A study of the sound transmission mechanisms of a finite thickness opening without or with an acoustic seal

This work is concerned with the transmission of sound through an opening in a wall of finite thickness where the opening may be filled with an acoustic seal represented by a pair of mass layers. Unlike past work where efforts were made on either developing a prediction model or investigating the influence of opening parameters on the sound transmission performance, the work presented here aims at studying the coupling mechanism of the system by considering the fluid-loaded opening as a resonant system. This treatment allows a better understanding of the system property at both resonance and off-resonance frequencies. It is found that, unlike that for the opening of vanishing thickness, the transmission loss for an opening of finite thickness exhibits obvious oscillation with maxima and minima in the frequency spectrum. The latter are associated with the resonance of the fluid-loaded opening. At resonance, the opening extracts the acoustic energy from a region much larger than its physical dimension, causing more energy to transmit through the opening. As a result, negative value of transmission loss occurs. At frequencies off the resonance, the way in which energy flows into the opening is found to be dependent upon the reactance of the wave impedance of the fluid-loaded system. For a sealed opening, the reactance of the wave impedance is greatly increased due to the large surface density of the mass layers of the acoustic seal, resulting in less energy injected into and thereby transmitted through the opening. The underlying physics leading to the improvement in sound insulation is sought by using a wave impedance expression from which equivalent system representation is derived in different frequency ranges.