TARGET COVERED WITH OPTIMIZED ACOUSTIC PAVING FOR DISCRETION AND STEALTH – CLEOPATRE

Acoustic discretion and stealth are major problems in underwater acoustics defense for next-generation ships, but the more general problematic of noise control also concerns the air domain, for example for planes, railway cars or engineering structures such as metallic bridges. The CLEOPATRE project is following the ANR ASTRID RAMSES project (Acoustic radiation engineered by resonant systems) in which the stiffeners inside the shell are geometrically modified and their distribution is optimized with the aim of jaming the acoustic response both in discretion and in stealth at low and very low frequencies. However, in conjunction with naval architects, the modifications tolerated for the stiffeners are too small to expect significant modifications in the acoustic responses. Therefore, the CLEOPATRE project aims to capitalize on previous developments to go further with more realistic geometries and propose solutions that do not impact naval architecture by focusing on the treatment of surfaces. Clearly, discretion and stealth are conventionally treated by coating the structure with layers of specific acoustic materials. Unfortunately, these two functions are not fulfilled by the same materials, which induces an additional complexity: generally the surfaces of the targets have specific treatments in specific regions according to the desired function. The CLEOPATRE project offers a solution under the form of a pavement of different materials, or even metamaterials, the distribution of which being optimized using the tools that will be developed for this purpose. Thus, we can see this approach as the use of a metamaterial of metamaterials, or metamaterial at two scales (the tile and the arrangement of the tiles) in order to optimize noise reduction whether radiated or diffracted. The analytical and numerical models developed will be used to design and optimize plates presenting controlled acoustic responses corresponding to realistic configurations and faithful to defense concerns. Six plates equipped with stiffeners and specific scale tiles will be manufactured and tested, in connection with targeted functions. The project will also propose to adapt the proposed solutions to periodically stiffened cylindrical shells as well as to the use of tiles made from metamaterials to extend the range of possibilities.