Acoustic Wavelength Research Articles

A 2D low-order thermoacoustic network model of annular combustor with baffles in the plenum

Experimental results showed that configuring different baffles (a regular flat, a single- or double-layer perforated plate and a Helmholtz resonator) in the plenum can well suppress the self-excited oscillation in annular combustor. In this paper, a 2D low-order network model is proposed to describe this phenomenon and predict the modal frequencies and growth rates of longitudinal and circumferential thermoacoustic modes for annular combustor with different baffles. In the plenum, only the circumferential acoustic propagation and the influence of baffles on it are accounted for as the longitudinal geometry size is much smaller than the acoustic wavelength of the longitudinal mode. Pure longitudinal acoustic propagation is considered in burners and both longitudinal and circumferential ones are accounted for in the combustion chamber. These three modules are connected by jump conditions. The numerical prediction matches the experimental results. Baffles using Helmholtz resonators feature the best damping performance. The circumferential mode is converted to two unfolded non-degenerate modes when configuring perforated plates baffles. Best damping effect for the configuration of the baffles in the plenum can be achieved based on the proposed model.

Acousto-optic sensing for near-field acoustic holography

Near-field acoustic holography (NAH) is a very powerful and widely used technique for the study of complex acoustic radiators. NAH enables to quickly understand how a complex source radiates into the medium. The technique is particularly suitable at low frequencies. At high frequencies, a dense transducer interspacing is required, and the measurement microphones can disturb the studied sound field when their size is comparable to the acoustic wavelength. In this study we examine the use of acousto-optic sensing in NAH. Acousto-optic sensing uses light beams as the sensing element, making it possible to acquire remote and non-invasive measurements without introducing extraneous objects in the vicinity of the source. The pressure, particle velocity and intensity fields, as well as the sound power radiated by a complex source, are determined from measurements in the near-field with an optical interferometer. The presented results demonstrate the potential of optical sensing to non-intrusively characterize sound fields, particularly at high frequencies.

Mechanics of biconcave red blood cells deformed by acoustic tweezers

The deformation of biconcave red blood cells (RBCs) in a fluid environment driven by ultrasonic standing waves is studied theoretically and numerically. The cell contains homogeneous cytosol enclosed by a thin biological membrane. The cell membrane is modeled as a hyperelastic thin shell whose enclosed volume is assumed to be preserved to ensure osmotic equilibrium. Regarding acoustic deformation, the acoustic induced force acting on the cell membrane is calculated based on the acoustic radiation stress theory. The equations governing the interaction of the cell deformation and acoustic wave propagation are then numerically implemented in an axisymmetric finite element framework. The simulation results show that when the biconcave red blood cells are trapped at the pressure node of the acoustic standing wave, they gradually transform into an oblate disc shape with the increase of the acoustic input. Parametric studies are performed to systematically investigate the effects of cell mechanical properties and acoustic wavelength on cell deformation. Given that the mechanical properties of RBCs play an important role in disease development and mechanotransduction, this work is helpful to diagnose RBC-related diseases and study mechanotransduction using acoustic deformation technique.

Simultaneous parametric estimation of shape and impedance of a scattering surface using a multi-frequency fast indirect boundary element method

Characterization of rough surface properties, such as surface shape or surface impedance, is relevant in many applications. Previous work has been done for characterizing these two aspects separately. This paper proposes a framework for characterizing rough surfaces of finite size in terms of their roughness shape and surface impedance in a single setup. The surface properties are estimated indirectly using a superposition of spatial sinusoidal components for the surface shape and a porous material model for the surface impedance. A fast multipole indirect boundary element method is employed to model the acoustic scattering problem. With the modeling flexibility from the indirect formulation, the target geometry can be simplified as a thin shell representation, which significantly improves computational efficiency. A subtraction method is proposed to mitigate the modeling error of this representation and enable the use of low driving frequencies (e.g. acoustic wavelength is comparable to or larger than the roughness scale) in the characterization. Considering the modeling accuracy of surface shape representation, a discretization criterion concerning both acoustic and spatial aspects is defined. The inverse problem is solved by means of a general-purpose optimizer, minimizing the difference between the simulated and reference acoustic field at multiple driving frequencies. Various numerical examples show that by using reference data with sufficient quality, the proposed framework can retrieve the surface shape and impedance separately, and also simultaneously.

