Scattering of sound from an infinite plate is significantly altered by the presence of an impedance discontinuity. Here, an approximate analytical solution is developed for the scattering of sound from a three-dimensional protuberance on a thin, infinite, submerged elastic plate. The protuberance is modeled as a solid elastic sphere of dimensions comparable to the incident acoustic wave[ka∼O(1)],and is attached to the plate along its equatorial plane. The plate is modeled using thin plate theory and supports the lowest-order flexural, compressional, and horizontally polarized shear(SH)modes, while the solid sphere is modeled with the full three-dimensional elastodynamic equations of motion. This idealized environmental model provides an understanding of the underlying physics of 3-D scattering, which is shown to be strongly frequency dependent. The analysis demonstrates that the attachment of the plate manifests itself in the scattered field in a frequency selective manner, and that the resonances of the elastic modes in the sphere leave their imprint on the elastic waves coupled into the plate. It is also shown that since the attached plate primarily restrains the motions of the sphere in the horizontal plane, the dominant contribution to the scattered field arises from the in-plane coupling forces.