Passive Acoustic Subwavelength Resonator (PASR)

We propose a Phase I effort that will provide a proof of concept demonstrating a class of Passive Acoustic Subwavelength Resonator (PASR) intended for deployment in undersea environments. The PASR concepts are designed to passively scatter acoustic fields at resonance within a tunable spectral band, potentially with tailored directivity. The PASR concepts feature a subwavelength scattering architecture that preserves its functionality over a range of ocean depths. Conventional resonant concepts that feature spectral tunability are typically difficult to make subwavelength in scale, while subwavelength aqueous resonators often feature a pressure-release boundary condition that compresses at depth causing alterations in the scattering response and limits deployment depth.  The proposed PASR concept is intended to combine subwavelength spectral tunability with an insensitivity to static pressure.  Deliverables will include experimental validation of powered components and rendered PASR concept validation in simulation. Two concepts that produce passive acoustic resonance will be considered: (1) an azimuthal lattice of thin flexural plates that achieve subwavelength resonance via a hyperbolic dispersion, and (2) oil-filled channel(s) coiled within a cylindrical cavity that produces subwavelength resonance due to an extended path length.  Both concepts will feature piezoelectric actuation using low-power DC biasing to tune the resonant properties, with the intention of tailoring the passive resonances of the PASR device after reconfiguration.  The plates of the azimuthal lattice will be composed of piezoelectric materials, while the oil-filled channels will contain piezoelectric geometries that can alter a channel’s mode shape.  The piezoelectric components in both concepts have the potential to be monitored as acoustic sensors, and possibly as vector sensors.  Furthermore, both concepts are expected to be insensitive to static pressure over a range of ocean depths.  The stiff component plates of the azimuthal lattice are in direct contact with the surrounding water thereby facilitating an acoustic interaction, while the oil-filled channels can be pressure-equalized using standard fabrication techniques.  The potential for passive, directional scattering will also be assessed by introducing geometric asymmetries into both concept architectures that take advantage of bianisotropic (Willis) acousto-elastic coupling. Our proposed concepts are well-suited for applications that require tunable acoustic spectral resonances using a compact, subwavelength design, such as for undersea communication, non-destructive evaluation, passive beamforming, or acoustic navigation and sensing for oil and gas exploration.  Following the successful demonstration of the PASR concepts in simulation, Phase II and III of this effort will provide prototyping, underwater testing, and a path toward commercialization of the technology.