Earth’s Upper Crust Seismically Excited by Infrasound from the 2022 Hunga Tonga–Hunga Ha’apai Eruption, Tonga

AbstractRecords of pressure variations on seismographs were historically considered unwanted noise; however, increased deployments of collocated seismic and acoustic instrumentation have driven recent efforts to use this effect induced by both wind and anthropogenic explosions to invert for near-surface Earth structure. These studies have been limited to shallow structure because the pressure signals have relatively short wavelengths (<∼300 m). However, the 2022 eruption of Hunga Tonga–Hunga Ha’apai (also called “Hunga”) volcano in Tonga generated rare, globally observed, high-amplitude infrasound signals with acoustic wavelengths of tens of kilometers. In this study, we examine the acoustic-to-seismic coupling generated by the Hunga eruption across 82 Global Seismographic Network (GSN) stations and show that ground motion amplitudes are related to upper (0 to ∼5 km) crust material properties. We find high (>0.8) correlations between pressure and vertical component ground motion at 83% of the stations, but only 30% of stations show this on the radial component, likely due to complex tilt effects. We use average elastic properties in the upper 5.2 km from the CRUST1.0 model to estimate vertical seismic/acoustic coupling coefficients (SV/A) across the GSN network and compare these to recorded observations. We exclude many island stations from these comparisons because the 1° resolution of the CRUST1.0 model places a water layer below these stations. Our simple modeling can predict observed SV/A within a factor of 2 for 94% of the 51 non-island GSN stations with high correlations between pressure and ground motion. These results indicate that analysis of acoustic-to-seismic coupling from the eruption could be used to place additional constraints on crustal structure models at stations with collocated seismic and pressure sensors. Ultimately, this could improve tomographic imaging models, which rely on methods that are sensitive to local structure.

A noise-based high-resolution time-reversal method for acoustic defect localization in water pipes.

Pressurized water supply pipeline systems (PWSPS) are quintessential to human development and sustenance, but suffer a multitude of unresolved defects (i.e., leaks, blockages, etc.) due to aging and inaccessibility. In this paper, the ubiquitous high-frequency background noise in PWSPS is harnessed to introduce a high-resolution, passive defect detection technique. The relation between the acoustic Green's and cross correlation functions for a pressurized water pipe is derived for the case of high frequency waves, i.e., acoustic wavelengths smaller than the pipe diameter. This relation is subsequently used to formulate a time-reversal technique for localizing anomalies such as small variations in pipe wall impedance and through-wall defects (i.e., leaks). It is shown that the derived relationship between the cross correlation and acoustic Green's functions enables very accurate defect detection and localization by measuring the background noise at two locations along a pipe. This is an important result given that (i) there is limited access to buried PWSPS to conduct high-frequency active defect detection, and (ii) traditional methods to actively probe pipes (e.g., valve maneuverer) are low-resolution (tens to hundreds of meters) and often result in pipe overloading and fatigue.

Estimating Effective Material Parameters of Inhomogeneous Layers Using Finite Element Method

