Improving the performance of structure-embedded acoustic lenses via gradient-index local inhomogeneities

We investigate the use of graded inhomogeneities in order to enhance the focusing and collimation performance of structure-embedded acoustic metamaterial lenses.The type of inhomogeneity exploited in this study consists in axial symmetric exponential-like gradients of either material or geometric properties that create gradient-index inclusions able to bend and redirect propagating waves.In particular,we exploit the concept of gradient index inclusions to achieve focusing and collimation of ultrasonic beams created by embedded drop-channel lenses in both bulk and thin-walled structures.In the latter,the implementation is possible thanks to geometric exponential tapers known as Acoustic Black Holes(ABH).ABH tapers allow accurate control of the characteristics of the acoustic beam emanating from the lens channel which in the conventional design is severely affected by diffraction.The concept of beam control via graded inclusions is numerically illustrated and validated by using a combination of methodologies including geometric acoustics,finite difference time domain,and finite element methods.

An acoustic Maxwell's fish-eye lens based on gradient-index metamaterials

We have proposed a two-dimensional acoustic Maxwell’s fish-eye lens by using the gradient-index metamaterials with space-coiling units. By adjusting the structural parameters of the units, the refractive index can be gradually varied, which is key role to design the acoustic fish-eye lens. As predicted by ray trajectories on a virtual sphere, the proposed lens has the capability to focus the acoustic wave irradiated from a point source at the surface of the lens on the diametrically opposite side of the lens. The broadband and low loss performance is further demonstrated for the lens. The proposed acoustic fish-eye lens is expected to have the potential applications in directional acoustic coupler or coherent ultrasonic imaging.

Direct and real-time sub-wavelength resolution photoacoustic imaging method based on acoustic lens with negative refractive index

The signal processing technology based on material with negative refractive index provides researchers with the latest ideas. As a new nondestructive bio-photonic technology, photoacoustic tomography is a kind of imaging method based on the differences of optical absorption within the biological organization However, photoacoustic tomography by the scanning sensor or by the sensors array at present has its inherent disadvantages that may lead to poor real-time performance and high cost in the imaging process. The characteristics of acoustic lens with negative refractive index such as focusing, filtering and directional control on acoustic wave, are very suitable for solving the problem in photoacoustic tomography. With an analysis on the nega-tive quality response of acoustic lens and the advantages of negative refractive imaging, we proposed an approach using the lens to change the current photoacoustic imaging methods. The experiment showed that the imaging effectiveness of photoacoustic tomography by the designed lens is very impressive that the pressure distribution of the absorber is basically consistent with the image of the absorber. In addition, the result of 0. 6 times wavelength in the experimental image is demonstrated on sub-wave-length photoacoustic imaging through the lens designed in this work.

Theoretical demonstration of the hybrid focusing points of sonic crystal flat lenses and possible applications

We demonstrate the hybrid focusing points of sonic crystals for a multi-source array applied to flat sonic crystal lenses. The contributions of different point source couples form hybrid focusing points. Ray-trace analyses are conducted for acoustic flat lenses with multi-source configurations. The finite-difference time-domain （FDTD） simulation of flat lenses with multi-source configurations demonstrates the establishment of pure and hybrid focusing points in a pyramidal constellation. The number of focusing points in the pyramidal constellation depends on the number of point sources. We propose an acoustic device for fine-tuning the location of a far-field hybrid focusing point and discuss its benefits for acoustic energy focusing application.

Acoustic Multi Emission Lens via Transformation Acoustics

Based on transformation acoustic methodology, we propose an algorithm for designing acoustic non-resonant lens antenna, which is competent to generate multiple directive beams that are pointing at the desired direction.Unattainable with previous works, the present approach is capable of adjusting the directivity of each radiated beam individually, which is of the utmost importance in several acoustic applications such as in sonar systems. A linear transformation function is intentionally used for eliminating the inhomogeneity of the obtained materials and to pave the way towards more general acoustic patterns. Several numerical simulations are performed to show the capability of the proposed method in manipulating the acoustic waves. To authenticate the concept,a structure that can generate four beams with different directivities is realized with non-resonant meta-fluid bi-layered structure through effective medium theory.

Review of research on sonar imaging technology in China

Over the past 20 years,sonar imaging technology particularly for the high-technology sector has been a focus of research,in which many developed countries,especially those with coast lines,have been competing with each other.It has seen a rapid development with increasing widespread applications that has played an important and irreplaceable role in underwater exploration with great prospects for social,economic,scientific,and military benefits.The fundamental techniques underlying sonar imaging,including multi-beamforming,synthetic-aperture and inverse synthetic-aperture sonar,acoustic lensing,and acoustical holography,are described in this paper.This is followed by a comprehensive and systematic review on the advantages and disadvantages of these imaging techniques,applicability conditions,development trends,new ideas,new methods,and improvements in old methods over recent years with an emphasis on the situation in China,along with a bold and constructive prediction to some development characteristics of sonar imaging technology in the near future in China.The perspectives presented in this paper are offered with the idea of providing some degree of guidance and promotion of research on sonar imaging technology.

A Non-Singular Boundary Element Method for Interactions between Acoustical Field Sources and Structures

Localized point sources(monopoles)in an acoustical domain are implemented to a three dimensional non-singular Helmholtz boundary element method in the frequency domain.It allows for the straightforward use of higher order surface elements on the boundaries of the problem.It will been shown that the effect of the monopole sources ends up on the right hand side of the resulting matrix system.Some carefully selected examples are studied,such as point sources near and within a concentric spherical core-shell scatterer(with theoretical verification),near a curved focusing surface and near a multi-scale and multi-domain acoustic lens.