Often, we have to study problems where the structure interacts with a surrounding fluid. The importance of this interaction depends on the fluid nature and on the structure of interest. A simple criterion to characterize the amount of coupling for a plate-like structure is given by (Lesueur 1988)

β ρ

h =



• hs the characteristic thickness of the structure • ρs the density of the structure • ρ0 the density of the fluid • c0 the sound speed in the fluid • ωc the characteristic frequency of the structure (critical frequency in

the case of a plate).