This study presents a finite element method (FEM) approach to estimate the effective medium parameters of 2-D and 3-D layers of arbitrary composition. The resonance frequency of a layer to be investigated is found by exciting the layer with plane waves and studying the reflected sound pressure from the layer as a function of frequency and incidence angle. This allowed for the calculation of compressional and shear wave velocities. The method was validated by applying the method to layers with known acoustic parameters and by comparing with results from the established analytical models. Composite layers with 1-3 and 2-2 connectivity are well described by established effective-medium theories, but these require the composite structures to be small compared to the acoustic wavelength. This limitation was overcome by the described FEM-based model, which could also capture deviations occurring in coarser composites. Conventional analytical models predict wave velocities as a function of void concentration, not considering positions of the voids. The described FEM approach predicted up to 5% variation in wave velocities for gold layers with identical volume fraction of voids, depending on the void distribution. This demonstrates that void positions influence wave velocity. The influence of connectivity between inclusions was studied by modeling tungsten inclusions in an epoxy matrix. It was found that composites with inclusions connected in a preferred direction had higher wave velocity in the direction of connectivity compared to randomly oriented inclusions. It is concluded that the presented FEM model reproduces the literature values for homogeneous materials and agrees with effective medium theories for fine-pitched composites. However, the strength of the model is its ability to go beyond this and model phenomena in real finite-size composites not captured by the classic effective medium models.

Low-frequency amplification of propeller tonal noise due to the scattering by a compact rigid cylinder

The present work addresses the scattering of the tonal noise of a low-speed propeller by a rigid cylinder, as a generic configuration representative of installed marine propellers. Both propeller and cylinder axes are parallel to each other. The diameters of the propeller and of the cylinder are much smaller than the acoustic wavelengths, as well as the propeller-cylinder distance. This corresponds to a compact regime of diffraction. Only the hydrodynamic tonal noise of the propeller at the first multiples of the blade-passing frequency is considered, assuming rigid blades, in a two-dimensional formulation. The direct and scattered sound fields are expressed in terms of spinning modes, with respect to the propeller and cylinder axes, respectively. Use is made of the exact Green’s function of the cylinder for the Helmholtz equation. The modes of orders ±1 are found the only efficient ones in the direct field, whereas higher-order modes rapidly decay. Yet, in the presence of the cylinder, higher-order modes are scattered into the contra-rotating mode of order 1 in the reference frame of the cylinder, with a strong amplification. A simple experiment, performed in air but with Helmholtz numbers typical of marine applications, confirms these results as key features of the asymptotic Green’s function of the cylinder. The same modal behavior is reproduced as closed-form simple expressions from a low-frequency approximation of the Green’s function. The results show that the installation effect is crucial for the tonal noise of marine propellers at very low frequencies.

Understanding the near-field photoacoustic spatiotemporal profile from nanostructures

Understanding the mechanism of photoacoustic generation at the nanoscale is key to developing more efficient photoacoustic devices and agents. Unlike the far-field photoacoustic effect that has been well employed in imaging, the near-field profile leads to a complex wave-tissue interaction but is understudied. Here we show that the spatiotemporal profile of the near-field photoacoustic waves can be shaped by laser pulses, anisotropy, and the spatial arrangement of nanostructure(s). Using a gold nanorod as an example, we discovered that the near-field photoacoustic amplitude in the short axis is ∼75 % stronger than the long axis, and the anisotropic spatial distribution converges to an isotropic spherical wave at ∼50 nm away from the nanorod's surface. We further extend the model to asymmetric gold nanostructures by arranging isotropic nanoparticles anisotropically with broken symmetry to achieve a precisely controlled near-field photoacoustic "focus" largely within an acoustic wavelength.

Modeling of airborne ultrasound reflection from water surface waves

Airborne ultrasound reflection from water surface waves is modelled to advance uses of acoustic signals to measure water surface waves and apply the measurements to explore interactions of water waves with rigid structures in a laboratory setting. When the ultrasound is incident on a moving periodic water surface wave, the reflected signal can be treated as diffraction from a moving corrugated reflection grating. Under the condition that the amplitude of the water surface waves is much less than the incident acoustic wavelength, diffraction theory leads to analytical formulas for the spectra of the acoustic signal relating to the water wave amplitudes and frequencies. Complementary modeling based on ray theory and wave superposition illustrates the diffraction and validates formulas of water wave reflection from a surface-piercing barrier structure, where two counter-propagating water waves are involved.

