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This paper delves into enhancing the performance of ScAlN-based Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) through the implementation of Polydimethylsiloxane (PDMS) acoustic lenses. The PMUT, encapsulated in PDMS, underwent thorough characterization through the utilization of an industry-standard hydrophone calibration instrument. The experimental results showed that the ScAlN-based PMUT with the PDMS lenses achieved an impressive sensitivity of −160 dB (re: 1 V/μPa), an improvement of more than 8 dB compared to the PMUT with an equivalent PDMS film. There was a noticeable improvement in the −3 dB main lobe width within the frequency response when comparing the PMUT with PDMS encapsulation, both with and without lenses. The successful fabrication of high-performance PDMS lenses proved instrumental in significantly boosting the sensitivity of the PMUT. Comprehensive performance evaluations underscored that the designed PMUT in this investigation surpassed its counterparts reported in the literature and commercially available transducers. This encouraging outcome emphasizes its substantial potential for commercial applications. [ABSTRACT FROM AUTHOR]

The main objective of presented study is to examine the influence of an acoustic lens on shape of the wavefront. To conveniently illustrate the difference between acoustic pressure wave propagation with and without the lens, an isodynamic transducer was chosen as a source. This kind of loudspeaker generates flat wavefront as a result of approximately uniform distribution of speed and phase on the entire diaphragm. The designed lens consisted of a matrix of individual waveguides. Manipulation of size and position of output matrix in relation to input matrix allowed for achieving the desired waveguide length distribution. Differences in lengths of lens's channels resulted in wavefront delay distribution at the output matrix. A numerical model of transducer and waveguide matrix was created to evaluate the behaviour of acoustic pressure wave propagation trough the designed lens. With stationary study, a spatial pressure distribution was calculated, in the near field and far field, in hemisphere in front of the lens as well as in hemisphere in front of just the transducer. The differences in wavefront shapes between the two cases were clearly visible in comparisons, confirming the expected pressure wave delay distribution of the lens. The resulting wavefront curvature was compared to the assumed one in theoretical design. Results of those comparisons proved the possibility of influencing the wavefront shape, by manipulating the output matrix with some caveats discussed in the paper. The data from numerical calculation of pressure propagation allowed for visualizing calculated sound pressure level distribution, adding the directivity evaluation to the comparisons. [ABSTRACT FROM AUTHOR]

This paper delves into enhancing the performance of ScAlN-based Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) through the implementation of Polydimethylsiloxane (PDMS) acoustic lenses. The PMUT, encapsulated in PDMS, underwent thorough characterization through the utilization of an industry-standard hydrophone calibration instrument. The experimental results showed that the ScAlN-based PMUT with the PDMS lenses achieved an impressive sensitivity of −160 dB (re: 1 V/μPa), an improvement of more than 8 dB compared to the PMUT with an equivalent PDMS film. There was a noticeable improvement in the −3 dB main lobe width within the frequency response when comparing the PMUT with PDMS encapsulation, both with and without lenses. The successful fabrication of high-performance PDMS lenses proved instrumental in significantly boosting the sensitivity of the PMUT. Comprehensive performance evaluations underscored that the designed PMUT in this investigation surpassed its counterparts reported in the literature and commercially available transducers. This encouraging outcome emphasizes its substantial potential for commercial applications.

