China and USA in race to submarine sonar stealth

Penn State University researchers have made progress on shielding large objects from sound. Chinese researchers have also published substantial progress on cloaking from sound. Both might lead to submarine sonar stealth.
Chinese researchers presented the design, architecture and detailed performance of a three-dimensional (3D) underwater acoustic carpet cloak (UACC). The proposed system of the 3D UACC is an octahedral pyramid which is composed of periodical steel strips. This underwater acoustic device, placed over the target to hide, is able to manipulate the scattered wavefront to mimic a reflecting plane. The effectiveness of the prototype is experimentally demonstrated in an anechoic tank. The measured acoustic pressure distributions show that the 3D UACC can work in all directions in a wide frequency range. This experimental verification of 3D device paves the way for guidelines on future practical applications.
They created realize a 3D UACC which can mimic a reflecting plane and hide the information of the 3D target. The required parameters, which contain anisotropic mass densities and isotropic bulk modulus, are obtained through steel strips array surrounded by water. The subwavelength steel-water unit cell can be regarded as fluid in long wavelength regime. Meanwhile, it ensures the carpet cloak as a broadband device. Then the performance of the carpet cloak is assessed experimentally by measuring the acoustic pressure fields in an anechoic tank. The results confirm the expected behavior and demonstrate the effectiveness and omnidirectionality of the designed 3D UACC. They believe that the verification of 3D
UACC gives more direct guidance on the future practical applications of acoustic metamaterials.

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