Metagrating-based acoustic wavelength division multiplexing enabled by deterministic and probabilistic deep learning models

Wavelength division multiplexing (WDM), the technology that utilizes different wavelengths of carrier signal as independent information channels and helps to realize enhanced information density and functional diversity, is desired in various acoustic applications for the growing demand of higher transmission efficiency and lower cost. In this paper, we marry the rising concept of acoustic metagrating with the technology of WDM and demonstrate that interesting and distinct wavelength-dependent functionalities with unitary efficiency in prespecified frequency ranges can be realized by a simple grating structure. To this end, we integrate the particle swarm optimization with the deep learning technique to resolve the uncertainty issue and multi-objective problem and develop both deterministic and probabilistic neural networks for the inverse design of a metagrating-based WDM device. As a result, the computational cost is greatly reduced, and the probabilistic model reveals the sensitivity and flexibility of the parameters of the metagrating with respect to the desired functionality. In this paper, we not only provide an intelligent inverse design paradigm of high-performance WDM devices for multiple manipulation purposes but also present a feasible solution for the development of integrated acoustic devices for wavelength-dependent applications.

Numerical Simulation of Acoustic Scattering by a Rotating Cylinder Based on the Enhanced Optimized Scheme

In this paper, we develop a new physical model which involves a Taylor vortex around a rotating cylinder. The scattering of sound waves from this new model is investigated by solving the linearized Euler equations under the polar coordinate system in the time domain. We use the enhanced optimized scheme for numerical simulations. Numerical results show that scattering causes a change in the spatial distribution of sound energy as the sound wave passes through a rotating cylinder. The directionality of the scattered field varies significantly with the length ratio of the acoustic wavelength to the radius of the rotating cylinder and the vortex intensity. The influences of the ratio and the strength on the directivities and intensities of scattered fields are analyzed. It is shown that the intensity of the scattering cross-section increases with the strength of the vortex. The pressure of the scattered sound fields has a phase shift induced by the couple scattering from the cylinder and the vortex.

High precision photoacoustic interferometer for the determination of the speed of sound in liquid media.

In this work, we introduce the concept and delineate the fundamental principles of photoacoustic interferometry (PAInt), aiming at the development of a novel methodology for the precise assessment of the speed of sound in liquid media. The PAInt apparatus integrates an intensity-modulated continuous wave laser beam at 20 MHz for the efficient generation of monochromatic photoacoustic wavefronts which interfere across the surface of a vertically displaced spherically focused piezoelectric element. In this context, the resulting interference pattern can reveal the acoustic wavelength in the liquid medium with remarkable accuracy, providing thus reliable estimations of the speed of sound in reference liquids (error ∼0.1%) such as distilled and sea water, acetonitrile, and ethanol.

Time-dependent deformation of biological tissue under ultrasonic irradiation

This work proposes a theoretical model for analyzing the time-dependent deformation of poroelastic materials, as a tissue model, under the action of an ultrasonic field, which considers the diffusion fluid transport that obeys Darcy's law. An acoustomechanical constitutive law is developed, which takes into account the mechanical stress generated by poroelastic deformation based on the classical poroelastic theory and the acoustic radiation stress generated by propagating ultrasonic waves. As an example, the time-dependent deformation of a poroelastic sheet subjected to two counter-propagating waves is calculated using the theoretical model, which is also verified by our finite element model. The results show that the sheet can be stretched or compressed, which depends on the ratio of the sheet thickness to the acoustic wavelength. Additionally, by exploring the relaxation process of the acoustic deformation, the role of the sheet thickness and porosity in determining the time of poroelastic relaxation is examined. This work is very helpful for understanding the deformation of tissues under ultrasonic fields, and also provides a method to extract the mechanical properties of tissues.

