Underwater sound radiation by a layered infinite elastic plate with discontinuity
The underwater sound radiation from layered structu re is determined by the properties of elastic waves in the structure and by interaction between structural/structur al waves and between structural/fluidal waves. In t his paper, a model of a layered infinite plate is used to addres s how the properties and interaction influence the near field and far field characteristics of the radiated sound. In par ticular, the effect of structural discontinuity int roduced to the layered plate by a finite signal conditioning plate on unde rwater sound radiation is investigated. The scatter ing of structural waves in the layered plate by the signal conditioni ng plate is used to explain the changes in the radi ated sound due to the discontinuity.
Full numerical solution for the far-field and near-field scattering from a fluid-loaded elastic plate with distributed mass or stiffness inhomogeneity
The scattering of a plane acoustic wave by a fluid-loaded thin elastic plate of infinite extent with a distributed mass or stiffness inhomogeneity is investigated. This paper is a follow up to previous work done by the same authors on the scattering from a fluid-loaded plate with a distributed mass inhomogeneity. In this paper both stiffness and mass distributed inhomogeneities are considered and a complete description of the full numerical solution is presented. Furthermore, both near-field and far-field scattering results are presented in this paper. The presence of the distributed inhomogeneity modifies the wave number transform of the equation of motion of the fluid-loaded plate to a Fredholm integral equation. This integral equation has singularities at the roots of the dispersion equation. To obtain a complete numerical solution of the Fredholm integral equation, a singularity subtraction technique is used, which is similar in essence to the hybrid analytic/numerical approach for the solution of the...
Aspects of the reflexion and free wave properties of a composite panel under fluid loading
Abstract The response of a composite panel to external forcing, with inclusion of fluid loading effects, is considered. Of the two strata comprising the composite panel one, backed by a vacuum, is of the conventional thin elastic plate or membrane kind, while the other, in contact with the fluid, is more like an elastic solid and may suffer significant compression. The behaviour of the acoustic field close to grazing incidence is examined, this behaviour being determined by that of the plane wave reflexion coefficient. In the absence of the upper stratum it is well known that the reflexion coefficient has the value −1 around grazing incidence, so that direct and reflected fields from an external source cancel and preclude the propagation of a genuine acoustic field over the surface (a situation known in optics as the “Lloyd's mirror” effect). It is shown that, at any given frequency, an impedance for the upper stratum can be prescribed which will lead to the value +1 for the grazing incidence reflexion coefficient, and will thus obviate the severe power loss which would otherwise occur in directions close to the surface. Next the free subsonic surface waves which can exist in the coupled three-part (two-layer panel plus fluid) system are examined. It is shown analytically that, if the upper layer has low impedance controlled by stiffness forces, a new surface wave can exist in the system. This wave essentially involves the stiffness of the upper layer and the mass of the fluid, and has a wavenumber much higher than that of the surface wave in a single conventional panel. It is also shown from numerical studies that two subsonic surface waves will exist over quite a wide range of parameters, though not necessarily with the wide wavenumber separation of the low impedance case. A discussion is given of the possible importance of the high wavenumber mode in the case of excitation by high wavenumber boundary layer turbulence, and of the significance of two free subsonic surface wave modes in calculations of energy transmission over composite panels of the kind modelled here.