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Passive Acoustic Subwavelength Resonator (PASR)


OUSD (R&E) MODERNIZATION PRIORITY: Microelectronics, Networked Command, Control and Communications TECHNOLOGY AREA(S): Materials/Processes, Sensors OBJECTIVE: The objective of this effort is to develop a passive, resonant acoustic scattering architecture for undersea operations that features resonances that are robust to changes in static pressure with depth, that are tunable in frequency in-situ, and that features a compact geometry that is deeply subwavelength compared to its resonant wavelength(s) in water. A secondary objective is to determine the feasibility of tailoring the scattered acoustic field to achieve patterned directionality at resonance. DESCRIPTION: Acoustic resonance occurs when the frequency of an acoustic field matches the natural frequency of vibration within a patterned geometric structure and becomes amplified. Research in the undersea domain has long focused on the acoustic enhancement of, suppression of, and/or coupling to vibrational modes within both man-made and naturally occurring structures. Subwavelength resonance occurs when the physical size of a resonant structure is smaller than the wavelength of an acoustic field in the medium that surrounds the structure. Minnaert resonance, where a gas bubble suspended in a liquid resonates at its natural frequency, is a prominent example of a naturally occurring subwavelength resonator in undersea environments [1]. Recent research in acoustic metamaterials, which often rely on subwavelength resonances in structured lattices [2-4], has led to breakthroughs impacting a broad range of device applications– however, much of this research has focused on airborne acoustics owing to the ease of fabrication and testing in air. Undersea environments present a unique set of challenges compared to air acoustics. Whereas many structured components can be assumed to be acoustically rigid in air, water has a lower impedance contrast with most elastic materials resulting in stronger acoustic coupling with the environment. Furthermore, in situations where a deployed system must operate over a range of depths, the functionality of the system must withstand and/or adapt to changes in static pressure. Given that resonances are typically dependent on the geometry of a structure, any geometric change under static load would be expected to alter or degrade the vibrational modes of the structure. Recently, piezoelectric metamaterials have been considered as a means of overcoming some of these challenges by providing ultra-wideband backscatter in aqueous environments [5-6]. However, these devices have not yet been optimized for compactness for a given resonant scattering response nor have they been made directionally tunable. This effort seeks to develop deeply subwavelength, resonant structures that scatter sound in undersea environments in a controlled and predictable fashion over a range of operational depths. Such structures should respond passively to externally impinging acoustic fields, and not simply be internally resonant in response to an on-board acoustic source. The resonant spectra should be tunable in-situ over a specified bandwidth with the goal of minimizing power requirements. The spectral response should be robust to changes in static pressure over a wide range of depths. In addition, this effort will investigate the feasibility of tailoring the resonant scattering to radiate directed acoustic beams in response to an external acoustic impinging field. As with the spectral response, the possibility of altering the directed field pattern in-situ should be investigated. Ultimately, the ideal deliverable of this effort should be compact, passive resonators that can be deployed within a range of undersea scenarios, and that maintain a consistent yet tunable scattering response over a broad range of ocean depth. PHASE I: Successful proposals for Phase I should principally address three key aspects of the program goals: (1) how the subwavelength resonance will be obtained in the structured geometry; (2) to what degree such resonances can be modulated in amplitude and frequency with optimal power efficiency; and (3) to what degree such resonances can be made insensitive to changes in static pressure when deployed at sea over a range of depths. Successful applicants should demonstrate in-depth knowledge in both aqueous resonant techniques and undersea deployed systems. Phase 1 will be research focused with a goal of demonstrating the resonant technology in a simulated environment using fully rendered designs. Experimental assessments of resonator components may also be necessary to demonstrate a proof of concept. Schedule/Milestones/Deliverables During Phase I of the effort the following deliverables should be included: • Month 1: Kickoff meeting and presentation • Month 3: Acoustic scattering models of the fully rendered resonant structure in a simulated aqueous environment to demonstrate the feasibility of the approach, assessments of the degree of spectral tunability; monthly reports and quarterly updated • Month 4: Experimental assessments of key components that produce the resonant functionality; monthly reports • Month 6: Final report that includes technical details of the project including a section addressing the possibility of achieving directed scattering using the chosen resonant methodology; monthly reports and quarterly update PHASE II: Upon successful completion of Phase I, in Phase II successful proposers will fabricate a fully functional prototype that will be tested in an aqueous environment. Insensitivity to static pressure will also be demonstrated, either in a pressure tank or through acoustic testing at a non-trivial depth. Assessments of the degree of scattering directivity will also be undertaken. Although the specific schedule of deliverables may depend on the chosen approach, the schedule/milestones could proceed as follows: • Month 3: Characterization of functional components, design iteration and modeling based on component results, assembly of initial prototype, modeling of designs with patterned or directed scattering. Monthly reports and quarterly update. • Month 6: Finalize initial prototype fabrication, acoustic testing in a water tank or deployed environment, experimental analysis of spectral response, narrow down design with patterned or directed scattering. Monthly reports and quarterly update. • Month 9: Iteration of prototype design based on initial results, assessment of spectral tunability and power requirements, pressure testing, fabrication of directed scattering design. Monthly reports and quarterly update. • Month 12: Fabrication and acoustic testing of improved and/or directed scattering designs, pressure testing, modeling assessments of improved performance metrics such as pressure insensitivity and ratio of component size to acoustic wavelength. Monthly and final reports. PHASE III DUAL USE APPLICATIONS: (U) There are many commercial uses for a passive acoustic subwavelength resonator (PASR) that could be explored in a Phase 3 effort. PASR technology could be used as low SWAP fiducials for underwater position, navigation, and timing (PNT) of autonomous vehicles doing deep water missions such as those commonly done in the oil and gas industry. Although the effort is aimed at aqueous environments, the technology may also be extended to air acoustics and used in wearable devices for augmented reality applications. Devices of this type could also offer a next generation capability for non-destructive testing by augmenting higher SWAP-C transmit arrays with passive resonators. REFERENCES: 1. [1] Greene, Chad A., and Preston S. Wilson. "Laboratory investigation of a passive acoustic method for measurement of underwater gas seep ebullition." The Journal of the Acoustical Society of America 131.1 (2012): EL61-EL66. 2. [2] Martin, Theodore P., et al. "Transparent gradient-index lens for underwater sound based on phase advance." Physical Review Applied 4.3 (2015): 034003. 3. [3] Martin, Theodore P., et al. "Elastic shells with high-contrast material properties as acoustic metamaterial components." Physical Review B 85.16 (2012): 161103. 4. [4] Titovich, Alexey S., and Andrew N. Norris. "Tunable cylindrical shell as an element in acoustic metamaterial." The Journal of the Acoustical Society of America 136.4 (2014): 1601-1609. 5. [5] Ghaffarivardavagh, Reza, et al. "Ultra-wideband underwater backscatter via piezoelectric metamaterials." Proceedings of the Annual conference of the ACM Special Interest Group on Data Communication on the applications, technologies, architectures, and protocols for computer communication. 2020. 6. [6] Afzal, Sayed Saad, et al. "Enabling higher-order modulation for underwater backscatter communication." Global Oceans 2020: Singapore–US Gulf Coast. IEEE, 2020. KEYWORDS: Systems, assembly, acoustic, fabrication, testing, resonance, metamaterials
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