Incident Acoustic Wave Research Articles

Simulating transmission through anisotropic pentamode materials submerged in water

Interest has increased in recent years on the potential for underwater metallic metamaterials, which can be linked to the advancements in metal 3-D printing. One metamaterial of particular interest is an underwater anisotropic pentamode, i.e., a structure designed to support a single longitudinal wave for a broad-range of frequencies whose speed will change depending on its orientation relative to the incident acoustic wave. In an attempt to close the gap between experiment and theory, COMSOL simulations have been developed modeling the transmission and reflection of a normal plane wave incident on an anisotropic pentamode metamaterial consisting of tetrahedral unit cells. The utilization of the Floquet periodic boundary conditions allows for the replication of a semi-infinite medium response composed of repeating unit cells while only using a finite CAD drawing. Meshing techniques and geometric work-arounds are discussed in an attempt to achieve a cost-effective computation absent of numerical instabilities for a broad range of frequencies. Results show dynamic stress concentration is focused mainly at the joints of the unit cells. Plates designed to separate the fluid medium from the internal structure are shown to exhibit higher modes of vibrations, however contribute little to the far field due to energy flux conservation.

Multiple selective absorption peaks of acoustic waves in a one-dimensional composite

We investigate a one-dimensional composite with a very low loss coefficient and find that multiple selective absorption peaks reach close to 100% absorption of incident acoustic waves when the acoustic waves propagate through this system. Especially, the unidirectional absorption is demonstrated when the propagation direction of acoustic waves is reversed. These high absorptions stem from the excitations of the strong resonances, which depend on not only the low loss in this composite but also the huge impedance mismatch at the outgoing interface. To further investigate those selective absorption, we construct an equivalent structural model, and our calculations show that these absorption peaks have the extraordinary response to the imaginary part of the effective mass density and bulk modulus. The results exhibit that the imaginary part of the effective mass density and bulk modulus at the frequencies of the absorption peaks abruptly appear, in the meanwhile the real part of those become zero. Based on these theoretical results, an unidirectional absorption device with multiple selection of frequencies can be designed.

Enhanced sound absorption in two-dimensional continuously graded phononic crystals

A two-dimensional continuously graded phononic crystal (CGPC) is proposed, which consists of continuously graded structures. A model considering viscous and thermal effects is established, and the predicted sound absorption coefficient and specific acoustic impedance of CGPC are qualitatively verified by experimentally measured results. The effective refractive indexes of CGPC monotonically increase along the incident acoustic wave at a specific frequency. The increased effective refractive indexes of CGPC result in enhanced acoustic scattering. Enhanced acoustic scattering lengthens the propagation path, subsequently more dissipated energy, which improves the sound absorption performance of CGPC, compared with uniform phononic crystal.

Acoustic focusing effect based on artificial periodic structure

To achieve high-efficiency acoustic focusing, an artificial periodic acoustic structure composed of two-dimensional three-component cell arrays was used to manipulate the transmission wave fronts of acoustic waves, and the acoustic field characteristics were numerically simulated by COMSOL finite element software. The results showed that whether the spherical acoustic wave generated by the point excitation source or the planar acoustic wave generated by the linear excitation source was used as the incident wave, an emission focus point can be generated at the top of the acoustic model. The intensity of the acoustic pressure at the focus point depended on the frequency of the incident acoustic waves. Under the same vibration period, the acoustic pressure will increase first and then decrease. It will reach a maximum value at the resonance frequency. In addition, when the model was cut into a right-angled trapezoidal structure with the axis of symmetry as a cross-section, the acoustic wave can produce a variable-direction focusing effect, and the “acoustic levitation” effect can also be realized. This series of anomalous acoustic phenomena can provide a new direction for acoustic directional transmission, acoustic wave detection, and acoustic stealth.

Acoustic Wave Incidence on a Medium Containing Multifractional Bubbly Liquid

The mathematical model that determines reflection and transmission of acoustic wave through a medium containing multifractional bubbly liquid is presented. For the water-water with bubbles-water-waterwith bubbles-water model the wave reflection and transmission coefficients are calculated. It is revealed that the special dispersion and dissipative properties of the layer of bubbly liquid can significantly influence on the reflection and transmission of acoustic waves in multilayer medium.

