Charles A. Rohde

Experimental Demonstration of Underwater Acoustic Scattering Cancellation.

We explore an acoustic scattering cancellation shell for buoyant hollow cylinders submersed in a water background. A thin, low-shear, elastic coating is used to cancel the monopole scattering from an air-filled, neutrally buoyant steel shell for all frequencies where the wavelength is larger than the object diameter. By design, the uncoated shell also has an effective density close to the aqueous background, independently canceling its dipole scattering. Due to the significantly reduced monopole and dipole scattering, the compliant coating results in a hollow cylindrical inclusion that is simultaneously impedance and sound speed matched to the water background. We demonstrate the proposed cancellation method with a specific case, using an array of hollow steel cylinders coated with thin silicone rubber shells. These experimental results are matched to finite element modeling predictions, confirming the scattering reduction. Additional calculations explore the optimization of the silicone coating properties. Using this approach, it is found that scattering cross-sections can be reduced by 20u2009dB for all wavelengths up to k0au2009=u20090.85.

Generation of topologically diverse acoustic vortex beams using a compact metamaterial aperture

Here, we present a class of metamaterial-based acoustic vortex generators which are both geometrically simple and broadly tunable. The aperture overcomes the significant limitations of both active phasing systems and existing passive coded apertures. The metamaterial approach generates topologically diverse acoustic vortex waves motivated by recent advances in leaky wave antennas by wrapping the antenna back upon itself to produce an acoustic vortex wave antenna. We demonstrate both experimentally and analytically that this single analog structure is capable of creating multiple orthogonal orbital angular momentum modes using only a single transducer. The metamaterial design makes the aperture compact, with a diameter nearly equal to the excitation wavelength and can thus be easily integrated into high-density systems. Applications range from acoustic communications for high bit-rate multiplexing to biomedical devices such as microfluidic mixers.

Demonstration of a directional sonic prism in two dimensions using an air-acoustic leaky wave antenna

Analysis and experimental demonstration of a two-dimensional acoustic leaky wave antenna is presented for use in air. The antenna is comprised of a two-dimensional waveguide patterned with radiating acoustic shunts. When excited using a single acoustic source within the waveguide, the antenna acts as a sonic prism that exhibits frequency steering. This design allows for control of acoustic steering angle using only a single source transducer and a patterned aperture. Aperture design was determined using transmission line analysis and finite element methods. The designed antenna was fabricated and the steering angle measured. The performance of the measured aperture was within 9% of predicted angle magnitudes over all examined frequencies.

Anisotropic acoustic metafluid for underwater operation

The paper presents a method to design and characterize mechanically robust solid acoustic metamaterials suitable for operation in dense fluids such as water. These structures, also called metafluids, behave acoustically as inertial fluids characterized by anisotropic mass densities and isotropic bulk modulus. The method is illustrated through the design and experimental characterization of a metafluid consisting of perforated steel plates held together by rubber coated magnetic spacers. The spacers are very effective at reducing the effective shear modulus of the structure, and therefore effective at minimizing the ensuing coupling between the shear and pressure waves inside the solid effective medium. Inertial anisotropy together with fluid-like acoustic behavior are key properties that bring transformation acoustics in dense fluids closer to reality.

Evaluation of the resolution of a metamaterial acoustic leaky wave antenna

Acoustic antennas have long been utilized to directionally steer acoustic waves in both air and water. Typically, these antennas are comprised of arrays of active acoustic elements, which are electronically phased to steer the acoustic profile in the desired direction. A new technology, known as an acoustic leaky wave antenna (LWA), has recently been shown to achieve directional steering of acoustic waves using a single active transducer coupled to a transmission line passive aperture. The LWA steers acoustic energy by preferential coupling to an input frequency and can be designed to steer from backfire to endfire, including broadside. This paper provides an analysis of resolution as a function of both input frequency and antenna length. Additionally, the resolution is compared to that achieved using an array of active acoustic elements.

