Acoustic cloaking the ability to direct sound completely around an object is an exciting possibility. Potential military applications include making underwater objects invisible to threat sonar and creating quiet zones where, for example, otherwise noisy manned underwater operations can be safely carried out or where battlefield noise can be contained. Commercial applications are almost limitless. Metamaterials have been well described in the literature and some electromagnetic prototypes have been demonstrated. The key to making an acoustic metamaterial is anisotropy of its effective material properties. Very recently, designs for underwater spherical and cylindrical cloaks have been developed requiring that the effective density of a material be described as a tensor, that is, that it have two different values depending on direction. In this proposed project, a means to construct metamaterials with this property using very simple, inexpensive and rugged dynamical elements commonly used for vibration reduction will be demonstrated. A promising naval application will be identified and a suitable metamaterial and cloak geometry will be designed for it. A test model representative of the metamaterial will be built and tested in air as a proof of principle. This model will be tested in water in the Phase I option.
Benefit: Acoustical cloaking could be a disruptive technology. Among its functionalities will be rendering an object invisible to sonar or other active means of acoustical detection, creation of quiet zones, and the ability to control the acoustics of spaces in novel ways. Little needs to be said about the first item. Stealth has always been a major component of U.S. underwater supremacy and this additional tool can only increase our capabilities. The ability to create quiet zones by directing sound around an object can be used to create more private and enjoyable environments and safer ones where noise levels are dangerously high, such as work environments of underwater divers, aircraft carrier deck hands, etc. Architecturally, the ability to cloak objects will eliminate them from consideration in acoustical terms. For example, cloaking could eliminate reflections from objects (e.g. pillars) in performance spaces. In short, the possibilities in military and civilian markets are potentially endless.
Keywords: reflectivity, reflectivity, target strength reduction, Noise control, acoustic metamaterial, Sonar, acoustic cloak