Steering the propagation direction of a non-linear acoustic wave in a solid material

In this research non-collinear wave mixing is used as a non-destructive testing method where the amplitude of the scattering wave contains information on the condition of a material. The practical implementation of non-collinear wave mixing as a non-destructive testing technique is limited by many factors such as the geometry and shape of the structure, the accessibility to the specimen’s surfaces and the ultrasonic sensors available to perform measurements. A novel approach to steer the propagation direction of a generated wave from the mixing of two incident acoustic waves is proposed. The angle of the scattering wave is controlled by the frequencies of the two interaction waves, rather than by the angle between these waves. The scattering amplitude was analytically solved for the longitudinal plus shear interaction process. The analytical solution was validated with experiments. The model qualitatively agrees with the experiments. Furthermore, the possibility to use a wider range of excitation frequencies of the incident waves was found. This is a great advantage in applications where the space and access to the specimen under test is limited.

Tunable perfect negative reflection based on an acoustic coding metasurface

Artificial manipulation of acoustic waves is a highly active research area, in which negative reflection is an exotic phenomenon. Emerging acoustic coding metasurfaces (ACMs) show great power in controlling acoustic waves. Here, we propose an ACM, where the state “0” or “1” is achieved by pushing up or pressing down a rigid rectangular strip. The ACM with a tunable coding sequence can adjust the direction of negative reflection as well as suppress the specular reflection. The feasibility of the ACM is demonstrated by totally reflecting the incident acoustic waves from 22° to the directions of −39°, −47°, and −61°. Furthermore, the ACM can spread the incident white noise into its acoustic spectral components like rainbows. This work may open diverse routes toward reconfigurable acoustic metasurfaces that are critical for practical functional devices, which are promising in potential applications including acoustic beam steering, retro-reflection, and surface wave sensing.

Reflection of Acoustic Wave Incident Normal to the Interface between Two Multifraction Gas Suspensions

The characteristics of the reflection and refraction of an acoustic wave at its normal incidence to an interface between two multifraction gas suspensions have been studied. We obtain the calculation formulas for the impedance of a multifraction gas suspension, the reflection coefficient, and refraction index. The dependences of the absolute values and arguments of the refraction index and reflection coefficient on the dimensionless frequency are obtained.

Acoustic transmissive cloaking using zero-index materials and metasurfaces

A design of acoustic transmissive cloaking is proposed using a hybrid cloaking shell. The shell consists of one layer of metasurface to manipulate both the incident and exit acoustic wavefronts and one layer of zero-index metamaterial to achieve acoustic energy tunneling. Incident acoustic wave will remain its wavefront after propagating through our hybrid cloaking shell constructed by experimentally realizable acoustic metamaterials. As there is no intrinsic limit to the object to be cloaked, the strategy of our transmissive cloaking geometry has wide potential applications in both airborne sound and underwater ultrasound.

Acoustic Absorption Characteristics of New Underwater Omnidirectional Absorber**Supported by the National Natural Science Foundation of China under Grant Nos 51575201 and 11204098.

We investigate a new underwater omnidirectional absorber with acoustic black hole effect to realize a broadband omnidirectional acoustic wave absorption. Based on multiple scattering theory, a two-dimensional axisymmetric model of underwater omnidirectional absorber comprised of an acoustic gradient refractive index structure and a hollow core is developed, and the mechanisms of omnidirectional absorption and dissipation of acoustic waves are studied. The numerical results indicate that the omnidirectional absorber developed here can achieve the omnidirectional absorption of incident acoustic waves in a broadband frequency and can effectively reduce the backscattering of acoustic waves. It potentially provides a new notion for underwater acoustic coating design.

Low frequency sound absorption by compliant Helmholtz resonators

A traditional Helmholtz resonator is a rigid-walled cavity and an open neck, which result in one acoustic resonance. In this study, a soft-walled cavity with an open neck, a compliant Helmholtz resonator, is investigated for broadband and large absorption of sound at low frequencies. By applying the soft materials, the dynamics of the compliant walls and the material impedance are coupled to the acoustic pressure filed in the cavity. Thus the impedance of the compliant resonator system as seen by an incident acoustic wave is determined by the interaction of the wall dynamics, material properties, and acoustic domain. Compared to a traditional Helmholtz resonator with the same geometric dimensions, the compliant Helmholtz resonator exhibits multiple resonances with large absorption of sound. In addition, strategic design of the structural composition and the material selection leads to resonant behavior at frequencies lower than the conventional Helmholtz resonance. Through exploring an analytical model developed to characterize the resonator, the threshold on material and structural compliance to yield traditional Helmholtz behavior is also illuminated.

