# Comparative Study of Sound Transmission Losses of Sandwich Composite Double Panel Walls
## 6 Summary and Conclusions  
A critical overview of the sound transmission loss of sandwich and composite double panel configurations investigated by several contemporary researchers has been presented in this review article. The STL as a vibro-acoustic index has been used to measure the potential of materials to absorb sounds transmitted through them. Single and double panel structures made from different materials have been used as noise reduction device in many novel engineering applications such as in the transportation, building and construction industries. However, double wall structures have gained popularity over their single panel counterparts owing to their wider applications and better transmission loss behavior. The vibro-acoustic characteristics, especially the STL of sandwich and composite double panel structures have been studied by an appreciable number of researchers. The main goal is to reduce the amount of sounds transmitted in such materials by investigating various parameter effects on their STL. In this review, the influence of these parameters on the STL of sandwich and composite double panel structures under acoustic effect has been critically examined. The different controlled regions in the STL versus frequency curve have been used to explain the effectiveness of the material to reduce transmitted sound waves. This review has attempted to classify the parameter effects on the STL as geometrical, material, structural and orientated parameters. It is revealed that geometrical parameters such as increasing plate/shell thicknesses, core heights, core radii, plate sizes, finite and infinite dimensions of the panels etc., have significant influence on the STL. The material parameters such as increasing Poisson ratios, shear modulus, damping loss factors etc., however, have little or no effects on the STL. Orientated parameters such as incident, azimuthal, inclination angles, stacking sequence etc., have varying significant effects on the STL of the structure. The structural parameters which include core types, metal or composite plate/shell types etc., have been shown to have great influence on the STL. This indicates more prospects for future research in adopting different material combination for enhancing STL. This idea, for example, was demonstrated by [PERSON] et al. [149] who attempted to combine the advantages of FGM carbon nanotubes by making them laminated composites sandwich plates. Their salient findings indicate an increase in the STL of this kind of material compared to the conventional ones. In spite of the promising prospects of enhancing the STL by using the FGM, the investigation of this type of material, however, is still in its nascent stage. Therefore, research efforts are needed to investigate more of the potentials of using this material type. In addition, the geometrical configuration of sandwich and composite DPS that have been mostly adopted for STL application are rectangular flat plates/shells and cylindrical or curved shells. More studies should be carried out to analyze and compare these topological effects on their STL using the same dimensional and parametric conditions. However, [PERSON] et al. [70] investigated the effects of three core topologies such as cellular core, trapezoidal core and triangular core, on their STL in the low frequency domain. In their results, it was observed that the STL was largely influenced with the different core topologies. Therefore, a quantifiable approach and shape optimization procedure can be performed to calculate the optimal STL of various geometrical shapes of both the plates and core. Quite a good number of researchers have endeavored to use analytical and experimental methods to model and validate, respectively the STL of flat and cylindrical double panel shells/plates. The analytical procedures give the exact solutions while the measured experimental results give the actual solution of the physical problem.