Acoustic Wavefront Research Articles

Lost and found sunquake in the 6 September 2011 flare caused by beam electrons

The active region NOAA 11283 produced two X-class flares on 6 and 7 September 2011 that have been well studied by many authors. The X2.1 class flare occurred on September 6, 2011 and was associated with the first of two homologous white light flares produced by this region, but no sunquake was found with it despite the one being detected in the second flare of 7 September 2011. In this paper we present the first observation of a sunquake for the 6 September 2011 flare detected via statistical significance analysis of egression power and verified via directional holography and time–distance diagram. The surface wavefront exhibits directional preference in the north-west direction We interpret this sunquake and the associated flare emission with a combination of a radiative hydrodynamic model of a flaring atmosphere heated by electron beam and a hydrodynamic model of acoustic wave generation in the solar interior generated by a supersonic shock. The hydrodynamic model of the flaring atmosphere produces a hydrodynamic shock travelling with supersonic velocities toward the photosphere and beneath. For the first time we derive velocities (up to 140 km s−1) and onset time (about 50 s after flare onset) of the shock deposition at given depths of the interior. The shock parameters are confirmed by the radiative signatures in hard X-rays and white light emission observed from this flare. The shock propagation in the interior beneath the flare is found to generate acoustic waves elongated in the direction of shock propagation, that results in an anisotropic wavefront seen on the solar surface. Matching the detected seismic signatures on the solar surface with the acoustic wave front model derived for the simulated shock velocities, we infer that the shock has to be deposited under an angle of about 30° to the local solar vertical. Hence, the improved seismic detection technique combined with the double hydrodynamic model reported in this study opens new perspectives for observation and interpretation of seismic signatures in solar flares.

Study of the refraction of sound by shear and boundary layers

Performing accurate acoustic measurements in aerodynamic wind tunnel is a challenge of great interest for the aeroacoustics community. Hence aeroacoustic measurements could be obtained in the same facility under the same aerodynamic conditions. This paper shows the refraction effects to be taken into account during aeroacoustics measurements in a closed test section, including flows up to high subsonic Mach numbers. In closed wind tunnel, the wall-mounted microphones are either flush-mounted or recessed in a cavity to avoid perturbations from the boundary layer. Besides, due to the wall, interference pattern generated by reflections occur. Thus, the acoustical wave front is strongly modified compared to the open test section configuration. Numerical computations, using the ONERA’s sAbrinA-v0 CAA code solving the Euler’s equations, allow us to accurately characterize the influence of a shear layer (recessed microphones) and of a boundary layer (flush-mounted microphones) and measurement setups, on the radiation from a monopolar source in the presence of a rigid wall. The CAA computations are made in the range 1 kHz to 10 kHz and for different boundary/shear layers thicknesses. The paper details a comparison between the Amiet’s model and the CAA computations. The comparisons are in a good agreement despite the absence of a wall in the Amiet’s model.

Inverse design method in acoustic wave manipulation

We present here the involvement of inverse design method with the acoustic wave modulation, to simplify how we get the required material parameters and design novel acoustic devices that can perform certain prescribed operation. As an example, the acoustic cloaking will be discussed. The existing acoustic invisibility design based on coordinate transformation method needs negative index media which can be realized through complicate structures. With inverse design method, we have designed a circular cloak device with positive index materials, that successfully make a circular scatterer acoustically invisible. This inverse design method takes a significant step towards realizing practical cloaking devices and would open up new possibilities for manipulating acoustic wavefront with simplified parameter distribution and optimized properties.

A Review of Tunable Acoustic Metamaterials

Acoustic metamaterial science is an emerging field at the frontier of modern acoustics. It provides a prominent platform for acoustic wave control in subwavelength-sized metadevices or metasystems. However, most of the metamaterials can only work in a narrow frequency band once fabricated, which limits the practical application of acoustic metamaterials. This paper highlights some recent progress in tunable acoustic metamaterials based on various modulation techniques. Acoustic metamaterials have been designed to control the attenuation of acoustic waves, invisibility cloaking, and acoustic wavefront engineering, such as focusing via manipulating the acoustic impedance of metamaterials. The reviewed techniques are promising in extending the novel acoustics response into wider frequency bands, in that tunable acoustic metamaterials may be exploited for unusual applications compared to conventional acoustic devices.

