Acoustic Wavelength Research Articles

Isolating long-wavelength fluctuation from structural relaxation in two-dimensional glass: cage-relative displacement

It has recently been revealed that long-wavelength fluctuation exists in two-dimensional (2D) glassy systems, having the same origin as that given by the Mermin–Wagner theorem for 2D crystalline solids. In this paper, we discuss how to characterise quantitatively the long-wavelength fluctuation in a molecular dynamics simulation of a lightly supercooled liquid. We employ the cage-relative mean-square displacement (MSD), defined on relative displacement to its cage, to quantitatively separate the long-wavelength fluctuation from the original MSD. For increasing system size the amplitude of acoustic long wavelength fluctuations not only increases but shifts to later times causing a crossover with structural relaxation of caging particles. We further analyse the dynamic correlation length using the cage-relative quantities. It grows as the structural relaxation becomes slower with decreasing temperature, uncovering an overestimation by the four-point correlation function due to the long-wavelength fluctuation. These findings motivate the usage of cage-relative MSD as a starting point for analysis of 2D glassy dynamics.

Ultrasound Detection Using Acoustic Apertures.

Ultrasound detection is commonly performed by piezoelectric transducers that are optimized for a specific application. Since the piezoelectric technology is not configurable, transducers designed for one application may not be compatible with other applications. In addition, some designs of ultrasound transducers may be difficult to implement owing to production constraints. In this paper, we propose a simple, low-cost method to reconfigure the geometry of ultrasound transducers. The technique is based on using apertures in thin sheets of acoustic blockers. We experimentally demonstrate this method for an ultrasound transducer with a central frequency of 1 MHz and show that it can emulate detectors of various sizes. An added advantage of this technique is its capability to achieve semi-isotropic detection sensitivity due to diffraction when the aperture size is comparable to the acoustic wavelength even when the angular sensitivity of the transducer is inherently limited.

Review of Progress in Acoustic Levitation

Acoustic levitation uses acoustic radiation forces to counteract gravity and suspend objects in mid-air. Although acoustic levitation was first demonstrated almost a century ago, for a long time, it was limited to objects much smaller than the acoustic wavelength levitating at fixed positions in space. Recent advances in acoustic levitation now allow not only suspending but also rotating and translating objects in three dimensions. Acoustic levitation is also no longer restricted to small objects and can now be employed to levitate objects larger than the acoustic wavelength. This article reviews the progress of acoustic levitation, focusing on the working mechanism of different types of acoustic levitation devices developed to date. We start with a brief review of the theory. Then, we review the acoustic levitation methods to suspend objects at fixed positions, followed by the techniques that allow the manipulation of objects. Finally, we present a brief summary and offer some future perspectives for acoustic levitation.

An efficient analytical model for baffled, multi-celled membrane-type acoustic metamaterial panels

A new analytical model for the oblique incidence sound transmission loss prediction of baffled panels with multiple subwavelength sized membrane-type acoustic metamaterial (MAM) unit cells is proposed. The model employs a novel approach via the concept of the effective surface mass density and approximates the unit cell vibrations in the form of piston-like displacements. This yields a coupled system of linear equations that can be solved efficiently using well-known solution procedures. A comparison with results from finite element model simulations for both normal and diffuse field incidence shows that the analytical model delivers accurate results as long as the edge length of the MAM unit cells is smaller than half the acoustic wavelength. The computation times for the analytical calculations are 100 times smaller than for the numerical simulations. In addition to that, the effect of flexible MAM unit cell edges compared to the fixed edges assumed in the analytical model is studied numerically. It is shown that the compliance of the edges has only a small impact on the transmission loss of the panel, except at very low frequencies in the stiffness-controlled regime. The proposed analytical model is applied to investigate the effect of variations of the membrane prestress, added mass, and mass eccentricity on the diffuse transmission loss of a MAM panel with 120 unit cells. Unlike most previous investigations of MAMs, these results provide a better understanding of the acoustic performance of MAMs under more realistic conditions. For example, it is shown that by varying these parameters deliberately in a checkerboard pattern, a new anti-resonance with large transmission loss values can be introduced. A random variation of these parameters, on the other hand, is shown to have only little influence on the diffuse transmission loss, as long as the standard deviation is not too large. For very large random variations, it is shown that the peak transmission loss value can be greatly diminished.

