# Comparative Study of Sound Transmission Losses of Sandwich Composite Double Panel Walls
## 3 Double Panel Air Cavity, Sandwich and Composite Structures
### Double Panel Air Cavity Structures  
Double panels with enclosed air cavity may be regarded as the simplest or pseudo form of an equivalent sandwich structures for vibro-acoustic application. Transmission loss results obtained from this equivalent double panel structure can be used as a basis for comparison of the STL results obtained from sandwich core structures. The enclosed air cavity between the two panels allows the transmission of sounds from one panel to the other and has been described to be equivalent to a mechanical spring [49]. It is worthwhile to note that the use of mechanical spring between the panels of the enclosed air cavity or the mass-air-mass (MAM) region of the DPS, reduces their STL especially in the  
Figure 1: Typical representation of sound transmission loss (STL) curve with different frequency characteristics controlled regions for (**a**) flat or rectangular double panel structures (DPS) (**b**) cylindrical or curved DPS. mid and high frequency region. To validate this statement, the authors of the present study performed a finite element simulation of double panel walls with enclosed air cavity of density \(\rho_{a}=1.21\) kg/m\({}^{3}\) and speed of sound \(c_{a}=340\) m/s. Each panel is simply supported with length \(a=350\) mm, width \(b=220\) mm and thickness \(t=1\) mm. The density, Poisson ratio and Young's modulus of the panels are 2814 kg/m\({}^{3}\), 0.33 and 71 GPa, respectively. The effects of spring and damper mechanisms are investigated and the results are compared with double panels without spring and damper effects as given in the finite element results of Figure 2a,b. With spring mechanism connected to the double panels, it is evident that the vibro-acoustic response deteriorates and the response is almost similar to the double panels with enclosed air cavity as shown in Figure 2a. Moreover, a careful inspection of the obtained result shows that at a particular frequency close to the resonance frequency, for the DPS with enclosed air cavity, the two panels move in opposite phase which results in a mass-air-mass resonance \((f_{MAM})\) as shown in Figure 2c. In addition, at resonance dip frequency, the STL of the MAM mechanism is higher than the mass-spring-mass (MSM) double panel mechanism as seen in the STL curve of Figure 3a. In addition, before the resonance dip frequency, it is seen that the STL of the two mechanical structures follow the same trend. The introduction of spring, therefore, increases the stiffness in the enclosed air cavity region. Thus, the result shows that an increment in the stiffness of the cavity or core of DPS undermines the effectiveness of the structure to reduce noise. To improve the STL of DPS with spring connection in the low frequency region, it is necessary to convert the structure to a mass-spring-damper (MSD) system [49]. Figure 3b shows the STL comparison between the MSD system and those obtained from MAM system. It is evident that at some low frequency range, the STL of the MSD system was increased by more than ten percent of the MAM and MSM systems. However, in the mid-high frequency zones (i.e., 900-1000 Hz), there is no improvement of the STL of MSD system and therefore, it can be seen to behave like the air cavity and spring connected DPS. Figure 2: Finite element simulation results of double panel enclosed air cavity structure (**a**) first vibration mode of mass–air–mass (MAM) system (**b**) first vibration mode of mass–spring–damper (MSD) system (**c**) vibration mode at MAM resonance. Figure 3: Result comparison of double panels and their corresponding finite element mesh configurations for the enclosed air cavity of (**a**) MAM and mass–spring–mass (MSM), (**b**) MAM and MSD systems. A few research works have been conducted to investigate the STL of DPS with enclosed air cavity [50; 51]. [PERSON] et al. [51] compared DPS with air cavity and sandwich structure with corrugated core.