Revelation of early detection of co-seismic ionospheric perturbations in GPS-TEC from realistic modelling approach: Case study

GPS-derived Total Electron Content (TEC) is an integrated quantity; hence it is difficult to relate the detection of ionospheric perturbations in TEC to a precise altitude. As TEC is weighted by the maximum ionospheric density, the corresponding altitude (hmF2) is, generally, assumed as the perturbation detection altitude. To investigate the validity of this assumption in detail, we conduct an accurate analysis of the GPS-TEC measured early ionospheric signatures related to the vertical surface displacement of the Mw 7.4 Sanriku-Oki earthquake (Sanriku-Oki Tohoku foreshock). Using 3D acoustic ray tracing model to describe the evolution of the propagating seismo-acoustic wave in space and time, we demonstrate how to infer the detection altitude of these early signatures in TEC. We determine that the signatures can be detected at altitudes up to ~130 km below the hmF2. This peculiar behaviour is attributed to the satellite line of sight (LOS) geometry and station location with respect to the source, which allows one to sound the co-seismic ionospheric signatures directly above the rupture area. We show that the early onset times correspond to crossing of the LOS with the acoustic wavefront at lower ionospheric altitudes. To support the proposed approach, we further reconstruct the seismo-acoustic induced ionospheric signatures for a moving satellite in the presence of a geomagnetic field. Both the 3D acoustic ray tracing model and the synthetic waveforms from the 3D coupled model substantiate the observed onset time of the ionospheric signatures. Moreover, our simple 3D acoustic ray tracing approach allows one to extend this analysis to azimuths different than that of the station-source line.

Born Kernels for the Helioseismic Fourier–Legendre Analysis Method

Measuring large-scale flows like the meridional flow with helioseismic methods plays a crucial role for understanding solar dynamics. In this work, we develop a detailed forward model for the Fourier–Legendre analysis method. This method has initially been applied for measuring the meridional flow by Braun & Fan, who observed frequency shifts between poleward and equatorward traveling p-modes. However, a detailed theoretical model based on a description of the solar internal wavefield is lacking. We therefore describe the effect of advection on the acoustic wavefronts in the first Born approximation following the framework of Gizon & Birch. We derive 2D sensitivity kernels for a suitable and unified definition of the frequency shift, and validate them on the example of a meridional flow. Finally, an inversion technique to estimate the meridional flow is developed on the basis of synthetic data from a simulation of linear wave propagation.

Systematic design and experimental demonstration of bianisotropic metasurfaces for scattering-free manipulation of acoustic wavefronts

Recent advances in gradient metasurfaces have shown that by locally controlling the bianisotropic response of the cells one can ensure full control of refraction, that is, arbitrarily redirect the waves without scattering into unwanted directions. In this work, we propose and experimentally verify the use of an acoustic cell architecture that provides enough degrees of freedom to fully control the bianisotropic response and minimizes the losses. The versatility of the approach is shown through the design of three refractive metasurfaces capable of redirecting a normally incident plane wave to 60°, 70°, and 80° on transmission. The efficiency of the bianisotropic designs is over 90%, much higher than the corresponding generalized Snell’s law based designs (81%, 58%, and 35%). The proposed strategy opens a new way of designing practical and highly efficient bianisotropic metasurfaces for different functionalities, enabling nearly ideal control over the energy flow through thin metasurfaces.

