Theoretical And Experimental Study On Underwater Anechoic Coating

Underwater anechoic coating is crucial to submarine stealth technology and is a major part of quiet-submarine design. In this paper, the mechanism of resonant sound absorber is studied theoretically. Dynamic mechanical properties of sound-absorbing materials are measured by use of DMTA ( Dynamic Mechanical Thermal Analyzer) .The performances of two large-scale underwater anechoic coating samples with cavities inside are measured in an anechoic water tank by use of wide-band compressed signal.The submarine hull coated with anechoic coating is assumed to be planar multi-layered system in the theoretical analysis. A perforated rubber sheet with doubly periodic voids sandwiched between a metal base and a cove layer constitute the theoretical model. The metal base may be a single steel sheet as in the case of a single-hull submarine or two steel sheets with a water layer sandwiched between them as in the case of a double-hull submarine.The finite element approach is used here to study its performance. A unit cell of the doubly periodic structure including a part of the close fluid domain, is modeled using finite elements and a mesh is generated including elastic, fluid, and interface elements. This cell is limited by six planes, two of them being its boundaries with the two semi-infinite fluid domains (they both exist for a fluid loaded structure and only one exists for a structure with air closure) and the remaining four being related to the periodicity. On the first two boundaries, the finite element nodal expansion of the pressure field is matched with two plane-wave expansions of the same field, including propagating and evanescent contributions. Between the nodal values of the displacement and pressure fields for the other boundaries, Bloch-Floquet type relations are enforced, which are related to the periodicity and are incorporated as specific degrees of freedom condensation in the finite element formalism. The computer code implementing this method is composed.Harmonic properties and anechoic behavior of resonant anechoic coating are studied. Various cavity size, cavity shape, perforation ratio and material parameter concerns are analyzed.The measurement of material dynamic mechanical properties is made by use of DMTA. This indirect method relies on the time-temperature superposition principle and the subsequent shift factors required to construct a master curve over a broad frequency range from measurements taken at multiple temperatures.The performances of two large-scale anechoic coating samples are measured in an anechoic water tank. The measuring signal is compressed by use of signal processing techniques to get a relatively sharp impulse with flat spectrum over a wide frequency band. By use of such a measuring signal, edge-diffraction can be effectively avoided and the wide-band measurement can be finished at one time. Good agreement is achieved between measured reflection coefficient and that of FEM analysis.Some very interesting results are obtained through the numerical analysis by which the mechanism of resonant sound absorption may be better understood. The FEM theory and the computer code have good adaptability to various forms of cavity shape and distribution in the sound-absorbing material and can be used in the performance prediction of new-type anechoic coating.