Acoustic Waves In Air Research Articles

Stretchable, stable, and structural color reversible enabled by mesoscopic regulation and design for flexible photonic crystal mechanical sensors

This paper reports a flexible mechanical sensor developed by combining an opal photonic crystal and a flexible medium through UV curing technology. Under the action of external force, the deformation of the flexible medium causes the change of the photonic crystal lattice constant, which leads to the displacement of the reflection peaks, thus realizing the detection of the deformation of the film. The flexible medium is mixed by photoinitiators and polymer monomers, and its mechanical properties are directly related to the mass fraction of photoinitiators and polymer monomers. The results show that the film can reach a maximum strain of about 180% when the mass fraction of photoinitiator is 3% and the volume ratio of the polymer functional monomers is 8:2. In addition, for the responsiveness test of the sensor under tensile, the results show that at 0%, 26%, and 78% of tensile strain, its structural color is blue-shifted sequentially, and its spectral peaks are 668 nm, 535 nm, and 488 nm, respectively. Finally, the sensor is applied to the detection of acoustic waves in air, and it is found that its spectral response peaks can be obtained for acoustic waves of different frequencies (250–650 Hz), and a spectral peak at 500 Hz. This flexible photon sensor has the potential to match the acoustic resistance of water. It will greatly expand the response research of ocean exploration in the low frequency range.

Diffraction-Limited Focusing of Acoustic Waves by a Mesoscopic Flat Janus Lens

Anisotropic focusing by a mesoscopic (Mie size parameter of about 18) acoustic cubic lens based on V-shaped plate structures has been simulated numerically and confirmed experimentally. It has been shown for the first time that this lens with an edge dimension of about three wavelengths ensures the focusing of an acoustic wave in air into a diffraction-limited region. In the inverse geometry of the structure, the lens completely reflects the incident acoustic wave.

Developing Novel Gas Discharge Emitters of Acoustic Waves in Air for Nondestructive Testing of Materials.

This study was devoted to the development of novel devices and a methodology intended for generating ultrasonic waves in an air medium by using atmospheric pressure gas discharge. In the proposed electrode system, the discharge process was accompanied by the generation of acoustic waves on the emitter surface and, consequently, in the ambient air. The gas discharge emitter vibrations were analyzed by applying the technique of Scanning Laser Doppler Vibrometry (SLDV). It was shown that the magnitude of displacements matched the corresponding characteristics of classical piezoelectric and magnetostrictive transducers. The use of the Fast Fourier transform procedure supplied amplitude-frequency spectra of vibrations generated by the gas discharge emitter. The amplitude-frequency spectrum analysis showed that the proposed emitter was able to generate acoustic waves in the air with frequencies from 50 Hz to 100 kHz, and such a device can be used for the nondestructive testing (NDT) of materials. The results of the statistical analysis of vibration displacements in the repetitive pulsed mode were discussed. A non-stable characteristic of the vibration displacement of the emitter membrane was demonstrated. The parameters of such instability were associated with the features of gas discharge processes. In the experiments, the proposed gas discharge emitter was used in combination with SLDV for inspecting carbon-fiber-reinforced polymer composites. The experiments demonstrated the possibility of using an air-coupled gas discharge transmitter to generate acoustic waves in NDT applications.

Megahertz ultrasonic source induced by femtosecond laser irradiation of graphene foam

The performance of a photo-thermo-acoustic (PTA) ultrasonic source can be significantly improved by utilizing graphene-based materials. Graphene is an excellent PTA material owing to its wide electromagnetic absorption spectrum, low heat capacity per unit area, and high thermal conductivity. In this study, a broadband graphene-foam-based PTA ultrasonic source covering the frequency range of 50 kHz to 1.8 MHz was excited by a near-infrared femtosecond laser beam. The lower and upper frequency limits of the acoustic wave that can be detected in the experiments were determined by the responsivity of the microphone and the attenuation of the acoustic wave in air, respectively. The experimental results show that the sound pressure of this ultrasonic source was independent of the laser polarization and incident angle. The peak-to-peak magnitude of the ultrasonic wave was proportional to laser energy when the single-pulse energy of the femtosecond laser varied from 0.4 to 1.0 mJ. Therefore, the sound intensity of the ultrasonic source could be easily controlled by modulating the laser energy. The experimental results also show that the ultrasonic wave emitted from the graphene foam had a dipole-like acoustic pressure distribution, and its principal emission direction was normal to the sample surface, regardless of the laser incidence angle. This characteristic may benefit future applications in directed message transfer/acquisition and nondestructive testing/imaging.