Space-coiling fractal metamaterial with multi-bandgaps on subwavelength scale

Acoustic metamaterials are remarkably different from conventional materials, as they can flexibly manipulate and control the propagation of sound waves. Unlike the locally resonant metamaterials introduced in earlier studies, we designed an ultraslow artificial structure with a sound speed much lower than that in air. In this paper, the space-coiling approach is proposed for achieving artificial metamaterial for extremely low-frequency airborne sound. In addition, the self-similar fractal technique is utilized for designing space-coiling Mie-resonance-based metamaterials (MRMMs) to obtain a band-dispersive spectrum. The band structures of two-dimensional (2D) acoustic metamaterials with different fractal levels are illustrated using the finite element method. The low-frequency bandgap can easily be formed, and multi-bandgap properties are observed in high-level fractals. Furthermore, the designed MRMMs with higher order fractal space coiling shows a good robustness against irregular arrangement. Besides, the proposed artificial structure was found to modify and control the radiation field arbitrarily. Thus, this work provides useful guidelines for the design of acoustic filtering devices and acoustic wavefront shaping applications on the subwavelength scale.

Comparison of conventional transducer and refracto-vibrometry measurements of ultrasonic transmission parameters from heel bones

Ultrasonic measurements of the heel bone (calcaneus) are used commonly for osteoporosis screening. Ultrasound pulses that pass through the calcaneus are detected with a receiving transducer. Parameters, such as normalized broadband ultrasound attenuation (nBUA) and speed of sound (SOS) calculated from the wave forms detected by the receiving transducer, are utilized to assess bone health. In the current study, refracto-vibrometry (RV), an interferometric method for optically measuring ultrasound, was compared with conventional transducer measurements of ultrasound transmission in-vitro through a human calcaneus sample. The measurement beam from a Polytec PSV-400 scanning laser Doppler vibrometer was directed through a water tank towards a stationary retroreflective surface. Acoustic wave fronts (density variations) which pass through the ~50μm diameter measurement laser cause variations in the integrated optical path length. The signals detected by the vibrometer at numerous scan points were used to determine the time evolution of ultrasonic wave fronts. Because there is not a physical receiving transducer, the RV measurements do not experience the same frequency response and reflection artifacts of conventional transducers. Parameters such as nBUA and SOS were calculated at multiple RV scan points, and the results were compared to parameters measured using a conventional ultrasound transducer.Ultrasonic measurements of the heel bone (calcaneus) are used commonly for osteoporosis screening. Ultrasound pulses that pass through the calcaneus are detected with a receiving transducer. Parameters, such as normalized broadband ultrasound attenuation (nBUA) and speed of sound (SOS) calculated from the wave forms detected by the receiving transducer, are utilized to assess bone health. In the current study, refracto-vibrometry (RV), an interferometric method for optically measuring ultrasound, was compared with conventional transducer measurements of ultrasound transmission in-vitro through a human calcaneus sample. The measurement beam from a Polytec PSV-400 scanning laser Doppler vibrometer was directed through a water tank towards a stationary retroreflective surface. Acoustic wave fronts (density variations) which pass through the ~50μm diameter measurement laser cause variations in the integrated optical path length. The signals detected by the vibrometer at numerous scan points were used to deter...

Videos of ultrasonic wave propagation through transparent acrylic objects in water for introductory physics courses produced using refracto-vibrometry

This project used refracto-vibrometry to produce full-field videos of ultrasonic wave propagation in water; these videos could be incorporated into high school and college physics courses to illustrate wave behaviors. Refracto-vibrometry (RV) is an interferometric method for optically measuring sound waves. The measurement beam from a Polytec PSV-400 scanning laser Doppler vibrometer was directed through a water tank towards a stationary retroreflective surface. Acoustic wave fronts (density variations) which pass through the measurement laser cause variations in the integrated optical path length. By superimposing tens of thousands of scan points, videos enable visualization of the ultrasound propagation. In one experiment, ultrasonic waves in water (1484 m/s) are incident on a submerged acrylic block at a non-zero angle. The waves are refracted as they travel at about 2700 m/s through the block. Students can determine this speed of sound by measuring the angle of refraction and lateral deflection of the wave fronts in the video. In another experiment, ultrasonic waves passed through an acrylic cylinder, producing a Mach cone in water. Measurement of the Mach cone angle enables determination of the acrylic speed of sound. Links for videos and sample worksheets will be provided.