A sensitivity-enhanced flexible acoustic sensor using side-polished fiber Bragg grating

A miniature flexible fiber Bragg grating (FBG) acoustic sensor with enhanced pressure sensitivity was fabricated by using a side-polished configuration of ∼54 μm depth and ∼15 mm length. The photoelectromechanical response of the sensor suspended as a dual-end-fixed beam was characterized based on the acoustic-induced strain interaction with fiber gratings. The opto-mechanical response of the sensor to bending strain was firstly investigated and exhibited about 11 times larger in wavelength shift, in comparison with a conventional bare FBG. Then acoustic testing demonstrated an acoustic pressure-induced wavelength shift of 0.041 nm/Pa and a noise equivalent acoustic signal level of ∼1.66 mPa/Hz1/2 in the frequency region around 2.5 kHz. Also, a relative flat frequency response with a fluctuation of less than 12.9 dB was achieved in the measurement between 400 Hz and 20 kHz, which is in well agreement with the result obtained by a reference condenser microphone. Along with strain-sensitive nanomaterials, the use of the side-polished FBG could be further extended for highly sensitive acousto-ultrasonic sensors in structural health monitoring and wearable applications.

Demonstration of acoustic source localization in air using single pixel compressive imaging

Acoustic source localization often relies on large sensor arrays that can be electronically complex and have large data storage requirements to process element level data. Recently, the concept of a single-pixel-imager has garnered interest in the electromagnetics literature due to its ability to form high quality images with a single receiver paired with shaped aperture screens that allow for the collection of spatially orthogonal measurements. Here, we present a method for creating an acoustic analog to the single-pixel-imager found in electromagnetics for the purpose of source localization. Additionally, diffraction is considered to account for screen openings comparable to the acoustic wavelength. A diffraction model is presented and incorporated into the single pixel framework. In this paper, we explore the possibility of applying single pixel localization to acoustic measurements. The method is experimentally validated with laboratory measurements made in an air waveguide.

Super-resolution photoacoustic imaging via flow-induced absorption fluctuations

In deep-tissue photoacoustic imaging, optical-contrast images of deep-lying structures are formed by recording acoustic waves that are generated by optical absorption. Although photoacoustics is perhaps the leading technique for high-resolution deep-tissue optical imaging, its spatial resolution is fundamentally limited by the acoustic wavelength, which is orders of magnitude longer than the optical diffraction limit. Here, we present an approach for surpassing the acoustic diffraction limit in photoacoustics by exploiting inherent temporal fluctuations in the photoacoustic signals due to sample dynamics, such as those induced by the flow of absorbing red blood cells. This was achieved using a conventional photoacoustic imaging system by adapting concepts from super-resolution fluorescence fluctuation microscopy to the statistical analysis of acoustic signals from flowing acoustic emitters. Specifically, we experimentally demonstrate that flow of absorbing particles and whole human blood yields super-resolved photoacoustic images, and provides static background reduction. By generalizing the statistical analysis to complex-valued signals, we demonstrate super-resolved photoacoustic images that are free from common photoacoustic imaging artifacts caused by band-limited acoustic detection. The presented technique holds potential for contrast-agent-free microvessel imaging, as red blood cells provide a strong endogenous source of naturally fluctuating absorption.

A diffraction based acoustic single pixel imager

We present a method for localizing an acoustic source with a single, omni-directional receiver paired with shaped aperture screens that allow for a spatially diverse set of measurements. Traditionally this is accomplished by patterning the screen with a series of sub-wavelength openings that allow the acoustic transmission an otherwise sound opaque material. Here we consider screens that have openings on the order of an acoustic wavelength or larger and incorporate a diffraction model into the single pixel imaging framework to account for these larger openings. The method is demonstrated on experimental data taken in air and an analysis of the error as a function of receiver position is presented. [This work was supported by ONR.]We present a method for localizing an acoustic source with a single, omni-directional receiver paired with shaped aperture screens that allow for a spatially diverse set of measurements. Traditionally this is accomplished by patterning the screen with a series of sub-wavelength openings that allow the acoustic transmission an otherwise sound opaque material. Here we consider screens that have openings on the order of an acoustic wavelength or larger and incorporate a diffraction model into the single pixel imaging framework to account for these larger openings. The method is demonstrated on experimental data taken in air and an analysis of the error as a function of receiver position is presented. [This work was supported by ONR.]