Theoretical and experimental study of micropolar elastic materials using acoustic waves in air

This study focuses on the development of a theoretical and experimental acoustic method and setup for studying the micropolar behavior of materials. Two types of low density closed-cell foams made from polystyrene (expanded and extruded) materials that were previously reported as materials exhibiting the non-classical elastodynamic behavior were investigated in order to reveal the particular comportment using acoustic waves. Two different types of transducers were employed to provide a large frequency band to study the influence of the Cosserat parameters. The frequency band from 1–40 kHz was covered using a tweeter loudspeaker whereas that from 30–120 kHz, by an air coupled ultrasonic transducer. The materials’ micropolar elastic behavior were revealed through the comparison between the theoretical and experimental transmission coefficients. At the cut-off frequency where the theoretical phase velocity of the transverse acoustic mode was established to be infinite, the acoustic wave transmission by the panels was found to be very small. This makes the Cosserat materials appealing to harmonic noise reduction.

Acoustic Steering Using Thermally Induced Optical Reflection of Sound (THORS)

Using the recently discovered THermally induced Optical Reflection of Sound (THORS) phenomenon, it is possible to generate optically induced, local density barriers in air by the absorption of intense, modulated laser light (the THORS phenomenon), which results in abrupt differences in compressibility of the air at these barriers that can efficiently reflect incident acoustic waves. In this note, we demonstrate the ability to optically manipulate and reflect acoustic waves in air as well as optimize the functional parameters (optical modulation and acoustic frequency) and characterize the effects of common physical parameters, including localized thermal gradients and incident angle of reflection on the efficiency of the resulting acoustic reflection. Finally, the ability to efficiently steer acoustic waves around a physical obstruction using THORS is also demonstrated.

Sound energy harvesting by leveraging a 3D-printed phononic crystal lens

We investigate the harvesting of sound waves by exploiting a 3D-printed gradient-index phononic crystal lens. The concept is demonstrated numerically and experimentally for focusing audio frequency range acoustic waves in air to enhance sound energy harvesting. A finite-element model is developed to design the unit cell dispersion properties and to construct the 3D lens for wave field simulations. Numerical simulations are presented to confirm the focusing of incident plane waves and to study the sensitivity of the refractive index profile to the direction of wave propagation. The theoretical predictions are validated experimentally using a scanning microphone setup under speaker excitation, and a very good agreement is observed between the experimental and numerical wave fields. A circular piezoelectric unimorph harvester is placed at the focal position of the lens, and its performance is characterized with a resistor sweep in the absence and presence of the lens, resulting in more than an order of magnitude enhancement in the harvested power with the lens. The 3D-printed lens presented here substantially enhances the intensity of sound energy via focusing, yielding micro-Watt level power output, which can find applications for wireless sensors and other low-power electronic components.