Characterization of ultrasound attenuation and angular distribution from trabecular bone phantom material using refracto-vibrometry

Ultrasound is being researched and used for diagnosing osteoporosis. Researchers use trabecular bone from humans as a test material, but such bone is bio-hazardous, not uniform, and of limited size. Recently it has been demonstrated that the open-cell polyurethane foam, known as “Sawbones” could be utilized as a substitute for trabecular bone in ultrasound studies. In the current study, rectangular slices of Sawbones trabecular bone phantom material were insonified with a 25 mm diameter 500 kHz ultrasound transducer oriented both normal to the surface and at an angle. Refracto-vibrometry (RV), an interferometric method for optically measuring ultrasound, was compared with conventional transducer measurements of ultrasound transmission through the bone phantom samples. The measurement beam from a Polytec PSV-400 scanning laser Doppler vibrometer was directed through a water tank towards a stationary retroreflective surface. Acoustic wave fronts (density variations) which pass through the ~50µm diameter measurement laser cause variations in the integrated optical path length. The measured signals were used to determine parameters such as normalized broadband ultrasonic attenuation (nBUA) at numerous scan points. This enabled measurements of the spatial and angular distribution of the transmitted ultrasonic field through the sample that are not possible using a conventional single-element ultrasonic transducer.

Ultrathin Ashoka Chakra like acoustic metastructure as a sound absorber

Absorption of undesirable sound is a fundamental issue with broad applications in a variety of situations, such as in a conference room, cinema hall, aircraft cabin noise, etc. A variety of sound absorbers, e.g., micro-slit, concrete walls, porous media, etc., are used for noise control but are required to have a thickness comparable to the working wavelength. Here, we report a novel ultrathin “Ashoka Chakra” like acoustic metastructure that can be utilized as a sound absorber. The developed acoustic metastructure demonstrates broad bandwidth and high absorption characteristics. Each unit cell contains 24 hollow spokes through which acoustic wavefront can move freely. Despite its simplicity, this approach provides tunability of the metastructure functionality such as operating frequency range by altering the structure dimensions. Finite element based simulations were carried out in order to optimize the metastructure’s dimensions to achieve maximum absorption. The efficacy of the metastructure is validated through excellent agreement between simulation and experimental measurements. Realization of the proposed acoustic metastructure can show promising applications in the field of sound absorption.

Retrieval of temperature profiles using radio acoustic sounding system (RASS) with the equatorial atmosphere radar (EAR) in West Sumatra, Indonesia

The radio acoustic sounding system (RASS) with the equatorial atmosphere radar (EAR) at Koto Tabang, Indonesia was adapted to test the effects of the acoustic source location and acoustic frequency range on the continuous measurement of height profiles of temperature in the tropical troposphere. We installed the acoustic transmitting system by using six high-power horn speakers and four subwoofers. We developed a three-dimensional ray-tracing method of acoustic waves to predict the shape of acoustic wavefronts, accounting for the effects of the background winds on acoustic wave propagation. Then, we selected the appropriate antenna beam directions for EAR that satisfy the Bragg condition between the radar and acoustic wave propagation vectors. We carried out eight campaign observations in 2016, testing the performance of EAR–RASS. We found that the location and acoustic frequency range affected the RASS echoes. We also tested the compensation method of the background wind velocity with EAR to obtain the true sound speed. We intensively analyzed the RASS results from August 29 to September 3, 2016, when radiosondes were launched 12 times from the EAR site. We successfully retrieved the temperature profiles from RASS from 2 to 6–14 km with time and height resolutions of about 10 min and 150 m, respectively. Some temperature profiles were obtained up to about the tropopause at 17 km, although the observation period was short. During the RASS campaign, we detected a few interesting events regarding temperature variations as well as large perturbations in the three components of wind velocity.

Directivity of GHz shear acoustic waves launched by the absorption of short laser pulses at the interface between a transparent and an absorbing material

When a short laser pulse is focused at the interface between a transparent and an absorbing material, an expansion source is created that generates mainly longitudinal acoustic pulses in the absorbing material. However, the interaction of these waves with the interface favours the excitation of shear acoustic pulses. In this paper, we present an analytical prediction of the directivity of shear waves launched in a semi-infinite absorbing medium, and we introduce corrections in order to account for detection in an absorbing layer. Experiments were performed with a picosecond ultrasonic set-up focusing femtosecond laser pulses to a micron spot size along the interface between sapphire and titanium. In this configuration, acoustic wave-fronts were reflected at the free surface of the micron titanium layer, and then optically detected at the interface. We also present an approach to filter and enhance the shear waves by processing measured wave-fronts recorded at multiple locations along the interface. In the resulting signals, the contribution of GHz shear acoustic waves, propagating mainly in directions that are oblique with respect to the interface, is enhanced.

