“…The OR for the "drum-like" silencer [22,23] of figure 2 (b) is similar in structure: (18) in this case, of course, every occurrence of Y m (a) is replaced by Y m (d) since the membrane lies along y = d. Hydrodynamic applications that involve layered fluids [24,25] and submersed plates [26] have also been addressed. Interestingly, the article by Mohapatra et al [25] considers a boundary value problem which is very similar to that of [1,2] and uses the same OR. Further extensions include the development of the theory for wave propagation in 3D waveguides with flexible boundaries [27]- [28] and porous linings [29].…”