The online monitoring method research of laser shock processing based on plasma acoustic wave signal energy

Aiming at the phenomenon of plasma acoustic wave during laser shock processing, the online monitoring method of laser shock processing based on plasma acoustic wave signal energy is proposed. The TC4 titanium alloy samples, after the vibration stress relief treatment, were treated by a Nd:YAG laser system with a wavelength of 1064nm, a pulse width of 12–18 ns, and a single pulse energy of 0–7 J. The plasma acoustic wave in air is sampled, stored, digitally filtered, and analyzed by the laser shock processing online monitoring system during laser shock processing. Then the system gets the plasma acoustic wave signal energy. The surface residual stress of material after the treatment of laser shock processing were measured by a Photo-LXRD X-ray diffraction device. The experimental results show that the residual compressive stress is generated on the surface of material after laser shock processing. Both the surface residual stress of the material and the plasma acoustic wave signal energy are increased as the laser pulse energy increases from 3J to 7J, and their growth trend is consistent. Finally, the empirical formula between the surface residual compressive stress of the material and the plasma acoustic wave signal energy is obtained by polynomial fitting, which will provides a theoretical basis for the development of laser shock processing online monitoring technology.

Visualization of acoustic waves in air and subsequent audio recovery with a high-speed schlieren imaging system: Experimental and computational development of a schlieren microphone

We present a high-speed single-mirror double-pass coincident schlieren system and corresponding algorithms for the visualization of acoustic waves and recovery of their associated audio signals. Schlieren systems are extensively used to visualize strong shockwaves, such as those from supersonic motion or explosions. Recently, they have also been used to visualize lower amplitude non-linear acoustic phenomena, such as the weak shockwaves arising from impact events including hand claps, belt snaps, and towel cracks. Time-invariant sounds produced by loudspeakers have also been imaged, in one case leading to frequency analysis, although these have been limited to high-frequency signals at very high sound pressure levels. The research presented here shifts the focus from sound-field visualization towards audio signal recovery. A comprehensive exploration of several parameters for imaging sound sources, including frequency, wave form, and amplitude, is presented. In addition, we address for the first time the recovery of phase information, which would be essential for speech intelligibility, and the more general case of non-contact sound field reconstruction. Through image and signal processing, it is shown that audio signals can be recovered from high-speed schlieren video whose acoustic waves appear to be below the limit of visibility, and were previously deemed unrecoverable by virtue of their frequency and sound pressure level. This includes sounds at frequencies and loudnesses relevant for human hearing, producing the first ‘schlieren microphone’.

Numerical and experimental study of acoustically enhanced heat transfer from a single particle in flue gas

ABSTRACTTo investigate the heat transfer of pulverized coal particles under the effect of an acoustic field (without a superposed steady component), we derived and solved the two-dimensional, unsteady, laminar conservation equations for mass, momentum, and energy transport in the flue gas phase of a power plant boiler. The local, space-averaged, and space- and time-averaged Nusselt (Nu) numbers for the coal particles are discussed for the full audible frequency range. Experiments were also conducted to study the heat transfer, in particular the heat exchange at the surface, considering the effect of the acoustic wave around a stationary copper sphere. The sphere was exposed to different sound pressure levels of acoustic waves in air, and the temperature of the sphere was recorded. The range of the studied sound frequencies is between 500 Hz and 3 kHz. Both, numerical and experimental investigations indicate that the acoustic field can increase the heat transfer significantly. Higher sound pressure levels cause higher heat-transfer rate. However, there is a special frequency, at which Nu reaches a maximum, due to the flow separation caused by flow acceleration. This special frequency increases with increasing sound pressure level.

Anomalous Reflection of Acoustic Waves in Air with Metasurfaces at Low Frequency

An acoustic metasurface made of a composite structure of cavity and membrane is proposed and numerically investigated. The target frequency is in the low frequency regime (570 Hz). The unit cells, which provide precise local phase modulation, are rather thin with thickness in the order around 1/5 of the working wavelength. The numerical simulations show that the designed metasurface can steer the reflected waves at will. By taking the advantage of this metasurface, an ultrathin planar acoustic axicon, acoustic lens, and acoustic nondiffracting Airy beam generator are realized. Our design method provides a new approach for the revolution of future acoustic devices.