In Vitro Visualization of Ultrasonic Wave Fronts Interacting with Heel Bones Using Refracto-Vibrometry

Ultrasonic measurements of the heel bone (calcaneus) are used commonly for osteoporosis screening. Pulses emitted by an ultrasound transducer are incident on the calcaneus, and the transmitted wave fronts are detected with a separate transducer. In the current study, full field videos were obtained using refracto-vibrometry of ultrasonic pulses interacting human calcaneus samples in an in vitro environment. Pulses were emitted by a 500 kHz Panametrics V303 transducer. The measurement beam from a Polytec PSV-400 scanning laser Doppler vibrometer laser was directed through a water tank towards a stationary retroreflective surface. Acoustic wave fronts (density variations) which pass through the measurement laser cause variations in the integrated optical path length. The time-varying signals detected by the vibrometer at numerous scan points were used to determine the time evolution of ultrasonic wave fronts. The resulting videos enable visualization of the propagating wave fronts and the backscattered and transmitted wave fronts. These videos enable direct investigation of wave front distortion due to reflection, refraction and diffraction effects

Research advances in acoustic metamaterials and metasurface

Acoustic metamateiral (AM) is an artificially structured material with the unique properties that cannot be found in nature materials, such as negative refraction, slab focusing, super-resolution imaging, cloaking, inverse Doppler effect, etc. In this paper we first review the research advances in AM in recent 20 years and then mainly discuss the properties of the meta-atom AM (MAAM), meta-molecule AM (MMAM), meta-atom cluster AM, and meta-molecule cluster AM. The MAAM consists of local resonant meta-atoms, whose resonant frequency is related to the geometry size of the structure. The MAAM presents the transmission dip and inversed phase near the resonant frequency. The meta-atoms discussed in the paper contain the split hollow sphere and hollow tube (HT), which can be used to realize the AM with single negative modulus and AM with single negative mass density near the frequency, respectively. The effective parameter of the MAAM is calculated from the transmission and reflection data in experiment according to the homogeneous-medium theory. By combining the two kinds of meta-atoms together, the assembled two-layered composite AM presents a transmission peak similar to the electromagnetic metamaterial in the overlapping resonant frequency region. The effective parameters calculated by experimental data demonstrate that the composite AM could realize simultaneously negative modulus and negative mass density near the peak frequency. In the double-negative band, this kind of double-negative AM can faithfully distinguish the acoustic sub-wavelength details (/7). Furthermore, by coupling the two kinds of meta-atoms in a structure, we design a flute-like meta-molecule structure of perforated hollow tube, which can be used to fabricate double-negative AM in high or low frequency band. The experimental results also show that the double-negative AM has the properties of flat focusing and negative refraction effect. Based on the weak interaction of the meta-atoms, the meta-atom cluster AM can be fabricated by arraying different sized meta-atoms. The meta-atom cluster AM composed of different sized meta-atoms of SHSs can realize multi-band or broadband negative modulus, and the different sized meta-atoms of HTs can realize broadband negative mass density. Similarly, the meta-molecule cluster AMs are constructed with seven kinds of flute-like perforated hollow tubes, which can overcome the limitations of arbitrary broadband negative bulk modulus and mass density to provide a region of inverse Doppler effects. It is also shown that the inverse frequency shift values will be enhanced with the increase of frequency. As the resonant unit can realize the effect of discontinuous phase, it can be used to design acoustic metasurface (AMS) to control the acoustic wavefronts at will and realize the anomalous manipulation of acoustic waves. Finally, we introduce the research status and tendency of AMS in coming years.

