Panel measurement method for the determination of scattering coefficients and edge diffraction in an open water tank.

Material effective properties of composite media or their acoustic performance are often required to be experimentally determined or verified. For underwater applications, systems can be characterized in an open water tank in which a conventional measurement configuration consists of the panel, an acoustic source, and two hydrophones, one on either side of the panel. However, panel measurements may be greatly complicated by waves diffracted by the panel edges. The three-point method is a technique that decomposes total pressure into contributions of the incident, reflected, transmitted, and edge-diffracted pressures. These contributions are determined using measurements at three positions. Reflection and transmission coefficients can then be obtained by removing the contribution of the edge-diffracted waves. The three-point method is applied to an aluminum reference panel for which the experimental scattering coefficients are in very good agreement with those of an infinite panel. The validity limits of the method are also studied. Finally, a pressure mapping shows that the three-point method leads to an accurate determination of the reflection and transmission coefficients for different positions despite strong variations of the edge-diffracted pressure.

Scattering from a finite ribbed plate with attached oscillators.

Double layer structures consisting of stainless steel and polymer rods are designed to blur and attenuate Bragg and Bloch-Floquet scattering from a periodically ribbed plate in a given frequency bandwidth. These structures can be considered as ribbed plate-spring-mass systems, the resonance frequencies of which are obtained from random and circular permutations of five basic oscillators. Analytical and finite element methods are used to find their parameters and tank tests have been carried out to ensure the accuracy of the numerical results and validate the relevance of such a model. It has been found that the typical features associated with the periodicity of the ribs are replaced by a "Christmas tree" appearance when the far field pressure backscattered from the model is displayed in the (frequency, aspect angle) plane. An analysis of backscattering patterns is presented and comparisons with Naval Research Laboratory results are made.