Three-dimensional elliptical cloak by impedance design of meta-composite cloak shell structure

Research into metamaterials with unique material properties has led to the development of acoustic cloaks. The use of a spatially transformed space makes it possible to change (i.e. redirect) the propagating path of an acoustic wave using an anisotropic shell material as a cloak. However, the cloak of an acoustic wave in air, based on spatial transformation, requires an anisotropic cloak shell with unique properties, such as a mass density less than air that is not found in nature and is hard to be realized. We overcome the difficulties by providing a new concept in acoustic metamaterial design based on the impedance of the cloak shell. Using this method, we propose a new meta-composite material structure and further, design an elliptic meta-composite cloak shell complying with a three-dimensional space transformation. The new meta-composite material comprises a specially designed structure of layered plastic block elements forming cavities and pipes, respectively. Analysis of acoustic waves passing through the cloak shell demonstrates the effectiveness of acoustic cloaking performance over a frequency range of 800 to 1250 Hz in a 1/3 octave band. The anisotropic properties of the proposed cloak make it possible to alter the direction of the acoustic waves propagating from the original path and direct them to travel along a pre-determined path. This meta-composite material design accomplishes the goal of realizing an invisible elliptical acoustic cloak in the air.

Bound states in the continuum

Bound states in the continuum (BICs) are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. Their very existence defies conventional wisdom. Although BICs were first proposed in quantum mechanics, they are a general wave phenomenon and have since been identified in electromagnetic waves, acoustic waves in air, water waves and elastic waves in solids. These states have been studied in a wide range of material systems, such as piezoelectric materials, dielectric photonic crystals, optical waveguides and fibres, quantum dots, graphene and topological insulators. In this Review, we describe recent developments in this field with an emphasis on the physical mechanisms that lead to BICs across seemingly very different materials and types of waves. We also discuss experimental realizations, existing applications and directions for future work. The fascinating wave phenomenon of ‘bound states in the continuum’ spans different material and wave systems, including electron, electromagnetic and mechanical waves. In this Review, we focus on the common physical mechanisms underlying these bound states, whilst also discussing recent experimental realizations, current applications and future opportunities for research.

Multiphysics Analysis and Experimental Validation of an air Coupled Piezoelectric Micromachined Ultrasonic Transducer with Residual Stresses

In this work, we present a complete multiphysics modelling (via the Finite Element Method, FEM) of an air coupled piezoelectric micromachined ultrasonic transducer (PMUT) with preliminary experimental validations. The PMUT is a suspended layered membrane, in which one of the layers is made of piezoelectric material. By means of an applied voltage over the piezoelectric layer thickness, the device emits acoustic waves in air. The model takes into account the multiple interactions between electrical, mechanical and acoustic fields, and in particular gives a realistic estimation of the device quality factor by means of a proper modelling of thermo-viscous losses in the fluid domain. The complexity of the model is increased by the presence of initial large deformations in the membrane and fabrication induced residual stresses. Preliminary experimental matchings are presented for static pre-deflection of the membrane due to residual stresses and for the eigenfrequency corresponding to acoustic wave emission.

Identification of the mechanical moduli of closed-cell porous foams using transmitted acoustic waves in air and the transfer matrix method

Closed-cell cellular polymer foams are the most efficient insulating materials commercially available. Their lightweight and mechanical resistance also make them ideal materials for packaging protection in the transport industry. A new method using guided acoustic waves in air transmitted through samples of cellular foam panels, has been developed to recover their P-wave moduli and Poisson’s ratios. These parameters were recovered from measured transmission coefficients of the foams through fitting to an elastodynamic transfer matrix method (TMM). The TMM integrates both the P and shear waves propagating in the layer. It was shown by using a finite element fluid–solid (elastic) interaction and an analytic P-waves-only model, that the two types of waves should be modeled to give a more precise representation of the data. The retrieved values were validated using vibration spectroscopy and from the measured velocity of a transient mechanical stress wave propagating in thin, long rod specimens cut from the panels. The problems inherent to these simple 1D characterization methods were pointed out and solved.