Simple Summary: Acoustic holograms were investigated as a method of delivering simultaneously controlled ultrasound hyperthermia to multiple tumor spheroids. Findings show that holographic lenses designed to deliver a uniform thermal dose rather than a uniform acoustic field can tune the natural focus of a transducer, allowing the exposure to ultrasound-mediated hyperthermia of several tumor spheroids simultaneously, and improving experimental throughput for future studies. Preliminary in-vitro findings suggest that hyperthermia delivered by ultrasound and polymerase chain reaction heating have different outcomes, with ultrasound being more lethal for the same thermal dose. Hyperthermia is currently used to treat cancer due to its ability to radio- and chemo-sensitize and to stimulate the immune response. While ultrasound is non-ionizing and can induce hyperthermia deep within the body non-invasively, achieving uniform and volumetric hyperthermia is challenging. This work presents a novel focused ultrasound hyperthermia system based on 3D-printed acoustic holograms combined with a high-intensity focused ultrasound (HIFU) transducer to produce a uniform iso-thermal dose in multiple targets. The system is designed with the aim of treating several 3D cell aggregates contained in an International Electrotechnical Commission (IEC) tissue-mimicking phantom with multiple wells, each holding a single tumor spheroid, with real-time temperature and thermal dose monitoring. System performance was validated using acoustic and thermal methods, ultimately yielding thermal doses in three wells that differed by less than 4%. The system was tested in vitro for delivery of thermal doses of 0–120 cumulative equivalent minutes at 43 °C (CEM43) to spheroids of U87-MG glioma cells. The effects of ultrasound-induced heating on the growth of these spheroids were compared with heating using a polymerase chain reaction (PCR) thermocycler. Results showed that exposing U87-MG spheroids to an ultrasound-induced thermal dose of 120 CEM43 shrank them by 15% and decreased their growth and metabolic activity more than seen in those exposed to a thermocycler-induced heating. This low-cost approach of modifying a HIFU transducer to deliver ultrasound hyperthermia opens new avenues for accurately controlling thermal dose delivery to complex therapeutic targets using tailored acoustic holograms. Spheroid data show that thermal and non-thermal mechanisms are implicated in the response of cancer cells to non-ablative ultrasound heating. [ABSTRACT FROM AUTHOR]

We numerically and experimentally realize broadband acoustic focusing lenses by employing units of phase manipulation composed of different numbers of cavity structures immersed in air, which is different from other units of phase manipulation by adjusting the structural parameters. The phase delays of transmitted units could cover a whole 2π range by using six cavity structures. Based on these units, we design a transmitted acoustic lens with different focal lengths, and the working bandwidth could reach about 2.1 kHz. Besides, the influence of air attenuation on the focusing performance and of the inclined angles of the channels in the cavity structure on the phase delays are discussed in detail. Moreover, we realize a transmitted acoustic lens with two types of units, a reflected acoustic lens and a three-dimensional transmitted acoustic lens. Compared with other acoustic lenses, the proposed acoustic lenses have the advantages of broad bandwidths, high focusing performance, easy fabrication, and high robustness, which have great potential applications in airborne acoustic focusing devices. [ABSTRACT FROM AUTHOR]

Recent studies have introduced a new class of two-dimensional acoustic metamaterials whose dispersion and propagation properties result from the use of geometric inhomogeneities in the form of Acoustic Black Holes (ABH). The ABH is an element able to smoothly bend acoustic rays and slow down elastic bending waves, therefore providing a variety of unconventional dispersion and propagation properties that are typically observed in more complex multi-material and locally resonant designs. Our approach enables full integration of acoustic lenses in thin-walled structural elements while preserving the structural character. The lenses are capable of different high-level functionalities such as focusing, collimation, and negative refraction. Numerical and experimental results show that such structures exhibit broadband operating conditions that span both the and the range. [ABSTRACT FROM AUTHOR]

Narrow bandwidth and bulky configuration are the main obstacles for the realization and application of invisible cloaks. In this paper, we present an effective method to achieve broadband and thin acoustic cloak by using an acoustic metasurface (AMS). In order to realize this cloak, we use slitted unit cells to design the AMS due to the advantage of less energy loss, broad operation bandwidth, and subwavelength thickness. According to the hyperboloidal phase profile along the AMS, the incident plane waves can be focused at a designed focal spot by the flat lens. Furthermore, broadband acoustic cloak is obtained by combining two identical flat lenses. The incident plane waves are focused at the center point in between of the two lenses by passing through one lens, and then recovered by passing through the other one. However, they cannot reach the cloaked regions in between of the two lenses. The simulation results can verify the nondetectability effect of the acoustic cloak. Our study results provide an available and simple approach to experimentally achieve the acoustic cloak, which can be used in acoustic nondetectability for large objects. [ABSTRACT FROM AUTHOR]

In the last two decades, it has been demonstrated that highly nonlinear solitary waves (HNSWs) can be used in many physics and engineering applications, such as acoustic lenses, impurity detectors, and nondestructive testing. HNSWs are compact nondispersive waves that propagate in nonlinear media such as 1D chains of spherical particles. In this paper, we propose to couple an array of granular particles that support the propagation of HNSWs, to a wafer-type lead zirconate titanate (PZT) transducer in order to harvest the energy of an object tapping the array. This latter is in contact with a polycarbonate block where the nonlinear waves become linear and coalesce at a designed focal point. Here, the PZT converts the acoustic energy into electricity that powers a load resistor. The performance of this harvester is compared to a similar system where the chains are replaced by solid rods. The results demonstrate that the granular system generates more electricity. [ABSTRACT FROM AUTHOR]

