3D-printed acoustic metasurface with encapsulated micro-air-bubbles for frequency-selective manipulation.

Understanding the behavior of water-immersed membranes in the megahertz range is critical to develop novel acoustic metamaterials compatible with biomedical ultrasound applications. Herein, we study the influence of water on the resonance frequency and quality factor near the megahertz range of silicon nitride membranes fully immersed in water using laser Doppler vibrometry. The resonance frequency of silicon nitride membranes significantly decreases in water compared to air. For a 40 µm wide membrane, the resonance frequency is reduced from 11.2 MHz in air to 1.24 MHz after immersion in water, which is confirmed by finite element method simulations. Our results indicate that the water mass loading plays a major role in the resonance frequency reduction, with a ratio of water mass to membrane mass of 102 and NAVMI factors of Γ∼1.3. We attribute the main losses to acoustic radiation with small contributions from viscous damping. We estimate that silicon nitride membranes with widths below 50 µm are required to build negative metamaterials operating above 1 MHz. The large NAVMI factors suggest strong coupling between membrane motion and acoustic waves in water, which is important to develop metamaterials able to manipulate acoustic fields.

Ultra low frequency (ULF) acoustic waves, propagating in the ionosphere, may produce a decrease of the ionospheric density of the F-layer. In turn, decreasing the density of the ionospheric plasma results in the increase of the transparency of the ionosphere for cosmic radio waves. In this paper, simulations of nonlinear frequency down-conversion of extremely low frequency (ELF) acoustic waves into ULF acoustic waves are presented. The propagation of excited ULF waves within the non-isothermal ionosphere is simulated, and the increase of the transparency of the ionospheric F-layer for cosmic radio waves due to plasma density modulation is also considered. We argue that this phenomenon takes place jointly with an increase of the transparency caused by the periodic structure of extremely low frequency (ELF) acoustic waves in the E-layer of the ionosphere. Both phenomena have previously been confirmed experimentally.

The advent of acoustic metasurfaces (AMs), which are the two-dimensional equivalents of metamaterials, has opened up new possibilities in wave manipulation using acoustically thin structures. Through the interaction between the acoustic waves and the subwavelength scattering, AMs exhibit versatile capabilities to control acoustic wave propagation such as by steering, focusing, and absorption. In recent years, this vibrant field has expanded to include tunable, reconfigurable, and programmable control to further expand the capacity of AMs. This paper reviews recent developments in AMs and summarizes the fundamental approaches for achieving tunable control, namely, by mechanical tuning, active control, and the use of field-responsive materials. An overview of basic concepts in each category is first presented, followed by a discussion of their applications and details about their performance. The review concludes with the outlook for future directions in this exciting field.

This thesis deals with electromagnetic inspired acoustic metamaterials, enabling sound-matter interactions in different wave scenarios that include propagation, guided-waves, radiation, refraction, reflection and transmission. To this end, a particular emphasis is placed on introducing novel applications for acoustic metamaterials operating on each one of the aforementioned wave scenarios. A few years ago, metamaterials have been introduced as a new class of composite artificial materials, engineered to produce unusual effective material properties not readily available in nature. Electromagnetic metamaterials are probably the oldest class of metamaterials, being nowadays in the process of reaching maturity and being proposed for interesting commercial applications. On the other hand, as in most young but not yet mature emerging fields of science, acoustic metamaterials are still providing lots of fertile and unexplored ground for research and study. Despite many inherent physical differences, the propagation of electromagnetic and acoustic waves are both governed by a similar mathematical model, the so-called Helmholtz wave equation. The main purpose of this work is to leverage this amazing similarity by translating recent advances in electromagnetic metamaterials into their corresponding, previously unseen, applications in acoustics. The first contribution of this thesis is to adapt the classic electromagnetic transmission-line theory to allow the design of acoustic metamaterials. The proposed circuit-based theory finds a direct application in the design of composite right/left hand transmission-line metamaterials, which yield novel guided-wave applications for acoustic metamaterials. Then, the developed theory is leveraged to model and achieve optimal design of different acoustic metamaterial-based devices, such as leaky-wave antennas and reflector-type metasurfaces. The second part of the thesis spins around acoustic leaky-wave antennas and their different functionalities, showing that they are able to act as acoustic dispersive prisms in the refraction mode and as acoustic single sensor direction finder in the radiation mode. Studying reflection phenomena in sound-metasurface interactions constitutes the next part of this thesis, where a membrane-capped cavity is introduced performing as an ultra-thin unit-cell for reflective acoustic metasurfaces. This leads to exciting applications of the concept, like acoustic reflectarray antennas and acoustic metasurface skin-cloaks Finally, the last part of this thesis deals with transmission phenomena in acoustic metasurfaces and, especially, in orbital angular momentum metasurfaces. This concept results in the design of an innovative labyrinthine-type helicoidal unit-cell that is used for phase coding a surface and transforming impinging acoustic wavefronts into transmitted helical wavefronts.

