Two new symmetric variational principles for “acoustoelastic” fluid-structure interaction problems are first derived, with simultaneous use of three variables: the structural displacements and accelerations and the fluid pressure. Thus, a total of four three-field symmetric formulations is obtained. As is then shown, these formulations can be reduced to four two-field symmetric variational principles. Each of them can lead to a single-field “limit case” formulation, which is the formulation of the “in vaccum” structure or of the “acoustic” fluid plus a frequency independent term, expressing an “added” property in its general form: the “incompressible” fluid (“in vacuum” structure plus “added mass”), the “hypercompressible” fluid (“in vacuum” structure plus “added stiffness”), the “hyperlight” structure (“acoustic” fluid plus “added compressibility”) and the “hyperflexible” structure (“acoustic” fluid plus “added rigidity”). Combinations of the above “limit case” formulations are also of interest. The impact of these formulations on the construction of modal methods is significant. A large number of methods can be constructed, since the modes of the “in vacuum”, of the “acoustic” and of the several “limit case” eigenproblems can provide basis functions for the structure and for the fluid variables, not only in the formulations of the full problem, but also in the “limit case” formulations. To demonstrate this, some new methods are proposed for the solution of the full and of the “incompressible” eigenproblems. Then, the methods based on the non-symmetric or on the three-field symmetric formulations are shown to have disadvantages, compared to the methods based on the two-field symmetric formulations. The examples provided demonstrate analytical expressions of “added properties” and verify the efficiency or the disadvantages of the model methods described. Finally, in the Appendices, the free surface conditions and the damping forces are incorporated in the various formulations and “singular cases” are treated (e.g., free bodies, “wall bounded” fluids).