The high‐efficiency transmitted acoustic focusing lenses based on the Helmholtz resonator (HR) metasurface is reported. The proposed lenses demonstrate superior focusing performances, which can provide deep‐subwavelength resolution. The proposed lenses demonstrate superior focusing performances, in which the performances of acoustic focusing metasurface is investigated in details and the bandwidth of the metasurface with hybrid resonant structures is broadened. For the former, the effects of different numerical apertures, different focusing distances, and different incidence angles are investigated. At the same time, off‐axis focusing and spherical wave focusing lenses are also demonstrated. For the latter, by spacing the HR elements with different resonant frequencies, a broad‐band acoustic lens is also designed and achieved, in which the favorable focusing performance can be realized in a relative wider frequency range. The proposed designs provide great potential applications in many scenarios, such as nondestructive testing, ultrasonic medical treatment and so on. [ABSTRACT FROM AUTHOR]

The realization of a near-frequency-independent acoustic metamaterial and the successful application of this material as a convex acoustic lens are introduced. We design a face-centered-orifice-cubic (FCOC) unit cell that has the characteristics of an acoustic wave retarder. The increment in the acoustic refractive index produced by the FCOC unit cell is observed by reducing the size of the orifice. This also shows that the refractive index remains nearly constant with frequency increments in the frequency regime that satisfies the homogeneous medium condition of the metamaterial. Using these unit cells, we create a two-dimensional acoustic metamaterial convex lens that correspond to the conventional optical convex lens and show that this lens focuses acoustic waves in the same manner that an optical lens focuses light waves. [ABSTRACT FROM AUTHOR]

In this study, we present an approach to ultrasonic beam-forming and high resolution identification of acoustic sources having critical implications for applications such as structural health monitoring. The proposed concept is based on the design of dynamically tailored structural elements via embedded acoustic metamaterial lenses. This approach provides a completely new alternative to conventional phased-array technology enabling the formation of steerable and collimated (or focused) ultrasonic beams by exploiting a single transducer. Numerical results show that the ultrasonic beam can be steered by simply tuning the frequency of the excitation. Also, the embedded lens can be designed to achieve sub-wavelength resolution to clustered acoustic sources, which is a typical scenario encountered in incipient structural damage. [ABSTRACT FROM AUTHOR]

Focused ultrasound with microbubbles (FUS-MBs) has shown that it can lead to an efficient drug delivery system (DDS) involving the oscillation and destruction of the MB but is limited in drug delivery due to its narrow pressure field. However, unfocused ultrasound with MBs (UUS-MBs) and an interchangeable acoustic lens can tune and enhance the pressure field for MB destruction to overcome the disadvantages of FUS-MB DDSs. We designed a lens suitable for an ultrasound-phased array probe and studied the optimal treatment conditions for MB destruction in vitro through an optical imaging setup. The DDS effects were evaluated in a rat hepatoma model using doxorubicin (DOX) treatment. A concave lens with a radius of curvature of 2.6 mm and a thickness of 4 mm was selected and fabricated. UUS-MBs with the acoustic lens at 60 Vpp for 32 cycles and a PRF of 1 kHz could induce MB destruction, promoting the DDS even under fluidic conditions. In the animal experiment, the UUS-MBs in the acoustic lens treatment group had a higher concentration of DOX in the tumor than the control group. Our system suggests uses an acoustic lens to increase DDS effectiveness by providing sufficient ultrasound irradiation to the MBs. [ABSTRACT FROM AUTHOR]

A liquid acoustic lens with variable focal length is described for photoacoustic microscopy. This liquid lens takes advantage of the elastic and deformable lens interface to tune its focal length in a pneumatical manner. The curvature of the lens interface as well as the dependant focal length was characterized as a function of the infusion volume of the liquid. Experiments were carried out to demonstrate the zooming ability of this liquid acoustic lens. Targets embedded at different depths were photoacoustically imaged without performing mechanically axial scanning. [ABSTRACT FROM AUTHOR]

