Acoustical scattering by multilayer spherical elastic scatterer containing electrorheological layer

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.

Arbitrary scattering of an acoustical high-order Bessel trigonometric (non-vortex) beam by a compressible soft fluid sphere

The present analysis extends the previous work on the axial acoustic scattering of a high-order Bessel trigonometric beam (HOBTB) from a fluid sphere [F.G. Mitri, J. Appl. Phys. 109 (2011) 014916] to the generalized case of arbitrary scattering from a fluid sphere placed off-axially. The scattered pressure is expressed using a generalized partial-wave series expansion involving the beam-shape coefficients (BSCs), the scattering coefficients of the fluid sphere, and the half-conical angle of the beam. The BSCs are evaluated using the numerical discrete spherical harmonics transform (DSHT). The properties of the off-axial acoustic scattering by a fluid red blood sphere (RBS), chosen as an example to illustrate the analysis, are discussed. 3D numerical computations for the directivity patterns in the near and far-field regions reveal unexplored phenomena that may be useful in applications related to particle entrapment, manipulation or rotation of soft matter using acoustic HOBTBs. Other potential applications may include medical or nondestructive ultrasound imaging with contrast agents, or monitoring of the manufacturing processes of sample soft matter systems with HOBTBs.

Optimizing imperfect cloaks to perfection

Transformation optics has been an essential tool for designing cloaking devices for electromagnetic and acoustic waves. All these designs have one requirement in common: material singularity. At the interface between the cloak and the cloaked region, some material properties have to approach infinity, while some others approach zero. This paper attempts to answer a central question in physically realizing such cloaks: is material singularity a requirement for perfect cloaking? This paper demonstrates that, through optimization, perfect cloaking can be achieved using a layered cloak construction without material singularity. Two examples are used for this demonstration. In one example, the initial design is based on the Cummer-Schurig prescription for acoustic cloaking that requires mass-anisotropic material. Another example uses the two isotropic layers to achieve the equivalent mass-anisotropy for each anisotropic layer. During the optimization processes, only material properties of cloaks' constituent layers are adjusted while the geometries remain unchanged. In both examples, the normalized total scattering cross section can be reduced to 0.002 (0.2%) or lower in numerical computations. The capabilities and other characteristics of the optimization in other tasks such as cloaking penetrable objects and isolating strong resonance in such objects are also explored.

Generalized theory of resonance excitation by sound scattering from an elastic spherical shell in a nonviscous fluid

This work presents the general theory of resonance scattering (GTRS) by an elastic spherical shell immersed in a nonviscous fluid and placed arbitrarily in an acoustic beam. The GTRS formulation is valid for a spherical shell of any size and material regardless of its location relative to the incident beam. It is shown here that the scattering coefficients derived for a spherical shell immersed in water and placed in an arbitrary beam equal those obtained for plane wave incidence. Numerical examples for an elastic shell placed in the field of acoustical Bessel beams of different types, namely, a zero-order Bessel beam and first-order Bessel vortex and trigonometric (nonvortex) beams are provided. The scattered pressure is expressed using a generalized partial-wave series expansion involving the beam-shape coefficients (BSCs), the scattering coefficients of the spherical shell, and the half-cone angle of the beam. The BSCs are evaluated using the numerical discrete spherical harmonics transform (DSHT). The far-field acoustic resonance scattering directivity diagrams are calculated for an albuminoidal shell immersed in water and filled with perfluoropropane gas, by subtracting an appropriate background from the total far-field form function. The properties related to the arbitrary scattering are analyzed and discussed. The results are of particular importance in acoustical scattering applications involving imaging and beam-forming for transducer design. Moreover, the GTRS method can be applied to investigate the scattering of any beam of arbitrary shape that satisfies the source-free Helmholtz equation, and the method can be readily adapted to viscoelastic spherical shells or spheres.