Combustion and extinction characteristics of an ethanol pool fire perturbed by low–frequency acoustic waves

We present a linear stability analysis of the perturbation modes in anisotropic magnetohydrodynamic (MHD) flows with velocity shear and strong magnetic field. Collisionless or weakly collisional plasma is described within the 16-momentum MHD fluid closure model that takes into account not only the effect of pressure anisotropy but also the effect of anisotropic heat fluxes. In this model, the low frequency acoustic wave is revealed into a standard acoustic mode and higher frequency fast thermo-acoustic and lower frequency slow thermo-acoustic waves. It is shown that thermo-acoustic waves become unstable and grow exponentially when the heat flux parameter exceeds some critical value. It seems that velocity shear makes thermo-acoustic waves overstable even at subcritical heat flux parameters. Thus, when the effect of heat fluxes is not profound acoustic waves will grow due to the velocity shear, while at supercritical heat fluxes the flow reveals compressible thermal instability. Anisotropic thermal instability should be also important in astrophysical environments, where it will limit the maximal value of magnetic field that a low density ionized anisotropic flow can sustain.

This study is focused on exploring the feasibility of an all-optic surface scanning method in determining the size and position of a submerged, laser generated, optoacoustic (OA) source. The optoacoustic effect in this case was generated when the absorption of a short electromagnetic pulse in matter caused a dielectric breakdown, a plasma emission flash and a subsequent acoustic wave. In the experiment, a laser pulse with λ = 1064 nm and 12 ns pulse length was aimed at a volume of deionized water. When the laser beam was focused by a f = 16 mm lens, a single dielectric breakdown spot occurred. When a f = 40 mm was used several breakdowns in a row were induced. The breakdowns were photographed using a double shutter camera. The acoustic wave generated by the dielectric breakdowns were detected at a point on the water surface using a laser Doppler vibrometer (LDV). First, the LDV signal was used to calculate the speed of sound with an accuracy of 10 m/s. Secondly, the location and length of the dielectric breakdown was calculated with an accuracy of 1 mm. The calculated position matched the breakdown location recorded by a camera. The results show that it is possible to use LDV surface measurements from a single spot to determine both the position and length of the OA source as well as the speed of sound in the medium. Furthermore, the LDV measurements also show a secondary peak that originates from the OA source. To unravel the origin and properties of this interesting feature, further investigations are necessary

A simple measurement system is developed to study the optical effect of the low-frequency liquid surface wave which excited by underwater acoustic source.The high stability and clear diffraction pattern was observed experimentally.The relationship between diffraction patterns divergence angle and the distance of the acoustic signal was derived.Furthermore,with the increase of distance,the diffraction patterns divergence angle will decrease.The damping characters of the liquid surface wave was theoretically obtained when underwater acoustic wave spread to the liquid surface.The analytical expression between diffraction patterns divergence angle and the liquid surface wave amplitude was theoretically derived.It was found that the surface wave amplitude is exponent attenuation with the change of the horizontal distance.The attenuation coefficients is dependent on the frequency of liquid surface acoustic wave,and the greater frequency,the smaller attenuation coefficient.

The design method with a non-resonant one-dimensional distributed transmissionline model for broadband electromagnetic metasurfaces is extended to acoustic metasurfaces and a broadband acoustic metasurface for refracting a normal incident acoustic plane wave is designed according to the introduced method. It is shown that we can theoretically determine the characteristic impedance and the line length for realizing the acoustic metasurface in the same way as in the case of electromagnetic metasurfaces. In order to confirm the validity of the introduced method, circuit simulations mimicking acoustic numerical simulations are carried out and the broadband operations of the designed acoustic metasurface are shown from the results of the complex voltage distributions corresponding to the acoustic pressure those.