In this paper, we propose a simultaneous multiple-slice photoacoustic (PA) imaging technique by utilizing an acoustic lens with parallel imaging ability. Our method can directly image multiple slices of the three-dimensional (3D) PA images in real-time without utilizing special reconstruction algorithms. Combining the advantages of an acoustic lens with long focal depth and a fast data acquisition system with high sampling rate, the new system can acquire all slices of a 3D object simultaneously. This is achieved by utilizing the time-resolved technique that allows to distinguish PA signals from different object planes. According to the experiment results, the reconstructed sectional images match the original samples well. The proposed technique provides a fast and reliable approach to multiple-section PA imaging. Our study can be applied to noninvasive imaging and medical diagnosis. [ABSTRACT FROM AUTHOR]

Tunable acoustic gradient index (TAG) lenses create tunable multiscale Bessel beams. These lenses are fluid-filled cylindrical cavities within which an acoustic radial standing wave is excited. This standing wave modulates the density, and thereby the refractive index within the lens. Spatial gradients in the refractive index can be used for lensing. A predictive model for the steady-state fluid mechanics behind TAG lenses driven with a sinusoidal voltage signal is presented here. The model covers inviscid and viscous regimes in both the resonant and off-resonant cases. The density fluctuations from the fluidic model are related to refractive index fluctuations. The entire model is then analyzed to determine the optimal values of lens design parameters for greatest lens refractive power. These design parameters include lens length, radius, static refractive index, fluid viscosity, sound speed, and driving frequency and amplitude. It is found that long lenses filled with a fluid of high refractive index and driven with large amplitude signals form the most effective lenses. When dealing with resonant driving conditions, low driving frequencies, smaller lens radii, and fluids with larger sound speeds are optimal. At nonresonant driving conditions, the opposite is true: High driving frequencies, larger radius lenses, and fluids with low sound speeds are beneficial. The ease of tunability of the TAG lens through modifying the driving signal is discussed, as are limitations of the model including cavitation and nonlinearities within the lens. [ABSTRACT FROM AUTHOR]

The theory of photoacoustic tomography imaging using an acoustic lens imaging system has been investigated, and the photoacoustic (PA) Fourier imaging property of an acoustic lens is presented. The theoretical results show that an acoustic lens is able to realize two-dimensional PA Fourier imaging and the focal depth of the acoustic lens is 20 mm. The PA signals with the advantage of time resolution can be resolved in time domain by time-resolved technique. Therefore, this long focal depth characteristic of the acoustic lens can be combined with time-resolved technique to realize PA tomography imaging. It is demonstrated by experiments that this method is able to provide PA tomography images of biological tissue. The images are clear and contrast sharply with their backgrounds. The depth resolution is 1.54 mm. [ABSTRACT FROM AUTHOR]

This work analyzes the influence of the distribution of transparent Fresnel regions over the focusing profile of Soret Zone Plates. It is shown that this effect becomes very significant in those fields where directional transducers are employed, such as microwaves or acoustics. A thorough analysis on both the lens transmission efficiency and the focusing enhancement factor is presented. Moreover, experimental measurements are also carried out, validating the theoretical model and demonstrating that the distribution of transparent Fresnel regions becomes a critical parameter in applications requiring directional emitters. [ABSTRACT FROM AUTHOR]

Abstract In this letter, the improvement in focus by the use of a pupil mask produced in an acoustic mesoscale cuboid particle filled with CO 2 is reported. Thereby, the result shows that the pupil mask increases the sound intensity and also increases the resolution (or a reduction of the full width at half maximum, FWHM) in focus compared to the non-masked one. These results are important because they confirm the effect of abnormal amplitude apodization for a one-wavelength dimension acoustic lens and demonstrate that it is possible to improve sound focusing of a cuboid gas-filled lens with one wavelength dimension. This is the smallest size of an acoustic lens ever considered in this type of literature, with side dimensions of the cube equal to one wavelength and a diameter to focus ratio of 2.5, the sound amplification in focus is 5.4 dB at 4125 Hz, with the resolution near to the diffraction limit. Highlights • Nonspherical acoustic wavelength-scale cuboid particle lens filled with CO 2 gas is reported. • The effect of anomaly amplitude apodization is used to improve the focusing characteristics of a flat lens. • This is the smallest size of an acoustic lens, ever considered in the literature. [ABSTRACT FROM AUTHOR]