Integral array of acoustic sensors for micro-liter liquid discrimination

It is shown experimentally that the sensing properties of normal acoustic waves propagating in a piezoelectric plate are sensitive to the mass load and the viscosity and electric conductivity of a liquid deposited on the plate. The sensing properties depend on (a) the direction of propagation, (b) the mode number for each direction, and (c) the plate thickness normalized to the acoustic wavelength for each mode and each direction. Based on these properties, a prototype of an acoustic liquid sensor array with different patterns of selectivity is developed and tested. The sensors are located on a single plane of a piezoelectric plate and do not use any sorbent film for sensing. The array is capable of identifying types and sorts of liquids according to their physical effects on the absorption of normal waves with different numbers. The total number of acoustic responses is presented in the form of histograms that are acoustic images of the analyzed substances’ flavors. Having no sorbent film improves the reliability and long-term consistency of sensing.

Sediment Size Effects in Acoustic Doppler Velocimeter-Derived Estimates of Suspended Sediment Concentration

Backscatter output from a 10 MHz acoustic Doppler velocimeter (ADV) was used to quantify suspended sediment concentrations in a laboratory setting using sand-sized particles. The experiments included (a) well-sorted sand samples ranging in size from 0.112 to 0.420 mm, obtained by the sieving of construction sand, (b) different, known mixtures of these well-sorted fractions, and (c) sieved natural beach sand with median sizes ranging from 0.112 to 0.325 mm. The tested concentrations ranged from 25 to 3000 mg•L−1. The backscatter output was empirically related to concentration and sediment size, and when non-dimensionalized by acoustic wavelength, a dimensionless sediment size gradation coefficient. Size-dependent upper and lower bounds on measurable concentrations were also established empirically. The range of measurable conditions is broad enough to make the approach useful for sand sizes and concentrations commonly encountered in nature. A new method is proposed to determine concentrations in cases of mixed-size sediment suspensions when only calibration data for well-sorted constituent sands are available. This approach could potentially allow better estimates when the suspended load is derived from but is not fully representative of the bed material, and when the size characteristics of the suspended material are varying in time over the period of interest. Differences in results between the construction and beach sands suggest that sediment shape may also need to be considered, and point to the importance of calibrating to sediments encountered at the site of interest.

Acoustic levitation of an object larger than the acoustic wavelength

Levitation and manipulation of objects by sound waves have a wide range of applications in chemistry, biology, material sciences, and engineering. However, the current acoustic levitation techniques are mainly restricted to particles that are much smaller than the acoustic wavelength. In this work, it is shown that acoustic standing waves can be employed to stably levitate an object much larger than the acoustic wavelength in air. The levitation of a large slightly curved object weighting 2.3 g is demonstrated by using a device formed by two 25 kHz ultrasonic Langevin transducers connected to an aluminum plate. The sound wave emitted by the device provides a vertical acoustic radiation force to counteract gravity and a lateral restoring force that ensure horizontal stability to the levitated object. In order to understand the levitation stability, a numerical model based on the finite element method is used to determine the acoustic radiation force that acts on the object.

Enhancing the sensitivity of three-axis detectable surface acoustic wave gyroscope by using a floating thin piezoelectric membrane

A new type of surface acoustic wave (SAW) gyroscope was developed on a floating thin piezoelectric membrane to enhance sensitivity and reliability by removing a bulk noise effect and by importing a higher amplitude of SAW. The developed device constitutes a two-port SAW resonator with a metallic dot array between two interdigital transducers (IDTs), and a one-port SAW delay line. The bulk silicon was completely etched away, leaving only a thin piezoelectric membrane with a thickness of one wavelength. A voltage controlled oscillator (VCO) was connected to a SAW resonator to activate the SAW resonator, while the SAW delay line was connected to the oscilloscope to monitor any variations caused by the Coriolis force. When the device was rotated, a secondary wave was generated, changing the amplitude of the SAW delay line. The highest sensitivity was observed in a device with a full acoustic wavelength thickness of the membrane because most of the acoustic field is confined within an acoustic wavelength thickness from the top surface; moreover, the thin-membrane-based gyroscope eliminates the bulk noise effect flowing along the bulk substrate. The obtained sensitivity and linearity of the SAW gyroscope were ∼27.5 µV deg−1 s−1 and ∼4.3%, respectively. Superior directivity was observed. The device surface was vacuum-sealed using poly(dimethylsiloxane) (PDMS) bonding to eliminate environmental interference. A three-axis detectable gyroscope was also implemented by placing three gyrosensors with the same configuration at right angles to each other on a printed circuit board.

