This work studies wave transmission along a fluid-loaded membrane supported by an array of ribs, one of which is driven harmonically. The influence of realistic rib dynamics is considered, including finite mechanical impedance as well as spring and damping forces. Under heavy fluid loading, when the coupling between ribs is dominated by the surface wavefield in the fluid, the system exhibits the usual stop/pass band structure. It is shown that there exists a critical rib mass, depending on drive frequency, at which the disturbance becomes completely localized at the driven rib. For frequencies at a band edge, the solution at the critical mass is no longer unique; in this parameter region, the solution is highly sensitive to small changes in frequency or rib properties. Varying the parameters in different ways gives rise to different limiting behavior at the critical points. Numerical simulations have been carried out to study this system. These simulations confirm the analytical predictions and help to determine when the surface wave approximation breaks down.