Highlights • A new ultrasonic transducer is designed to achieve an excellent beam profile. • The new structure incorporates both a shaded electrode and a multifocal lens. • Optimal structure is determined using the ratio of focal range to minimum beam width. • Designed beam profile has a consistently narrower beam width over a longer distance. • The design is validated by fabricating and characterizing a prototype transducer. Abstract A new ultrasonic transducer structure is designed by incorporating both a shaded electrode and a multifocal lens to achieve an excellent beam profile radiating from the transducer's elevation. The beam profile of the transducer is investigated by the finite element method, and then the new structure is designed by simultaneously analyzing such performance parameters as focal range, beam width, and sidelobe level. The optimal structure is determined using the ratio of focal range to minimum beam width, focal range ratio, along the radiation axis as a new figure of merit. The resultant beam profile has a consistently narrower beam width over a longer distance from the transducer surface as compared to the initial profile. The focal-range ratio is increased by 19%, which results in the reduction of the minimum beam width by 59%. The design is validated by fabricating and characterizing a prototype transducer of the new structure and comparing its measured performance with that of a transducer of the initial basic structure. [ABSTRACT FROM AUTHOR]

Neuromodulation is a fundamental method for obtaining basic information about neuronal circuits for use in treatments for neurological and psychiatric disorders. Ultrasound stimulation has become a promising approach for noninvasively inducing neuromodulation in animals and humans. However, the previous investigations were subject to substantial limitations, due to most of them involving anesthetized and fixed small-animal models. Studies of awake and freely moving animals are needed, but the currently used ultrasound devices are too bulky to be applied to a freely moving animal. This study is the first time to design and fabricate a miniature and lightweight head-mounted ultrasound stimulator for inducing neuromodulation in freely moving mice. The main components of the stimulator include a miniature piezoelectric ceramic, a concave epoxy acoustic lens, and housing and connection components. The device was able to induce action potentials recorded in situ and evoke head-turning behaviors by stimulating the primary somatosensory cortex barrel field of the mouse. These findings indicate that the proposed method can be used to induce noninvasive neuromodulation in freely moving mice. This novel method could potentially lead to the application of ultrasonic neuromodulation in more-extensive neuroscience investigations. [ABSTRACT FROM AUTHOR]

Highlights • A free parameter (reference radius r 0 ) allows obtaining a much finer focus. • When r 0 is bigger than r 1 , an expansion of the focus along the longitudinal axis is achieved. • When r 0 smaller than r 1 , a control of the Sidelobe Level can be achieve. • The prospect of these results are important in some medical applications. • Possibility to control a spatial frequency to obtain a finer focus. Abstract The manipulation of focal patterns of acoustic underwater Soret Zone Plate lens in far fields, such as manipulation (optimization) of Sidelobe Level and the design of long depth of focus by selecting the simple free parameter called reference radius (phase) has been demonstrated.Two effects have been studied by means of numerical simulations. Regarding the first effect, simulations demonstrate diffraction limited focal spot (0.47 wavelength) and 3 dB reduction of the first Side Lobe Level using Soret ZP with an optimal reference radius and without causing neither main lobe broadening or gain reduction. In the second effect, by using numerical simulations an increasing of depth of focus, more than 2 times, in comparison with classical Soret ZP with high numerical aperture (F/D = 2.5), was observed. [ABSTRACT FROM AUTHOR]

Zone plate lenses are used in many areas of physics where planar geometry is advantageous in comparison with conventional curved lenses. There are several types of zone plate lenses, such as the well-known Fresnel zone plates (FZPs) or the more recent fractal and Fibonacci zone plates. The selection of the lens material plays a very important role in beam modulation control. This work presents a comparison between FZPs made from different materials in the ultrasonic range in order to use them as magnetic resonance imaging (MRI) compatible materials. Three different MRI compatible polymers are considered: Acrylonitrile butadiene styrene (ABS), polymethyl methacrylate (PMMA) and polylactic acid (PLA). Numerical simulations based on finite elements method (FEM) and experimental results are shown. The focusing capabilities of brass lenses and polymer zone plate lenses are compared. [ABSTRACT FROM AUTHOR]

A thin planar acoustic lens composed of concentric annular slits on a circular steel plate covered by a membrane made up of aluminum foil is numerically designed, and its performance in focusing low-frequency sound in air is experimentally demonstrated. Resonant fundamental modes of the membrane-backed cavities are calculated via the Finite-Element Method. By carefully adjusting the radii of the concentric cavities, the phase discontinuity across the cavities can be employed to achieve constructive interference of sound waves at a focal length of 200 mm, which is close to the acoustic wavelength, at 2190 Hz. The designed lens has an overall radius which is approximately one fourth of the radius of a conventional acoustic Fresnel lens employing a wavelength path length difference between adjacent cavities. [ABSTRACT FROM AUTHOR]