Comparison of finite element and analytical modeling of scattering of an acoustic wave by particles in a fluid

Ultrasonic wave propagation through dispersions of particles in liquids is of interest for particle characterization and process monitoring applications. Interpretation of the measurements relies on a theoretical model; we typically use a multiple scattering model which builds on models of scattering by independent particles. We report finite element modeling of an acoustic wave propagating through a liquid and interacting with a particle, using the linearized thermo-acoustic equations for propagation in a viscous liquid. We demonstrate that the interaction of the acoustic field with the particle leads to decaying thermal and shear wave fields in the region very close to the particle. Since the length scale of the thermal and shear decay is orders of magnitude smaller than the propagational mode acoustic wavelength, fine meshing is necessary in the region of the particle/fluid boundary. The simulation results are compared with analytical solutions for scattering of a plane wave by a single spherical particle, provided by Epstein and Carhart [JASA, 25, 533, (1953)] and Allegra and Hawley [(JASA, 51, 1546 (1972)].

On the metrology of the speed of sound in liquids

Decentralization of the standards base in measuring the speed of sound in liquids by expressing the speed of sound in fractions of the speed of light in vacuum is considered. A method to take into account the diffraction effects by measuring the speed of sound in the case of a constant acoustic wavelength and binding the absolute speed of sound values to the calculated thermodynamic values obtained by the nonacoustical method when the adiabatic compressibility is equal to the isothermal one is proposed. Errors of the reproducibility of the unit of the speed of sound in water using the proposed method are estimated.

Global acoustic propagation and the link between hydroacoustics and infrasound

The ocean is nearly transparent for acoustic propagation at low frequencies, leading to the detection of signals (seismic events, volcanoes and man-made signals) at distances as large as the ocean basin. When the ocean depth approaches the acoustic wavelength (~ 1500m for 1 Hz energy) the potential for conversion of energy from ocean acoustic propagation to atmospheric infrasonic propagation exists. This was observed in an earthquake off the coast of Australia (Evers et al. GRL 2014) energy was observed as seismic waves, hydro-acoustic waves and then infrasound waves, the latter two on the International Monitoring System (IMS) of the Comprehensive Test Ban Treaty Organization. In this paper, global scale hydro-acoustic propagation modeling from earthquakes and volcanoes will be presented with an emphasis on the locations where conversion to atmospheric propagation will is possible. Recent observations at the Aloha Cables Observatory (Butler JASA 2016) indicate the possibility of a loss of energy from 1-5 Hz as hydro-acoustic signals propagate over the Oahu-Kauai ridge (the Kauai “Keyhole”). Hydro-acoustic and infrasound modeling of this event will be presented.The ocean is nearly transparent for acoustic propagation at low frequencies, leading to the detection of signals (seismic events, volcanoes and man-made signals) at distances as large as the ocean basin. When the ocean depth approaches the acoustic wavelength (~ 1500m for 1 Hz energy) the potential for conversion of energy from ocean acoustic propagation to atmospheric infrasonic propagation exists. This was observed in an earthquake off the coast of Australia (Evers et al. GRL 2014) energy was observed as seismic waves, hydro-acoustic waves and then infrasound waves, the latter two on the International Monitoring System (IMS) of the Comprehensive Test Ban Treaty Organization. In this paper, global scale hydro-acoustic propagation modeling from earthquakes and volcanoes will be presented with an emphasis on the locations where conversion to atmospheric propagation will is possible. Recent observations at the Aloha Cables Observatory (Butler JASA 2016) indicate the possibility of a loss of energy from 1-5 ...