Perimeter Modes of Nanomechanical Resonators Exhibit Quality Factors Exceeding 109 at Room Temperature

Systems with low mechanical dissipation are extensively used in precision measurements such as gravitational wave detection, atomic force microscopy and quantum control of mechanical oscillators via opto- and electromechanics. The mechanical quality factor ($Q$) of these systems determines the thermomechanical force noise and the thermal decoherence rate of mechanical quantum states. While the dissipation rate is typically set by the bulk acoustic properties of the material, by exploiting dissipation dilution, mechanical $Q$ can be engineered through geometry and increased by many orders of magnitude. Recently, soft clamping in combination with strain engineering has enabled room temperature quality factors approaching one billion ($10^9$) in millimeter-scale resonators. Here we demonstrate a new approach to soft clamping which exploits vibrations in the perimeter of polygon-shaped resonators tethered at their vertices. In contrast to previous approaches, which rely on cascaded elements to achieve soft clamping, perimeter modes are soft clamped due to symmetry and the boundary conditions at the polygon vertices. Perimeter modes reach $Q$ of 3.6 billion at room temperature while spanning only two acoustic wavelengths -- a 4-fold improvement over the state-of-the-art mechanical $Q$ with 10-fold smaller devices. The small size of our devices makes them well-suited for near-field integration with microcavities for quantum optomechanical experiments. Moreover, their compactness allows the realization of phononic lattices. We demonstrate a one-dimensional Su-Schrieffer-Heeger chain of high-$Q$ perimeter modes coupled via nearest-neighbour interaction and characterize the localized edge modes.

Characteristics, relationships and precision of direct acoustic-to-seismic coupling measurements from local explosions

SUMMARY Acoustic energy originating from explosions, sonic booms, bolides and thunderclaps have been recorded on seismometers since the 1950s. Direct pressure loading from the passing acoustic wave has been modelled and consistently observed to produce ground deformations of the near surface that have retrograde elliptical particle motions. In the past decade, increased deployments of colocated seismometers and infrasound sensors have driven efforts to use the transfer function between direct acoustic-to-seismic coupling to infer near-surface material properties including seismic velocity structure and elastic moduli. In this study, we use a small aperture (≈600 m) array of broadband seismometers installed in different manners and depths in both granite and sedimentary overburden to understand the fundamental nature and repeatability of seismic excitation from 1 to 15 Hz using horizontally propagating acoustic waves generated by 97 local (2–10 km) explosions. In agreement with modelling, we find that the ground motions induced by acoustic-to-seismic coupling attenuate rapidly with depth. We confirm the modelled relation between acoustic and ground motion amplitudes, but show that within one acoustic wavelength, the uncertainty in the transfer coefficient between seismic and acoustic energy at a given seismic station increases linearly with separation distance between the seismic and acoustic sensor. We attribute this observation to the rapid decorrelation of the infrasonic wavefield across small spatial scales and recommend colocating seismic and infrasound sensors for use in studies seeking to invert for near-surface material properties. Additionally, contrary to acoustic-to-seismic coupling theory and prior observations, we find that seismometers emplaced in granite do not record retrograde elliptical particle motions in response to direct pressure loading. We rule out seismometer tilt effects as a likely source of this observations and suggest that existing models of acoustic-to-seismic excitation may be too simplistic for seismometers placed in high rigidity materials.

Recent Advancement of Phase Shifted Fiber Bragg Grating Sensor for Ultrasonic Wave Application: A Review

Fiber Bragg grating (FBG) is gaining traction in the field of optical fiber sensors, either as a detection tool for ultrasonic waves or as a detection target. The significant contributions of the last few years are summarized in this overview, that includes setups based on wavelength and intensity. This article summarizes recent research and development activities in ultrasonic wave applications utilizing FBG sensors and makes recommendations for future research. Several different FBG strain sensor packing configurations are discussed. This article presents a comprehensive theoretical and mathematical analysis of a phase-shifted fiber Bragg grating (<inline-formula> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula>-PS-FBG) for use in a structural health monitoring system that employs a longitudinal ultrasonic strain wave obtained via a natural single FBG. The effect of grating regulating factors (grating length, index change, and apodization), as well as ultrasonic acoustic wavelengths, on the <inline-formula> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula>-PS-FBG&#x2019;s wavelength shift sensitivity have been investigated. Additionally, the slow-light effect in the <inline-formula> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula>-PS-FBG acoustic sensor was investigated. Sensitivity, detection range, and limit of detection are all thoroughly examined and discussed with the objective of comparing various technologies and determining future directions in the field of phase shifted fiber optic sensors.

