Enhanced transmission loss through lattice-supported micro-membranes

Acoustic micro-membranes (AMMs) have theoretical advantages in terms of light weight yet high sound transmission loss (STL) at low frequency, but an actual medium- or large-scale specimen usually shows a significant performance degradation. Those lumped-parameter models developed so far lose their accuracy on theoretically describing the actual sound transmission loss of such an assembly. In this paper, the influence of the frame vibration is explored, and an acoustic theory based on a distributed parameter system is established. By coupling the vibration of the frame and the membranes, this theory shows how the acoustic impedance of the assembly depends on the number of membrane cells and the dimensions of the membranes and frame. As the assembly becomes larger, its global response is no longer dominated by the membrane but by the frame, while the latter determines the upper limit of the STL of the whole assembly. Compared to the well-developed lumped-parameter model that works only for small-scale AMMs, this work provides a theoretical foundation for designing larger size yet high loss AMMs, and may pave the way for their use in noise control engineering.