Elastic metasurfaces for splitting SV- and P-waves in elastic solids

Although recent advances have made it possible to manipulate electromagnetic and acoustic wavefronts with sub-wavelength metasurface slabs, the design of elastodynamic counterparts remains challenging. We introduce a novel but simple design approach to control SV-waves in elastic solids. The proposed metasurface can be fabricated by cutting an array of aligned parallel cracks in a solid such that the materials between the cracks act as plate-like waveguides in the background medium. The plate array is capable of modulating the phase change of SV-wave while keeping the phase of P-wave unchanged. An analytical model for SV-wave incidence is established to calculate the transmission coefficient and the transmitted phase through the plate-like waveguide explicitly. A complete 2π range of phase delay is achieved by selecting different thicknesses for the plates. An elastic metasurface for splitting SV- and P-waves is designed and demonstrated using full wave finite element simulations. Two metasurfaces for focusing plane and cylindrical SV-waves are also presented.

Acoustic metasurfaces for scattering-free anomalous reflection and refraction

Manipulation of acoustic wavefronts by thin and planar devices, known as metasurfaces, has been extensively studied, in view of many important applications. Reflective and refractive metasurfaces are designed using the generalized reflection and Snell's laws, which tell that local phase shifts at the metasurface supply extra momentum to the wave, presumably allowing arbitrary control of reflected or transmitted waves. However, as it has been recently shown for the electromagnetic counterpart, conventional metasurfaces based on the generalized laws of reflection and refraction have important drawbacks in terms of power efficiency. This work presents a new synthesis method of acoustic metasurfaces for anomalous reflection and transmission that overcomes the fundamental limitations of conventional designs, allowing full control of acoustic energy flow. The results show that different mechanisms are necessary in the reflection and transmission scenarios for ensuring perfect performance. Metasurfaces for anomalous reflection require non-local response, which allows energy channeling along the metasurface. On other hand, for perfect manipulation of anomalously transmitted waves, local and non-symmetric response is required. These conclusions are interpreted through appropriate surface impedance models which are used to find possible physical implementations of perfect metasurfaces in each scenario. We hope that this advance in the design of acoustic metasurfaces opens new avenues not only for perfect anomalous reflection and transmission but also for realizing more complex functionalities, such as focusing, self-bending or vortex generation.

Manipulation of acoustic wavefront by gradient metasurface based on Helmholtz Resonators

We designed a gradient acoustic metasurface to manipulate acoustic wavefront freely. The broad bandwidth and high efficiency transmission are achieved by the acoustic metasurface which is constructed with a series of unit cells to provide desired discrete acoustic velocity distribution. Each unit cell is composed of a decorated metal plate with four periodically arrayed Helmholtz resonators (HRs) and a single slit. The design employs a gradient velocity to redirect refracted wave and the impedance matching between the metasurface and the background medium can be realized by adjusting the slit width of unit cell. The theoretical and numerical results show that some excellent wavefront manipulations are demonstrated by anomalous refraction, non-diffracting Bessel beam, sub-wavelength flat focusing, and effective tunable acoustic negative refraction. Our designed structure may offer potential applications for the imaging system, beam steering and acoustic lens.

Modulating Sound with Acoustic Metafiber Bundles

Acoustic metamaterials and metasurfaces provide great flexibility for manipulating sound waves and promise unprecedented functionality, ranging from transformation acoustics, acoustic cloaking, acoustic imaging to acoustic rerouting. However, the design of artificial structures with both broad bandwidth and multifunctionality remains challenging with traditional design approaches. Here we present a design and realization of a broadband acoustic metafiber bundle. Very different from previously reported acoustic metamaterials and metasurfaces, not only the metafiber structure is simple, flexible and tunable, but also the metafiber bundle has the advantages of broad bandwidth, high transmission, no resonance-induced energy loss and unchangeable output wavefront owing to eigenmodes in the passbands of the metafiber. Besides, it could also achieve arbitrary complex modulations of cylindrical and plane acoustic wavefronts. The metafiber bundles realize the exciting multifunctionality of both acoustic metamaterials and metasurfaces in a broad frequency range, which provides diverse routes to design novel acoustic devices with versatile applications.