Matthew D. Guild

Cancellation of acoustic scattering from an elastic sphere

Recent research has suggested the possibility of creating acoustic cloaks using metamaterial layers to eliminate the acoustic field scattered from an elastic object. This paper explores the possibility of applying the scattering cancellation cloaking technique to acoustic waves and the use of this method to investigate its effectiveness in cloaking elastic and fluid spheres using only a single isotropic elastic layer. Parametric studies showing the influence of cloak stiffness and geometry on the frequency dependent scattering cross-section of spheres have been developed to explore the design space of the cloaking layer. This analysis shows that an appropriately designed single isotropic elastic cloaking layer can provide up to 30 dB of scattering reduction for ka values up to 1.6. This work also illustrates the importance of accounting for the elasticity of the object and the relevant limitations of simplistic quasi-static analyses proposed in recent papers.

Cloaking of an acoustic sensor using scattering cancellation

In this Letter, a bilaminate acoustic cloak designed using scattering cancellation methods is applied to the case of an acoustic sensor consisting of a hollow piezoelectric shell with mechanical absorption. The bilaminate cloak provides 20–50 dB reduction in scattering strength relative to the uncloaked configuration over the typical range of operation for an acoustic sensor, retains its ability to sensing acoustic pressure signals, and remains within the physical bounds of a passive absorber. Further, the cloak is shown to increase the range of frequencies over which there is nearly perfect phase fidelity between the acoustic signal and the voltage generated by the sensor. The feasibility of achieving the necessary fluid layer properties is demonstrated using sonic crystals with the use of readily available acoustic materials.

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 20 dB for all wavelengths up to k0a = 0.85.

Acoustic metamaterial absorbers based on multilayered sonic crystals

Through the use of a layered arrangement, it is shown that lossy sonic crystals can be arranged to create a structure with extreme acoustic properties, namely, an acoustic metamaterial. This artificial structure shows different effective fluids and absorptive properties in different orientations. Theoretical, numerical, and experimental results examining thermoviscous losses in sonic crystals are presented, enabling the fabrication and characterization of an acoustic metamaterial absorber with complex-valued anisotropic inertia. To accurately describe and fabricate such an acoustic metamaterial in a realizable experimental configuration, confining structures are needed which modify the effective properties, due to the thermal and viscous boundary layer effects within the sonic crystal lattice. Theoretical formulations are presented which describe the effects of these confined sonic crystals, both individually and as part of an acoustic metamaterial structure. Experimental demonstrations are also reported ...

Cancellation of the acoustic field scattered from an elastic sphere using periodic isotropic elastic layers.

Periodic layers have recently been proposed as a means to obtain the material properties requisite for the creation of near‐perfect spherical acoustic cloaking layers designed using coordinate transformation techniques. Previous work has suggested using radially periodic fluid layers that display an anisotropic density tensor in the long wavelength homogenization limit. Unfortunately, the high degree of anisotropy and functional grading necessitated by the coordinate transformation method and the use of fluid‐like layers suggested by those techniques present significant practical challenges to their construction. An alternative to the coordinate transformation method is a non‐resonant scattering cancellation technique that has been shown to provide up to 30 dB scattering reduction over a finite bandwidth using isotropic elastic layers. [M. D. Guild, A. Alu, and M. R. Haberman, J. Acoust. Soc. Am. 127, 1952 (2010).] The research presented here investigates the use of periodic isotropic elastic layers using...

Enhanced inertia from lossy effective fluids using multi-scale sonic crystals

In this work, a recent theoretically predicted phenomenon of enhanced permittivity with electromagnetic waves using lossy materials is investigated for the analogous case of mass density and acoustic waves, which represents inertial enhancement. Starting from fundamental relationships for the homogenized quasi-static effective density of a fluid host with fluid inclusions, theoretical expressions are developed for the conditions on the real and imaginary parts of the constitutive fluids to have inertial enhancement, which are verified with numerical simulations. Realizable structures are designed to demonstrate this phenomenon using multi-scale sonic crystals, which are fabricated using a 3D printer and tested in an acoustic impedance tube, yielding good agreement with the theoretical predictions and demonstrating enhanced inertia.

Sound speed in water-saturated glass beads as a function of frequency and porosity

Sound propagation in water-saturated granular sediments is known to depend on the sediment porosity, but few data in the literature address both the frequency and porosity dependency. To begin to address this deficiency, a fluidized bed technique was used to control the porosity of an artificial sediment composed of glass spheres of 265 μm diameter. Time-of-flight measurements and the Fourier phase technique were utilized to determine the sound speed for frequencies from 300 to 800 kHz and porosities from 0.37 to 0.43. A Biot-based model qualitatively describes the porosity dependence.

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.

Acoustic Cloaking with Plasmonic Shells

This chapter presents an overview of the scattering cancellation approach applied to acoustic waves, inspired by the use of plasmonic cloaks for electromagnetic waves. Using an analogous analytical approach, we show here that isotropic and homogeneous acoustic metamaterial covers may provide strong scattering reduction over moderately broad bandwidths of operation in a variety of acoustic scenarios of interest. This chapter outlines the basic physics of this approach, along with numerical examples for moderately sized elastic and fluid objects, thereby providing insights into the anomalous suppression of acoustic scattering produced by this cloaking technique.

Design of an ideal spherical hydrophone using a plasmonic acoustic cloak.

An ideal acoustic sensor provides an electrical output that is proportional to and in phase with an incident acoustic pressure field, without significantly altering the impinging field. The common approach to producing such a sensor is simply to minimize the size of the sensing elements relative to the wavelength of the measured field. Unfortunately, this approach has the drawback of reducing sensitivity. Recent work on plasmonic acoustic cloaking [Guild et al., J. Acoust. Soc. Am. 128, 2374 (2010)], however, suggests the possibility of creating an ideal acoustic sensor with dimensions on the same length scale as the incident wavelength, obtained by drastically suppressing its scattering without affecting its ability to measure the impinging signal. Unlike other cloaking methods that reroute the incident field around the cloaked object, plasmonic cloaks allow the cloak interior to interact with the incident field, thereby permitting the realization of highly noninvasive acoustic sensors. This work present...