Browsing by Subject "Transformation acoustics"

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Characterization of underwater acoustic metamaterials inspired by transformation acoustics
(2021-08-16) Cushing, Colby Walker; Haberman, Michael R. (Michael Richard), 1977-; Wilson, Preston S.; Norris, Andrew N; Cummer, Steven A; Hamilton, Mark F
Underwater acoustic metamaterials (AMMs) use engineered subwavelength structures to produce novel effective material properties to control water-borne acoustic wave propagation. Transformation Acoustics (TA) is a mathematical construct that is used to specify the material properties that are necessary to achieve exotic manipulation of acoustic fields, and as such, AMMs can be the means through which physical realization of these devices can occur. An acoustic cloak is one possible device that can be designed using TA, but it is one that has garnered the most attention in academic studies due to the multi-faceted complexity of designing, fabricating, and experimentally validating performance. For an object to be indistinguishable from the background medium, an acoustic cloak requires either anisotropic effective dynamic density or anisotropic effective dynamic stiffness, but further requires these properties to be spatially dependent. Devices predicted by TA, such as cloaks, can be expensive and difficult to fabricate due to the geometric complexities of the substructures required to realize the required material properties. In some cases, state-of-the-art additive manufacturing methods are the only means of fabricating such devices. Devices predicted by TA are also difficult to characterize because of their frequency dependent dynamic behavior. Further, since the AMMs of interest in this work operate primarily at low frequencies, freefield methods are not practical. Therefore, this work investigated the characterization of select underwater AMMs predicted by TA using new and existing experimental apparatuses and techniques while evaluating the applicability of different fabrication processes to physically realize testable samples. Three materials were investigated in this work: (i) a layered AMM comprised of aluminum, air-filled, honeycomb panels coupled to one another by compliant rubber rods demonstrating anisotropic effective dynamic density, (ii) a hexagonal elastic lattice comprised of bimode unit cells that exhibits a focusing beam pattern in an underwater acoustic field by exploiting spatially varying density and stiffness, and (iii) an additively manufactured, titanium, diamond-shaped, elastic pentamode lattice that demonstrates anisotropic sound speeds due to anisotropic effective dynamic stiffness. Significant effort was expended on experimental validation of the AMM's acoustic properties, but this work also explored the relationships between homogenization theory, numerical simulation, and fabrication methods, in order achieve more robust understanding of the underwater acoustic AMMs studied here.