Traditional acoustic lenses modulate the ultrasonic beam due to their curved surfaces and the refractive material of which they are made. In this work, a different type of acoustic lens, based on Polyadic Cantor Fractals (PCF), is presented and thoroughly analyzed. These new Polyadic Cantor Fractal Lenses (PCFLs) are completely flat and easy to build, and they present interesting modulation capabilities over the acoustic profile. The dependence of the focusing profile on the PCFL design parameters is fully characterized, and it is shown that certain design parameters provide a dynamic control, which is critical in many medical applications such as thermal ablation of tumors. [ABSTRACT FROM AUTHOR]

Focused transducers composed of flat piezoelectric ceramic coupled with an acoustic lens present an economical alternative to curved piezoelectric ceramics and are already in use in a variety of fields. Using a displacement/pressure (u/p) mixed finite element formulation combined with parametric level-set functions to implicitly define the boundaries between the materials and the fluid–structure interface, a method to optimize the shape of acoustic lens made of either one or multiple materials is presented. From that method, two 400 kHz focused transducers using acoustic lens were designed and built with different rapid prototyping methods, one of them made with a combination of two materials, and experimental measurements of the pressure field around the focal point are in good agreement with the presented model. [ABSTRACT FROM AUTHOR]

An acoustic metasurface made of a composite structure of cavity and membrane is proposed and numerically investigated. The target frequency is in the low frequency regime (570 Hz). The unit cells, which provide precise local phase modulation, are rather thin with thickness in the order around 1/5 of the working wavelength. The numerical simulations show that the designed metasurface can steer the reflected waves at will. By taking the advantage of this metasurface, an ultrathin planar acoustic axicon, acoustic lens, and acoustic nondiffracting Airy beam generator are realized. Our design method provides a new approach for the revolution of future acoustic devices. [ABSTRACT FROM AUTHOR]

In medical ultrasound transducers, the transmission mode (pass-through) approach has been used to estimate the characteristics of the acoustic lens. However, it is difficult to measure the acoustic lens properties with high precision because of human, systemic, or mechanical measurement errors. In this paper, we propose a low-cost estimation instrument for acoustic lens properties connected with a customized database. In the instrument, three-axis and one-axis transmitting and material fixtures accurately align the transmitting and receiving transducers separately. Through the developed instrument, we obtained a precise standard deviation of the attenuation coefficient and velocity of the acoustic lens material of 0.05 dB/cm and 2.62 m/s, respectively. Additionally, the simultaneous alignment between the fixtures is controllable with developed programs, thus generating very accurate information of the acoustic lens about the testing ultrasound transducer. In our instrument, the database could support users in managing the result data efficiently. User programs developed using LabVIEW provide the capability to obtain precise values of the attenuation coefficient and velocity, which represent the fundamental material characteristics of the acoustic lens of the medical ultrasound transducers. The developed review program of the customized database can also search the acoustic lens information and store the experimental results. [ABSTRACT FROM AUTHOR]

Complex diffraction limited acoustic fields can be generated from a single element transducer using inexpensive 3-D printable acoustic kinoforms. This is extremely promising for a number of applications. However, the lack of ability to vary the field limits the potential use of this technology. In this work, this limitation is circumvented using multi-frequency acoustic kinoforms for which different acoustic fields are encoded onto different driving frequencies. An optimisation approach based on random downhill binary search is introduced for the design of the multi-frequency kinoforms. This is applied to two test cases to demonstrate the technique: a kinoform designed to generate the numerals “1,” “2,” and “3” in the same plane but at different driving frequencies, and a kinoform designed to generate 3 sets of eight foci lying on a circle with a driving-frequency-dependent radius. Field measurements from these samples confirmed that multi-frequency acoustic kinoforms can be designed that switch between different arbitrary, pre-designed, acoustic field patterns in the target plane by changing the driving frequency. [ABSTRACT FROM AUTHOR]

