# Comparative Study of Sound Transmission Losses of Sandwich Composite Double Panel Walls
## 3 Double Panel Air Cavity, Sandwich and Composite Structures
### Sandwich Composite Structures
#### 3.3.1 Composite Flat Panels  
The influence of various parameters on the STL of flat sandwich composite structures as depicted in Figure 5 has been investigated by a good number of researchers [64; 68; 69; 70; 71]. In recent times, [PERSON] et al. [72] performed an analytical solution of the STL of flat sandwich composite structures by expressing the STL as the mode shape superposition. The authors also studied how certain parameters such as the angle of elevation, temperature and azimuth of incident sound influenced the STL. In their findings, it was observed that as the temperature increased, the peaks and dips tend to decrease and flow towards the lower frequency range. In addition, with increased elevation angle, the STL decreased while the effect of the azimuth angle is noticed especially at high frequency region. Further, [PERSON] et al. [73] performed an analysis of sandwich plates whose panels were made of flat composite materials and supported by pyramidal truss core. In their study, they investigated the effect of incident angle, geometrical shape and material properties on the STL of the sandwich composite structures. Their results showed that as the elevation angle of the truss increased, the STL curve became smoother. They also found that the stacking geometry had little effect on the STL in the low frequency region but greater effect on the STL in the high frequency region. In addition, it was observed that altering the mechanical properties of the plates or their structural size will shift and depress their resonance frequency; and when the modulus was also altered, a depression in the extreme values were noticed. An experimental investigation of the vibro-acoustic response of flat, thin and thick sandwich composite panels was performed by [PERSON] and [PERSON] [74]. The general laminate model and the equivalent orthotropic panel model were used to predict the STL of the structures. It was found that while the equivalent orthotropic panel model was able to predict the STL perfectly well with the experiment, the general laminate model unexpectedly underestimated the STL due to the possibility of damping effect at coincidence frequency region. The STL due to laminates layup effects of flat composite panels and stiffeners effect between the panels were investigated by [PERSON] et al. [75]. The authors utilized four composite material layers to demonstrate these effects and observed that the STL curve were noticeable varied with the different laminate layup at middle and high frequency regions. Moreover, the STL results were largely dependent of each coincidence frequency of the laminate layup. By altering the laminate layup, the stiffness of the sandwich composite plates were also altered and the position of the coincidence frequencies were shifted. However, in the low frequency region, the effects of laminate layup on the STL were insignificant due to mass inertia of the stiffened structure. Moreover, at this region, the composite panels with stiffened core produced higher STL than those with air cavity owing to mass law effect. However, with increased frequencies, the composite panels were seen to transmit more sound energy than those with air cavity which in turn resulted into lower STL along the mid and high frequency regions. As demonstrated by [PERSON] et al. [76], it was also shown that the addition of ribs on the radiating face-sheet of flat composite panel has the tendency to lower the STL of the sandwich composite structure. However, in applications where the need arises to include stiffeners on the face-panel of the flat composite structure, it was demonstrated by [PERSON] et al. [75] that by meticulous spacing of the stiffener, the STL can be improved.