Acoustic metasurfaces have shown great promise for applications in many acoustic wave modulation fields, while the use of intelligent optimization algorithms has facilitated the design of acoustic metasurfaces. In this paper, we utilize the stop band properties of phononic crystals to design a metasurface that can achieve both transmission focusing and reflection focusing, as well as a metasurface that utilizes acoustic focusing to filter the incident acoustic wave. In order to achieve the best effect, this paper uses the taboo genetic algorithm to optimize the arrangement order of the structural units controlling different focuses in the acoustic metasurface, and the optimization achieves a good focusing effect, which expands the design of acoustic metasurfaces as well as the application scenarios of acoustic focusing.

In this paper, a double-layer acoustic metasurface (DAM) composed of a fixed lower acoustic metasurface (LAM) and a rotatable upper acoustic metasurface (UAM) is proposed for the generation of mode-reconfigurable acoustic orbital angular momentum (OAM). The UAM and LAM are divided into multiple sections, in which the hybrid structures combining cascaded Helmholtz resonators and a straight pipe are adopted to achieve specific phase compensation. By rotating the UAM, the incident acoustic plane wave can be efficiently converted into the vortex acoustic waves of reconfigurable topological charges ranging from −5 to +5 with distinguishable purity. Furthermore, the influences of the parameters on the purity of the generated topological charges have been investigated and discussed, such as the distance between LAM and UAM, rotatable angle error, and operating frequency. With the capability of reconfigurable OAM modes, the proposed DAM can be used to efficiently increase capacity or to conveniently switch between different channels in underwater vortex acoustic communications.

Nonchemical flame control using acoustic waves from a subwoofer and a lightweight carbon nanotube thermoacoustic projector was demonstrated. The intent was to manipulate flame intensity, direction and propagation. The mechanisms of flame suppression using low frequency acoustic waves were discussed. Laminar flame control and extinction were achieved using a thermoacoustic ‘butterfly’ projector based on freestanding carbon nanotube sheets.

A non-contact inspection method of tile debonding using tuned acoustic wave and laser doppler vibrometer

Acoustic metasurfaces use the phase gradient of a single layer to reflect/refract waves in any direction. This study shows that other than wave steering, acoustic metasurfaces can exhibit wave splitting or trapping through the geometry design. Previous studies focused on the generalized Snell’s law when developing metasurfaces and attempted to prevent wave leakages. On the contrary, this study exploits the periodicity of metasurfaces to accomplish acoustic wave splitting, which leads to a similar concept to metagrating. For acoustic wave trapping, we show that through proper arrangements, an acoustic wave can be localized in a specific region without using any boundaries based on the generalized Snell’s law. A design formula is provided to trap waves from any incident angle or at any frequency. The analytical and numerical results are in good agreement, verifying the effectiveness of the proposed concept for wave splitting and trapping. This study shows the versatile applications of acoustic metasurfaces and is useful for interferometry and energy harvesting.

Advances in additive manufacturing technology are promising for the creation of materials to control acoustic waves. This work presents the design of an acoustic metasurface (AMS) created from a periodic two-dimensional array of multi-material scatterers embedded in an elastomeric matrix. Finite element analysis (FEA) of the AMS shows that the absorption of acoustic energy is dependent on direction of incident wave propagation if the material properties are asymmetric within the scatterers while the acoustic transmission remains symmetric. Asymmetry of acoustic absorption is highly sensitive to small changes in scatterer geometry and property distribution. The AMS design must therefore consider inherent variability in manufacturing processes and the resultant stochastic asymmetric absorption performance of the as-built AMS, which is computationally expensive to determine using FEA. A machine learning classifier is trained to replace FEA and enable efficient robust design of the AMS via Monte Carlo simulations. The classifier is trained to be valid for a variety of characteristic manufacturing variations and the trivial expense to call it leads to significantly faster design with awareness of manufacturing variability. This work demonstrates how robust design approaches can be used to select fabrication methods suitable for acoustic materials. [Work supported by NSF and ONR.]