truss core sandwich structure is currently applied in aerospace and marine industries owing to their higher specific stiffness, lower density, lower thermal conductivity and better sound insulation properties. The different forms of these sandwich core supports that have been adopted by contemporary researchers to evaluate STL are the octahedral core, tetrahedral core, pyramidal core, and 3D-kagome core. [PERSON] et al. [63] investigated an octahedral type of core support for the sandwich panel. The authorscited the advantage of using this particular type of truss core that it has the potential of addressing the acoustic and mechanical properties of the sandwich structure not only in one direction but also in the vertical direction, i.e., two orthogonal directions. The authors also investigated the effects of boundary conditions, core thickness, inclination angle and material parameters on the STL of the octahedral structure and observed significant influence of these parameters on the transmission loss. The tetrahedral, pyramidal and 3D-kagome cores were classified as truss cores by [PERSON] et al. [64]. They performed numerical study on the STL of these truss cores and verified the finite element approximation with a theoretical solution. The effects of various parameters (i.e., truss core radius, height, damping loss factor, external mean flow and material combination types) on the STL of the different truss cores were investigated.