Coupling and attenuation of waves in plates by randomly distributed attached impedances

Coupling and attenuation of waves in plates by randomly distributed attached impedances" The average response of an infinite thin plate with statistically homogeneous attached random impedances is examined. The added impedances, which represent typical heterogeneities that might occur on complex shells, provide light coupling between the extensional, shear, and flexural waves. The mean plate response is formulated in terms of the Dyson equation which is solved within the assumptions of the first-order smoothing approximation, or Keller approximation, valid when the heterogeneities are weak. Scattering attenuations are derived for each propagation mode. It is shown that the attenuation of one wave type due to coupling to another is proportional to the modal density of the other wave type. Thus the attenuation of extensional and shear waves is predominantly due to mode conversion into flexural waves and is proportional to the large modal density of flexural waves. The flexural degrees of freedom serve as a sink for the energy of the membrane modes and constitute for them an effective fuzzy structure. The specific case of delta-correlated springs is considered for purposes of illustration.