Far-field superresolution imaging using shaped acoustic vortices

The limitations on resolution due to the effects of diffraction have presented a significant barrier to generating and observing small features with acoustic or electromagnetic waves. Previously proposed methods to overcome this limit, and therefore achieve superresolution, have largely been restricted to operating within the near-field region of the aperture. In this work, we will describe how acoustic helicoidal waves generated using a phased acoustic aperture (such as a traditional phased array or acoustic metasurface) can create acoustic vortices that are well below the resolution limit, and how this can enable far-field superresolution acoustic imaging. The acoustic vortices generated in this manner propagate from the near-field into the far-field through an arrangement of stable integer mode vortices, thereby enabling the generation of far-field superresolved features in the acoustic pressure field. Through the use of non-axisymmetric vortex beam distributions, splitting of the on-axis vortex occurs. This leads to arbitrary off-axis arrangements of vortices, enabling more complicated superresolved structures to be created such as squares, triangles and multi-point stars. In this paper, theoretical and numerical results will be presented for an acoustic aperture which is capable of generating superresolved far-field features in the radiated acoustic pressure, and results will be shown illustrating the superresolution capability of this novel technique.

3D printed sound absorbers using functionally-graded sonic crystals

In this work, a thin functionally-graded sound absorber that achieves an absorption coefficient near unity is demonstrated. The sound absorber consists of a multilayer arrangement of an interwoven sonic crystal lattice with varying filling fractions, backed by a thin elastic coating that acts as a flexural acoustic element. The overall thickness of the sound absorber is about one tenth of the wavelength in air, and it was 3D printed with a thermoplastic polyurethane. Samples were fabricated and acoustically tested in an air-filled acoustic impedance tube, from which absorption and effective acoustic properties were obtained. A theoretical formulation for the effective acoustic properties of the sonic crystal lattice was used to guide the design process, and excellent agreement was found between measured and theoretically predicted results. A range of sonic crystal filling fractions and thicknesses were tested to verify the fabrication process and robustness of the theoretical formulation, and both were fo...

Reconfigurable metasurfaces for directional acoustic sensing

We design and acoustically simulate additively manufactured, flat acoustic membranes (also called metasurfaces) which can be reconfigured into 3-dimentional solids. Using finite element simulations, we design frequency selective acoustic ‘window’ membranes. These transmit narrow frequency bands near flexure resonances. The frequency range of coverage was chosen to be in the audible range and spans from 2,500Hz to 10,000Hz with first order resonances only. We demonstrate selective, non-overlapping acoustic transmission through each membrane window in its flat configuration, and directional selectively when the flat metasurface is folded into the truncated-octahedron with an omnidirectional microphone placed on the interior of the solid form. This work was supported by the Office of Naval Research.

Acoustic vortex beam generation using a compact metamaterial aperture

We have previously demonstrated an acoustic vortex wave antenna (VWA) based on a metamaterial aperture. This system produced arbitrary angular mode number waveforms,1 but did not produce pure integer mode vortex waves. In this work we extend our previous result to a design which radiates pure integer vortex modes. By combining an acoustic leaky wave antenna with a ring resonator waveguide we produce integer mode acoustic vortex waves. Further, we computationally show that this spatial mode can be transferred between two opposing acoustic vortex wave antenna in a pitch - catch configuration.

A diffraction based acoustic single pixel imager

We present a method for localizing an acoustic source with a single, omni-directional receiver paired with shaped aperture screens that allow for a spatially diverse set of measurements. Traditionally this is accomplished by patterning the screen with a series of sub-wavelength openings that allow the acoustic transmission an otherwise sound opaque material. Here we consider screens that have openings on the order of an acoustic wavelength or larger and incorporate a diffraction model into the single pixel imaging framework to account for these larger openings. The method is demonstrated on experimental data taken in air and an analysis of the error as a function of receiver position is presented. [This work was supported by ONR.]We present a method for localizing an acoustic source with a single, omni-directional receiver paired with shaped aperture screens that allow for a spatially diverse set of measurements. Traditionally this is accomplished by patterning the screen with a series of sub-wavelength openings that allow the acoustic transmission an otherwise sound opaque material. Here we consider screens that have openings on the order of an acoustic wavelength or larger and incorporate a diffraction model into the single pixel imaging framework to account for these larger openings. The method is demonstrated on experimental data taken in air and an analysis of the error as a function of receiver position is presented. [This work was supported by ONR.]