We are interested in shape sensitivity analysis for an optimization problem arising in medical applications of high intensity focused ultrasound. The goal is to find the optimal shape of a focusing acoustic lens so that the desired acoustic pressure at a kidney stone is achieved. Coupling of the silicone acoustic lens and nonlinearly acoustic fluid region is modeled by the Westervelt equation with nonlinear strong damping and piecewise constant coefficients. We follow the variational approach to calculating the shape derivative of the cost functional which does not require computing the shape derivative of the state variable; however assumptions of certain spatial regularity of the primal and the adjoint state are needed to obtain the derivative, in particular for its strong form according to the Delfour-Hadamard-Zolésio structure theorem. [ABSTRACT FROM AUTHOR]

A layout optimization method for a two-dimensional acoustic lens system used in underwater imaging is presented. To this end, a shape and topology optimization is formulated for the design problem of a lens system for the first time. The layout of a lens system to be optimized includes the number of lenses, shape of lens surfaces, distances between lenses, and lens materials. A phase field function is employed to implicitly parameterize the boundaries of the lenses, which move according to design sensitivities during optimization. Multiple lenses with different materials are optimized using a single phase field function. Because the ratio of the acoustic wavelength with respect to lens dimensions is large, diffraction effects should be taken into account. Accordingly, the performance of a lens system should be analyzed using wave acoustics and not the ray tracing method. The optimization problem is formulated to remove the aberrations of coma and field curvature. The validity of the proposed optimization method is demonstrated by solving benchmark design problems including a lens system with a large field of view. [ABSTRACT FROM AUTHOR]

We report a flat acoustic lens with a periodical structure to transform a divergent beam into a Bessel-like beam. Using the Schlieren imaging technique, the propagation process of acoustic wave in the periodical structure was experimentally observed. The pressure distribution in the main lobe is in good agreement with the Bessel function and the positions of the side lobes are close to the peak of the Bessel function. To observe the directivity of the beam, simulations were performed using the finite-element method. The simulation results indicated that the transmitted acoustic intensity at the central axis was several times greater with the lens than without it. The applicability of the lens for detecting the location of an acoustic source was also investigated. [ABSTRACT FROM AUTHOR]

This paper develops effective numerical models to study the wave dynamics of highly nonlinear tensegrity metamaterials. Recent studies have highlighted the geometrically nonlinear response of structural lattices based on tensegrity prisms, which may gradually change their elastic response from stiffening to softening through the modification of mechanical, geometrical, and prestress variables. We here study the nonlinear dynamics of columns of tensegrity prisms subject to impulsive compressive loading. An effective nonlinear rigid body dynamics is employed to simulate the dynamic response of such metamaterials. We illustrate how to pass from the matrix to the vector form of the equations of motions, on accounting for a rigid response of the compressive members (bars). Numerical simulations show that the wave dynamics of the examined metamaterials supports compression solitary pulses with profile dependent on the elastic properties of the tensile members (strings), the given impact velocity, and the applied prestress. We conclude that tensegrity columns can be effectively used as tunable acoustic lenses, which are able to generate acoustic solitary waves with adjustable profile in a host medium. [ABSTRACT FROM AUTHOR]

An inhomogeneous acoustic metamaterial lens based on spatial variation of refractive index for broadband focusing of underwater sound is reported. The index gradient follows a modified hyperbolic secant profile designed to reduce aberration and suppress side lobes. The gradient index (GRIN) lens is comprised of transversely isotropic hexagonal microstructures with tunable quasistatic bulk modulus and mass density. In addition, the unit cells are impedance-matched to water and have in-plane shear modulus negligible compared to the effective bulk modulus. The flat GRIN lens is fabricated by cutting hexagonal centimeter scale hollow microstructures in aluminum plates, which are then stacked and sealed from the exterior water. Broadband focusing effects are observed within the homogenization regime of the lattice in both finite element simulations and underwater measurements (20–40 kHz). This design approach has potential applications in medical ultrasound imaging and underwater acoustic communications. [ABSTRACT FROM AUTHOR]

Due to a low lateral resolution in elevation plane, the application of linear ultrasonic phased array transducers (ULPAT) is limited. The objective is to enhance the spatial resolution by means of spatial dual focusing technique of ULPAT in two orthogonal planes - azimuth and elevation. A design of a special delay line possessing a convex cylindrical lens has been proposed. The lens enables fixed geometrical focusing of the excited beam in the elevation plane, while in the other, azimuth, plane the beam is electronically focused at an arbitrary point by the active aperture of the ULPAT. The simulation results demonstrate the extended possibilities of resolution enhancement. Experiments show that the undetectable defect is detected using the designed acoustic lens. In addition, the interfaces of the intermediate layer of metal composite were detected. [ABSTRACT FROM AUTHOR]

