Analysis of acoustic cloaks for anti-sonar camouflage based on local resonance in acoustic metamaterials

Abstract

Non-visual camouflage plays a significant role in the art of military deception. One of the fields of interest is auditory camouflage, where the goal is to remove the acoustic signature of an object, whether it is generated by the object itself or scattered from a surveillance device like sonar. A recently proposed approach to auditory camouflage is acoustic cloaking, where the object is made 'invisible' in acoustic sense by surrounding it with a cloak of acoustic metamaterial. Acoustic metamaterial is basically an artificial structure tailored to enable control of acoustic wave dispersion through Bragg scattering, where the features of the structure have subwavelength dimensions. The operation of an acoustic cloak is based on negative effective dynamic mass and bulk modulus which can be obtained by local resonances. This leads to a possibility to fully tailor the path of acoustic waves (infrasound, audible waves or ultrasound) around the camouflaged object, effectively enabling one to make waves avoid the object and render it invisible. In this contribution we perform a full finite element modeling of the elements of an acoustic cloak, analyze it and consider coordinate transformation necessary to ensure acoustic concealment of a macroscopic object. We investigated spatial distribution of acoustic waves for two different scatterers, one of them being a cylindrical object with circular basis, another one a cylinder with elliptical basis. All our calculations were performed for a realistic sea water medium, modeled by an empirical formula. We considered the frequency dispersion of the acoustic field in different spectral ranges, from infrasound to audible frequencies. For elliptic cloaks we applied a very simple approach that nevertheless furnished better acoustic cloaking than some more complex layered profiles previously published.