Effects of Remote Boundary Conditions on Clamping Loss in Micromechanical Resonators

Clamping loss in micromechanical resonators can strongly depend on the boundary conditions far away from the actual vibrating structure because the acoustic wavelength greatly exceeds the device dimensions. We demonstrate a scheme for post-fabrication tuning of the clamping loss in flexural-mode and bulk-mode resonators by modifying the boundary conditions of the chip with the frame. The measured quality factor increases by more than an order-of-magnitude for the microcantilevers and more than a factor of three for the bulk-mode resonators when frame contact is minimized via suspension of the chip by wirebonds. We propose a two-degree-of-freedom fluctuation-dissipation model to describe the thermomechanical noise and forced response in the presence of this tunable anchor damping. By studying the thermomechanical displacement spectrum with tunable clamping loss, we show that variable clamping loss tunes the mechanical quality factor, modifying both the resonator transfer function and thermomechanical noise force. We delineate the dependence of the tunable clamping loss mechanism on microcantilever beam length and ambient temperature from 300 K down to 40 K, and observe potential temperature dependence to clamping loss with reducing temperature. [2021-0141]

Born approximation of acoustic radiation force used for acoustofluidic separation

Acoustofluidic separation often involves biological targets with specific acoustic impedance similar to that of the host fluid, and with dimensions on the order of the acoustic wavelength. This parameter range, combined with the use of standing waves to separate the targets, lends itself to use of the Born approximation for calculating the acoustic radiation force. Considered here is the configuration analyzed by Peng et al. [J. Mech. Phys. Solids 145, 104134 (2020)], in which two intersecting plane waves radiated into the fluid by a standing surface acoustic wave exert a force on a eukaryotic cell modeled as a multilayered sphere. The angle of intersection is determined by the velocity of the surface wave and the sound speed in the fluid. The acoustic field in this case is a standing wave parallel to the substrate and a traveling wave perpendicular to the substrate. For all parameter values considered by Peng et al., including spheres several wavelengths in diameter, the Born approximation of the acoustic radiation force parallel to the substrate is in good agreement with a full theory based on spherical wave expansions of the incident and scattered fields. [C.A.G. and T.S.J. were supported by ARL:UT McKinney Fellowships in Acoustics.]

Node formation mechanisms in acoustofluidic capillary bridges

Using acoustofluidic channels formed by capillary bridges two models are developed to describe nodes formed by leaky and by evanescent waves. The liquid channel held between a microscope slide (waveguide) and a strip of polystyrene film (fluid guide) avoids solid-sidewall interactions. With this simplification, our experimental and numerical study showed that waves emitted from a single plane surface, interfere and form the nodes without any resonance in the fluid. Both models pay particular attention to tensor elements normal to the solid–liquid interfaces they find that; initially nodes form in the solid and the node pattern is replicated by waves emitted into the fluid from antinodes in the stress. At fluids depths near half an acoustic wavelength, most nodes are formed by leaky waves. In the glass, water-loading reduces node–node separation and forms an overlay type waveguide which aligns the nodes predominantly along the channel. One new practical insight is that node separation can be controlled by water depth. At 0.2 mm water depths (which are smaller than a ¼ wavelength) nodes form from evanescent waves. Here a suspension of yeast cells formed a pattern of small dot-like clumps of cells on the surface of the polystyrene film. We found the same pattern in sound intensity normal, and close, to the water-polystyrene interface. The capillary bridge channel developed for this study is simple, low-cost, and could be developed for filtration, separation, or patterning of biological species in rapid immuno-sensing applications.