Development and performance analysis of a low-cost MEMS microphone-based hearing aid with three different audio amplifiers

In this paper, MEMS-based capacitive microphone and low-cost amplifier are designed for low-cost power-efficient hearing aid application. The developed microphone along with the associated circuitry is mounted on a common board in the form of pocket-type (body-worn) device. The designed microphone consists of a flexible circular silicon nitrite (Si3N4) diaphragm and a polysilicon-perforated back plate with air as dielectric between them. The incident acoustic waves on the sensor cause deflection of the diaphragm to alter the air gap between the perforated back plate (fixed electrode) and the diaphragm (moving plate) which causes a change in capacitance. The acoustic pressure applied to the microphone is from 0 to 100 Pa for an operating range of 100 Hz–10 kHz which corresponds to the audible frequency range in case of human beings. The main purpose of this work is to increase the longevity of battery used in conventional hearing aids. The designed MEMS microphone with Si3N4 diaphragm is capable of identifying acoustic frequencies (100 Hz to 10 kHz) which correspond to a specific change in absolute pressure from 0 to 100 Pa for 2-micron-thick diaphragm with a sensitivity of about 0.08676 mV/Pa. The design of the sensor and the characteristics analysis are performed in FEM-based simulation software, which are later validated in real time. The prototype is designed using MEMS microphone and low-cost amplifier ICs with biasing components in the form of pocket-type (body-worn) hearing aid. In order to study the performance of proposed device, three different market-available amplifiers with controllable gain are used. Finally, the performance of the hearing aid is studied through audio spectrogram analysis to choose the best-suited amplifier among the three.

The acoustic impedance of a laminar viscous jet through a thin circular aperture

The unsteady axisymmetric flow through a circular aperture in a thin plate subjected to harmonic forcing (for instance under the effect of an incident acoustic wave) is a classical problem first considered by Howe (Proc. R. Soc. Lond. A, vol. 366, 1979, pp. 205–223), using an inviscid model. The purpose of this work is to reconsider this problem through a numerical resolution of the incompressible linearized Navier–Stokes equations (LNSE) in the laminar regime, corresponding to $Re=[500,5000]$. We first compute a steady base flow which allows us to describe the vena contracta phenomenon in agreement with experiments. We then solve a linear problem allowing us to characterize both the spatial amplification of the perturbations and the impedance (or equivalently the Rayleigh conductivity), which is a key quantity to investigate the response of the jet to acoustic forcing. Since the linear perturbation is characterized by a strong spatial amplification, the numerical resolution requires the use of a complex mapping of the axial coordinate in order to enlarge the range of Reynolds number investigated. The results show that the impedances computed with $Re\gtrsim 1500$ collapse onto a single curve, indicating that a large Reynolds number asymptotic regime is effectively reached. However, expressing the results in terms of conductivity leads to substantial deviation with respect to Howe’s model. Finally, we investigate the case of finite-amplitude perturbations through direct numerical simulations (DNS). We show that the impedance predicted by the linear approach remains valid for amplitudes up to order $10^{-1}$, despite the fact that the spatial evolution of the perturbations in the jet is strongly nonlinear.

Directional Transmission Characteristics of Acoustic Waves Based on Artificial Periodic Structures

Acoustic artificial periodic structures, also known as phononic crystal, have been widely used in special acoustic problems such as low-frequency vibration reduction and noise reduction due to their bandgap characteristics. Different from previous researches, which focus on the bandgap characteristics, we designed a locally resonant artificial periodic structure, which can not only use the bandgap characteristics to design a directional transmission channel of acoustic waves, but also use its bandpass characteristics to generate multidirectional transmission of acoustic waves. The vibration mechanism of two-dimensional, three-components phononic crystal lattice structures was given, and a variety of directional transmission models of acoustic waves were constructed based on the unit cell. The simulation results showed that acoustic directional transmission characteristics were produced by the locally periodic resonance structure, and the transmission direction depends on the design of the structure. By taking advantage of the superposition properties of acoustic waves, the incident acoustic wave in the horizontal direction can be converted into the outgoing acoustic wave in the vertical direction without changing the position of the model, and this capability greatly expands the application range of the artificial periodic structure. In addition, the transmission characteristics of the directional transmission effect have a strong correlation with the frequency; that is, limited by the wavelength, the acoustic pressure distribution mode at different frequencies will be different. At the resonant frequency of the structure, the transmission efficiency of the acoustic wave will be highest. Moreover, similar to the traditional artificial periodic structure, the acoustic model proposed in this paper also has bandgap characteristics. Using bandgap characteristics, we designed an acoustic directional transmission channel to achieve uniform directional transmission of acoustic waves. These results greatly enrich the application space of artificial acoustic structures and provide new ideas for acoustic communication, acoustic detection, and acoustic stealth.