Estimating the influence of ice thickness and elasticity on long-range narrow-band reverberation in a representative Arctic environment

A full-field perturbation approach [A. Ivakin (2016), J. Acoust. Soc. Am., 140(1), 657-665] is modified for an ice-covered ocean and applied to estimating narrow-band long-range reverberation caused by roughness of the ice-water interface. First-order approximation of the approach is used which requires the roughness amplitudes be small compared to the acoustic wavelength. An upward refracting sound speed water column profile typical for Arctic conditions is assumed. To obtain the zeroth-order Green’s function and transmission loss field used in the reverberation model, elastic parabolic equation solutions [J. Collis, S. Frank, et al. (2016), J. Acoust. Soc. Am., 139(5), 2672-2682] are generated in range-independent environments. Ice is represented by two layers. The first approximates acoustic properties of a relatively thin, water-saturated transitional ice layer and is described as a stratified fluid with sound speed increasing with the distance from the ice-water interface. Second layer is introduced as a homogeneous isotropic elastic medium with fixed complex shear and longitudinal wave speeds. Effects of ice properties are discussed and demonstrated by comparing reverberation calculated for different ice layer thicknesses and shear speeds varying from zero (for a fluid model) to typical ice values. [Work supported by ONR.]

Finite element modeling of thermal and shear decay fields around particles in liquids in an ultrasound field

Ultrasonic multiple scattering models of wave propagation through inhomogeneous particulate media frequently assumed only acoustical interactions between particles, neglecting thermal and hydrodynamic (shear-mediated) interactions. This assumption is, however, only valid at low concentrations of particles where the particle separation distances are large. Each particle has the propensity to have a short-ranged thermal or shear field around it, caused by the acoustic field, and at moderate to high concentrations these can overlap. Our recent work has shown that, in these conditions in the long acoustic wavelength regime, when the shear or thermal wavelengths are similar to the particle radius, the overlapping shear and thermal fields can result in a significant reduction in effective attenuation. The effect can be described as the conversion of compressional waves to shear/thermal waves and back again at particle/liquid boundaries. Herein, we use finite element modelling of single and groups of particles t...

Shape and topology optimization of acoustic lens system using phase field method

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.

Realization of acoustic wave directivity at low frequencies with a subwavelength Mie resonant structure

We realized high-efficiency acoustic directivity at low frequencies based on monopolar Mie resonance. This is caused by micro structures that have a high refractive index relative to the background medium. The structures can strongly control the radiation pattern though the acoustic wavelength is much larger than its dimensions. We herein discuss how to enhance the directivity through modifying the structure's parameters. Furthermore, our structure is proposed for use in obtaining an acoustic collimated beam without sidelobes. The structure characteristics and applications are demonstrated both theoretically and experimentally. Potential applications of our structures include acoustic device miniaturization, noise control, and medical ultrasonics.

Faxén form of time-domain force on a sphere in unsteady spatially varying viscous compressible flows

An explicit expression for the time-dependent force on a stationary, finite-sized spherical particle located in an unsteady inhomogeneous ambient flow is presented. The force expression accounts for both viscous and compressible effects. Towards this end, a time-harmonic plane travelling wave of a given frequency propagating in a viscous compressible flow over a sphere is considered. Linearized compressible Navier–Stokes equations are solved to obtain an analytical expression for the force exerted on the particle in the frequency domain. The force obtained in the Laplace space due to a travelling wave of a given frequency and wavenumber is then generalized to any arbitrary incoming flow. This is achieved by relating the radial and tangential velocity components in the Laplace space to the surface-averaged radial velocity and volume-averaged velocity vectors respectively in the time space. Moreover an expression relating the surface-averaged radial velocity and volume-averaged velocity vector has been provided. The total force is written as a summation of the undisturbed and disturbed force (quasi-steady, inviscid-unsteady and viscous-unsteady) contributions. The force contributions thus obtained are expressed as comprising of two parts – that arising due to spatial variation in the ambient flow and the other arising due to temporal variation. The current formulation is applicable to inhomogeneous ambient flows, however in the limit of negligible Reynolds and Mach numbers. The results are applicable even for particles of sizes larger than the acoustic wavelength. The accuracy of the explicit time-domain force expression is first tested by computing the force on an 80 mm diameter particle due to a weak planar expansion fan. Extension of this formulation when nonlinear effects become important is also proposed and tested by considering strong expansion fans. The results thus obtained are compared against corresponding axisymmetric numerical simulations.