  The interaction of an acoustic plane wave with a pair of plates connected by
periodically spaced stiffeners in water is considered. The rib-stiffened
structure is called a "flex-layer" because its low frequency response is
dominated by bending stiffness. The quasi-static behavior is equivalent a
homogeneous layer of compressible fluid, which we identify as air for the
purposes of comparison. In this way an air layer is acoustically the same as a
pair of thin elastic plates connected by a periodic spacing of ribs. At
discrete higher frequencies the flex-layer exhibits perfect acoustic
transmission, the cause of which is identified as fluid-loaded plate waves
propagating back and forth between the ribs. Both the low and finite frequency
behavior of the flex-layer are fully explained by closed-form solutions for
reflection and transmission. The analytical model is extended to two
flex-layers in series, introducing new low and high frequency phenomena that
are explained in terms of simple lumped parameter models.