A kind of tunable acoustic metamaterial for low frequency absorption

Low frequency noise is always an important factor affecting people’s quality of life. At present, the most widely used sound absorbing materials include polyurethane foam, trimeric amine, mineral cotton, textiles, cotton and special sound insulation materials. However, the sizes of these materials are generally large, and the sound absorption efficiencies are often low, especially in a low frequency range (below 2000 Hz). Acoustic metamaterial is a kind of artificial composite material, which is constructed by microunits whose dimensions are much smaller than the working wavelength. The results show that if the strong coupling condition between the resonant scatter and the waveguide is satisfied, the sound energy flowing through the metamaterial will be completely offset by the internal loss of the resonant scatter. Therefore, it is believed that this kind of acoustic metamaterial can solve the absorption problem of low-frequency sound waves. In order to solve this problem, researchers have conducted a lot of exploratory researches. However, most of the structural units that are constructed with acoustic metamaterials are passive, that is, once the material is processed and shaped, its properties are fixed and cannot be changed. This defect greatly limits the development of acoustical metamaterials, so it is urgent to study acoustical metamaterials whose material properties and the working frequency bands are flexibly adjustable. Although tunable acoustic metamaterials have been studied, few people have extended this research to the field of low-frequency tunable sound absorption. In our previous work, we systematically studied the acoustic properties of two kinds of acoustic artificial " meta-atoms”, namely, open hollow sphere model with negative equivalent elastic modulus and hollow tube model with negative equivalent mass density. The research shows that these two kinds of " meta-atoms” both have obvious sound absorption effect. According to our previous studies, in this paper we couple these two kinds of " meta-atoms” into a whole, and design a new nested model of open loop. The model has the advantages of simple structure and easy preparation. Through theoretical analysis, numerical simulation and experimental testing, it is found that the strong coupling resonance effects between these " meta-atoms” can be excited by the low frequency incident acoustic wave in the nested structure, thus achieving nearly perfect sound energy absorption. In addition, the relative impedance of the metamaterial can be changed by simply rotating the inner splitting ring around the axis, therefore the position of the absorption peak can be freely controlled in a wide frequency band. Because of its deep sub-wavelength size, the metamaterial is very useful for miniaturizing and integrating the low-frequency acoustic absorption devices. What is more, this model also lays a foundation for designing the broadband absorbers.

Acoustic Focusing Imaging Characteristics Based on Double Negative Locally Resonant Phononic Crystal

To achieve high-efficiency and high-resolution acoustic focusing, we used artificial periodic acoustic structures composed of cell arrays to manipulate the acoustic transmission wave front, and used finite element software to simulate the acoustic field characteristics. We found that when focuses generated by arc-shaped acoustic excitation sources are incident on one side of the model, the energy will be localized within the structure to reduce its transmission loss, and then the high-resolution emission focus will be generated on the other side of the model. The magnitude of the acoustic pressure at the focus point will change with the frequency of the incident acoustic wave and will reach an extreme value at the resonant frequency. In addition, we explored the acoustic transmission characteristics of structures at different widths and found that it is only necessary to increase the width of the model by an integer multiple of half a wavelength to achieve low loss focusing effects at various distances. The acoustic phenomena studied in this paper may provide new methods for the application of photoacoustic signal detection, ultrasound medical imaging and ultrasonic nondestructive testing.