Effect of waveguide impedance on radiation in underwater acoustic leaky wave antennas

Leaky wave antennas (LWAs) have been examined for decades as a way to steer electromagnetic waves as indexed by input frequency, enabling rapid video scanning of an environment with a simple compact device. Recently, an LWA device constructed of a rigid waveguide with open shunts was shown to produce a similar steering effect for acoustic waves in air [Naify et al., Appl. Phys Lett. 102, 203508 (2013)]. The shunts serve two purposes, to couple acoustic energy from the waveguide to the surrounding area, as well as control directionality of the radiated wave by changing the impedance, and thus modulating the phase speed of the waves in the waveguide. While this impedance contrast of the waveguide wall relative to that of the surrounding fluid is very large for an air background, when designing a structure for use in water the material of the waveguide can drastically change the directionality of the radiation profile. This study examines the effect of waveguide wall impedance on directionality of an acoustic LWA using both numerical and finite element methods. Three material impedance conditions are examined, including acoustically rigid, and two materials with finite impedance. [Work sponsored by the Office of Naval Research.]

Numerical simulation of acoustic waves in air and poroelastic media using the partition of unity finite element method

Foams and fibrous materials are used in a large range of applications such as automotive or building acoustics. Their properties can be described with either a poroelastic model or an equivalent fluid model. These models, also used in geophysics, are now widely spread and are also available in some commercial finite element software applications. However, the discretization level required to achieve reasonable accuracy is not always acceptable in the mid-frequency range. In such case, the Partition of Unity Finite Element Method (PUFEM) using plane wave functions seems appropriate to avoid this limitation. The PUFEM has been recently applied to rigid frame materials with and without coupling with an acoustic cavity. It has also been applied efficiently to poroelastic materials. The present work focuses on the coupling between a poroelastic material, i.e., described with Biot's equations, and an air cavity. Some practical examples are tested to demonstrate the efficiency of the PUFEM for solving noise control problems at medium frequency, but also to underline the precautions that must be taken when dealing with an air-porous interface.

Numerical simulations of acoustic emissions due to offshore‐pile installation

AbstractThe use of renewable energy is growing due to economic reasons and a spreading sense of ecology. One possible source for renewable energy is wind energy. It is predicted that the number of offshore wind parks will be increasing within the next ten years. The foundation piles of offshore wind power plants are normally installed by impact driving. This leads to an emission of acoustic noise which possibly influences or even damages the sense of hearing of marine mammals like seals and whales.In this contribution the propagation of acoustic waves in air and water due to pile driving is investigated using the finite element method (FEM) with an explicit time integration scheme. In the analyses a mass representing the pile hammer falls on an elastically modeled offshore monopile. The surrounding media is varied between air and water in order to investigate the acoustic emissions due to the pile driving process with respect to the medium. The numerical results are compared to measurements which can be found in literature to verify the numerical models. Furthermore, as a practical example the numerical model can be used in order to simulate noise reduction techniques such as the bubble curtain. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthetic Aperture Imaging of Surface Laid Targets by Sound

This paper discusses the design and implementation of a synthetic aperture acoustic imaging system for investigating solid objects in an outdoor environment. Measurement results are reported for a setup consisting of a rail-mounted transceiver, digital data acquisition and control system, and local storage with data post processing capabilities. This imaging system works much like typical sonar and RF synthetic aperture radar technologies but operates above ground and uses acoustic waves in air for its interrogation.

Chaotic Time-Reversed Acoustics: Sensitivity οf the Loschmidt Echo to Perturbations

We experimentally demonstrate a new acoustic sensor based on the concept of quantum mechanical scattering fidelity and the Loschmidt echo applied to classical acoustic waves in air. The sensor employs a one-recording-channel time-reversal mirror that exploits spatial reciprocity to sensitively measure the classical analog of the scattering fidelity of an enclosed region. The experiments are carried out in a stairwell using a simple speaker and microphone. The input is a 7.0 kHz signal that is amplitude modulated with a 1 ms long pulse. We examine the sensitivity of the time-reversed reconstructed pulse to phase noise, long term drift, and to typical perturbations caused by the rotation of an object in the scattering environment.