Artificially structured materials with unit cells at sub-wavelength scale, known as metamaterials, have been widely used to precisely control and manipulate waves thanks to their unconventional properties which cannot be found in nature. In fact, the field of acoustic metamaterials has been much developed over the past 15 years and still keeps developing. Here, we present a topical review of metamaterials in acoustic wave science. Particular attention is given to fundamental principles of acoustic metamaterials for realizing the extraordinary acoustic properties such as negative, near-zero and approaching-infinity parameters. Realization of acoustic cloaking phenomenon which is invisible from incident sound waves is also introduced by various approaches. Finally, acoustic lenses are discussed not only for sub-diffraction imaging but also for applications based on gradient index (GRIN) lens. [ABSTRACT FROM AUTHOR]

This study investigates the application of air-coupled ultrasonics for nondestructive, non-contact characterization of polymer and polymer-matrix composite samples. A lot of progress has been made with this technique which does not require any liquid couplant, hence water sensitive samples can be studied without deterioration. We demonstrate the power of highly focused transducers for topography imaging of structured polymer surfaces such as a craquelure, fingerprint or an acoustic lens with a lateral resolution up to 0.1 mm and an elevation resolution of 2 μm. With a further technique, transmission resonance, the thickness of plates can be imaged with a resolution of 3 μm. Air-coupling is also used for checking the integrity of a polymer-matrix composite plate, revealing invisible internal impact damage. Finally, a single-sided Lamb-wave technique is used to detect sub-wavelength artificial defects such as notches and through-holes in plates. [ABSTRACT FROM AUTHOR]

Shape optimization of two-dimensional lenses for acoustic cameras used in underwater acoustic imaging is presented. To understand elastic shear wave effects of a lens on the pressure field of the medium, we couple two physics models, namely a pressure acoustics model and a linear elastic model and formulate it in the finite element analysis framework. As a result, the focal length of a lens is calculated to be longer than that in the absence of shear effects. The accuracy of our model is tested against experimental results, and compared with finite element analysis of a single physics model as well as the hybrid method - a method using geometrical acoustics and wave acoustics. Pressure at the focal point is optimized using geometric parameters for lens surfaces as design variables. Performance differences due to temperature change or obliquely incident waves are controlled using design constraints. The effectiveness of our optimization formulation is verified by solving single- and dual-lens design problems. [ABSTRACT FROM AUTHOR]

Photoacoustic (PA) imaging is a modality for achieving high-contrast images of blood vessels or tumors. Most PA imaging systems use complex reconstruction algorithms under conventional linear array transducers. We introduced the optical simulating method to improve the acoustic lens design and obtain a PA imaging system with improved spatial revolution (a 0.5-mm point spread function and a lateral image resolution of more than 1 mm) is realized using a 4f aspherical acoustic lens. The acoustic lens approach improved the image resolution and enabled direct reconstruction of three-dimensional (3-D) PA images. The system demonstrated a lateral resolution of more than 1 mm, a field of view of 8.5 deg, and a depth of focus of 10 mm. The system displays great potential for developing a real-time 3-D PA camera system for biomedical ultrasound imaging applications. [ABSTRACT FROM AUTHOR]

A lens acoustic microscope is proposed in which a two-dimensional high-frequency ultrasound array is used instead of a single converter. In this microscope, electronic scanning of the region of naturally occurring focus of the acoustic lens can be performed and the focus can be held electronically at different distances, including inside solid objects. It was demonstrated that the multielement microscope has potentially higher performance compared with a single-element scanning microscope and is not inferior in resolution power. [ABSTRACT FROM AUTHOR]

Impedance is an important issue in the design of acoustic lenses because mismatched impedance is detrimental to real focusing applications. Here, we report two designs of acoustic lenses that focus acoustic waves in water and air, respectively. They are tailored by acoustic meta-surfaces, which are rigid thin plates decorated with periodically distributed sub-wavelength slits. Their respective building blocks are constructed from the coiling-up spaces in water and the layered structures in air. Analytic analysis based on coupled-mode theory and transfer matrix reveals that the impedances of the lenses are matched to those of the background media. With these impedance-matched acoustic lenses, we demonstrate the acoustic focusing effect by finite-element simulations. [ABSTRACT FROM AUTHOR]