Acoustic Streaming and Its Applications

Broadly speaking, acoustic streaming is generated by a nonlinear acoustic wave with a finite amplitude propagating in a viscid fluid. The fluid volume elements of molecules, d V , are forced to oscillate at the same frequency as the incident acoustic wave. Due to the nature of the nonlinearity of the acoustic wave, the second-order effect of the wave propagation produces a time-independent flow velocity (DC flow) in addition to a regular oscillatory motion (AC motion). Consequently, the fluid moves in a certain direction, which depends on the geometry of the system and its boundary conditions, as well as the parameters of the incident acoustic wave. The small scale acoustic streaming in a fluid is called “microstreaming”. When it is associated with acoustic cavitation, which refers to activities of microbubbles in a general sense, it is often called “cavitation microstreaming”. For biomedical applications, microstreaming usually takes place in a boundary layer at proximity of a solid boundary, which could be the membrane of a cell or walls of a container. To satisfy the non-slip boundary condition, the flow motion at a solid boundary should be zero. The magnitude of the DC acoustic streaming velocity, as well as the oscillatory flow velocity near the boundary, drop drastically; consequently, the acoustic streaming velocity generates a DC velocity gradient and the oscillatory flow velocity gradient produces an AC velocity gradient; they both will produce shear stress. The former is a DC shear stress and the latter is AC shear stress. It was observed the DC shear stress plays the dominant role, which may enhance the permeability of molecules passing through the cell membrane. This phenomenon is called “sonoporation”. Sonoporation has shown a great potential for the targeted delivery of DNA, drugs, and macromolecules into a cell. Acoustic streaming has also been used in fluid mixing, boundary cooling, and many other applications. The goal of this work is to give a brief review of the basic mathematical theory for acoustic microstreaming related to the aforementioned applications. The emphasis will be on its applications in biotechnology.

Influence of long-range interatomic and interlayer interactions on dispersion of acoustic waves by multilayer graphene

In this study, effects of the long-range interatomic and interlayer interactions on the dispersion of acoustic waves by multilayer graphene are investigated. A hybrid model that accounts for the long-range interatomic and interlayer interactions is developed. In the context of this model, the interatomic interactions are modeled based on the general nonlocal theory. The general nonlocal theory effectively models the dispersions of acoustic waves where it defines two independent nonlocal fields for the longitudinal and transverse acoustic waves. Interlayer interactions are defined to model the van der Waals interaction between the graphene layers. A new approach is proposed to determine the mechanical properties and the equivalent thickness of a single-layer graphene. The Young's modulus and Poisson's ratio of a single-layer graphene is obtained by ∼345MPa and ∼0.08, respectively. The equivalent thickness of a graphene sheet is determined by ∼0.057nm. An experimental validation for the dispersions of four-layer graphene is carried out where a good assent is observed. A parametric study is then presented to demonstrate the significant influence of the long-rang interatomic and interlayer interactions on the dispersion of incident acoustic waves by multilayer graphene.

Large-Eddy Simulation of Acoustic Flame Response to High-Frequency Transverse Excitations

The objective of this numerical study is to analyze the excitation mechanisms of high-frequency thermoacoustic instabilities. Two distinct acoustic wave/flame interaction scenarios are considered, which can arise in a technical combustion system. First, an acoustic oscillation is enforced with the pressure antinode at the burner centerline, termed as pressure forcing. Second, a transverse acoustic oscillation is enforced with the velocity antinode at the burner centerline, termed as velocity forcing. The excitations are created by the interaction of two incident acoustic waves for a range of excitation frequencies of practical relevance. The susceptibility of the flame to acoustic oscillations is quantified using the Rayleigh index. Detailed postprocessing is performed to investigate the underlying thermoacoustic mechanisms and to quantify the contributions of each possible mechanism to the total Rayleigh index of the system as a function of different excitation frequencies. It is observed that the main driving mechanism with pressure forcing can be related to the density oscillation and that the main driving mechanism with velocity forcing can be related to the flame surface oscillation relative to the position of the acoustic pressure node. For both forcing cases, the laminar flame speed is varied to change the length of the flame. With pressure forcing, the highest Rayleigh index is found for a specific flame length, associated to a specific laminar flame speed. With velocity forcing, the Rayleigh index increases when the flame speed is reduced and the flame length is increased, respectively.

Ultra-Broadband Acoustic Diode in Open Bend Tunnel by Negative Reflective Metasurface

We theoretically and numerically propose an open bend tunnel with the capability of realizing ultra-broadband unidirectional transmission. The designed tunnel can isolate acoustic wave incidence from opposite directions and substance like the fluids or objects can exchange freely by employing acoustic gradient metasurface. The underlying mechanism is due to apparent negative reflection in ultra-broadband frequency range when the incoming angle impinging on the metasurface is over the critical incidence. The numerical results keep a good agreement with the theoretical analyses. The proposed design could be employed to generate various situations, like broadband noise control, architectural acoustics and ultrasound imaging.