# Comparative Study of Sound Transmission Losses of Sandwich Composite Double Panel Walls
## 5 Parameter Effects and Optimization Strategy on the Sound Transmission Loss of Double Panel Walls  
### Influence of Various Parameters on the Sound Transmission Loss of Double Wall Structures  
Parameter effects on the STL of acoustic panels/shells have been used by most authors to determine their noise absorbing efficiencies. For example, [PERSON] et al. [64] obtained interesting results of various parameter effects on the STL of sandwich truss cores. The authors expanded a full analysis of some parameter effects on the STL of three truss cores which included the pyramidal, 3D-kagome and tetrahedral cores. The pyramidal truss core was reported to give the best STL and was used as a standard for investigating the influence of other parameters. This is because, the STL of the pyramidal truss core at the dip and peak frequencies were higher than the 3D-kagome and tetrahedral cores. Structural parameters such as different damping loss factors and different Mach numbers; as well as geometrical parameters such as different core radii and different core heights were studied. First, different material composition types of the flat panels were compared. From the result of their findings, the pyramidal core gave the highest STL over other truss core types at all dip frequencies and its curves shifted to lower frequencies. With an increasing damping ratio, the authors reported no significant increase in STL at all frequencies except at the resonance dip frequencies. At these dip frequencies, the dips flattened and moved upward with increasing damping ratio. On the contrary, the effect of different Mach numbers at all resonance dip frequencies was insignificant; however, there were significant differences in STL at the non-resonance frequency (NRF) regions. At these NRF regions, the STL increased with decreasing Mach numbers. In addition, different material compositions of the sandwich panels resulted in various magnitude of STLs in the resonance or non-resonance frequency regions. With different core radii, the frequency curves shifted to lower frequencies but the STL increased with increasing radius at the NRF regions. Finally, the STL increased at dip frequencies with increasing core and the frequency curves was shifted to higher frequencies. Apart from the parameter effects on the STL of the acoustic structure performed by [PERSON] et al. [64], a number of parameters have also been tested by different researchers. They can be grouped into geometrical parameters, structural parameters, material parameters and orientated parameters, as represented in Table 2. The table gives a quick overview of the parameter effects on STL studied by researchers in recent times. For example, in the structural parameter group, different sandwich enclosed gas cavity types were investigated by [PERSON] and [PERSON] [86]. The six enclosed gas cavity types were oxygen, air, argon, neon, helium and hydrogen. Their findings evidently depicted a significant effect of STL of the different cavity gases used especially in the low frequency region. While hydrogen and helium gases showed the highest STL at this region, the air and oxygen cavity gas types showed the least STL. The reason for this was attributed to the fact that the speeds of sound in hydrogen and helium gases are higher than the speed of sound in air. In the same vein, [PERSON] and [PERSON] [46] investigated the effects of these gases on the STL of cylindrical shell. Their results verified those performed by [86] in that, the hydrogen and helium gases produced the highest STL results than other gases in the low and mid frequency domains. To further explain the grouping of Table 2, some orientated parameters such as the incident angles and stacking sequences were investigated by [PERSON] et al. [69]. With increased incident angles, the STL in the low and high frequency range decreased while there is no change in STL in the mid frequency mass controlled region.