Low-frequency Acoustic Waves Research Articles

Subwoofer and nanotube butterfly acoustic flame extinction

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

A Comparison of 2 Respiratory Devices for Sputum Clearance in Adults With Non-Cystic Fibrosis Bronchiectasis.

Airway clearance techniques are a vital part of routine care for patients with bronchiectasis. There is no clear superior modality. The Flutter combines oscillations (6-20 Hz) and positive expiratory pressure; the Lung Flute combines positive expiratory pressure and low frequency acoustic waves (18-22 Hz), to augment clearance. This project aimed to compare these devices. This was a randomized crossover study of adult subjects with stable non-cystic fibrosis bronchiectasis (expectorating > 25 mL/d). Subjects attended 2 separate out-patient visits, 1 week apart, and completed a supervised sputum clearance regime and Lickert scale (8 questions regarding subjects' perception of the experience using each device). Total sputum expectorated during supervised intervention (T1) and after 30 min from the end of T1 (T2) was recorded as wet sputum weight. Total wet sputum weight desiccated in a microwave (10 min at 300 watts), allowed measurement of total dry sputum weight. Data were compared using paired t test. We recruited 40 subjects with a mean ± SD age of 63 ± 16 y. Overall, there was no significant difference in wet sputum weight (Flutter, 5.78 ± 6.47 g; Lung Flute, 5.75 ± 0.22 g) and dry sputum weight (Flutter, 0.40 ± 0.86 g; Lung Flute, 0.22 ± 0.21 g). At T1, wet sputum weight was higher for the Flutter (5.10 ± 6.26 g) compared with the Lung Flute (3.74 ± 3.44 g) (P = .038). At T2, wet sputum weight was higher for the Lung Flute (2.02 ± 3.01 g) compared with the Flutter (0.68 ± 0.75 g) (P = .001). Subjects perceived the Flutter as being significantly better at clearing secretions (P = .01), easy to understand (P = .03), and simple to use (P = .01) compared with the Lung Flute. Both devices were well-tolerated and successfully augmented secretion clearance. Most subjects preferred the Flutter because of increased speed of secretion clearance, and greater ease of use.

Sound attenuation on an ocean shelf at short ranges from a source in the presence of surface waves

The effect of surface roughness on the attenuation of low-frequency acoustic waves on a shallow ocean shelf is analyzed using numerical simulation. We focus here on transmission loss during propagation at short (less than 50 water layer depths) ranges from the sound source. The effect is considered both for a soft and hard bottom, when the sound velocity in the bottom is, respectively, lower or higher than the sound velocity in seawater. It is shown that to correctly predict losses at a short range in the presence of a rough upper boundary, it is necessary to take into account the interaction of both propagation and leaky modes. In the case of a hard bottom compared to a low-velocity one, the effect of surface roughness on propagation turned out to be the most pronounced.

Deep ocean sound speed characteristics passively derived from the ambient acoustic noise field

The propagation of acoustic waves in the ocean strongly depends on the temperature. Lowfrequency acoustic waves can penetrate the ocean down to depths where few in situ measurements are available. It is therefore attractive to obtain a measure of the deep ocean temperature from acoustic waves. The latter is especially true if the ambient acoustic noise field can be used instead of deterministic transient signals. In this study the acoustic velocity, and hence the temperature, is derived in an interferometric approach from hydrophone array recordings. The arrays were separated by over 125 km, near Ascension Island in the Atlantic Ocean, at a depth of 850 m. Furthermore, the dispersive characteristics of the deep ocean sound channel are resolved based on the retrieved lag times for different modes. In addition, it is shown how the resolution of the interferometric approach can be increased by cross correlating array beams rather than recordings from single-sensor pairs. The observed acoustic lag times between the arrays corresponds well to modelled values, based on full-wave modelling through best-known oceanic models.

Agglomeration and capture of fine particles in the coupling effect of pulsed corona discharge and acoustic wave enhanced by spray droplets

Fine particles from industrial processes are considered to be potentially harmful to humans and environment, and massive amounts of fine particles are continuously emitted into the air due to the low removal efficiency of current conventional dust capture devices. A novel pretreatment process using the coupling effect of pulsed corona discharge and acoustic waves enhanced by spray droplets for fine particle agglomeration and capture was investigated in this study. The application of pulsed corona discharge in an acoustic field led to an increase in the efficiency of fine particle agglomeration to 74.7%, and the improvement was much more obvious for low frequency acoustic waves. The spray flow rate delivered by the spray nozzles with a size of 0.30mm or 0.40mm was incrementally increased. Furthermore, the penetration efficiency initially decreased, then increased, and finally decreased when droplets were sprayed in either a pulsed corona discharge field or an acoustic wave field. The penetration efficiency of fine particles substantially decreased to 10% with the enhancement of spray droplets coupled with both a pulsed corona discharge and an acoustic wave, both at their most efficient settings. The penetration efficiency further decreased with the addition of sodium dodecyl sulfonic salt surfactant to the spray droplets, in particular, at a concentration of 0.3%.

Acoustic streaming in two-dimensional freely suspended smectic liquid crystal films

We study horizontal streaming excited by means of a low-frequency and low-intensity acoustic wave in 2D freely suspended films of thermotropic smectic liquid crystals. Acoustic pressure induces fast periodic transverse oscillations of the film, which produce in-plane stationary couples of vortices slowly rotating in opposite directions owing to hydrodynamic nonlinearity. The parameters of the vortices are measured using a new method, based on tracking solidlike disk-shaped islands. The horizontal motion occurs only when the amplitude of the acoustic pressure exceeds the threshold value, which can be explained by Bingham-like behavior of the smectic film. The measurements above threshold are in good agreement with existing theoretical predictions. We demonstrate experimentally that in-plane flow is well controlled by changing the acoustic pressure, excitation frequency, and geometry of the film. The observations open the way to using the phenomenon in nondisplay applications.

Распространение упругих волн в плоском слое-волноводе с учётом упрощённой модели континуума коссера.

The problem of propagation of high-frequency and low-frequency acoustic waves in a plane elastic waveguide layer on the basis of a simplified model of the Cosserat continuum is considered. In view, the presence of micro polarity of the medium, boundary value problems for plane and antiplane deformations for different combinations of boundary conditions on the waveguide surface are formulated. In a long-wave and a short-wave approximations the obtained results are compared with the results of the classical theory of elasticity. The conditions for a possible localization of the wave energy near the surface of the waveguide are found. It is shown that in the antiplane deformation problem, the considering of micropolarity of the material is not leads to the possibility of existence of high localized forms. The frequency bands of localized and harmonic waveforms are found. In the plane deformation problem, considering of micropolarity of the material under different surface conditions may cause as a distortion of the frequency band of the localized Rayleigh wave existence so as the emergence of a new frequency band of possible localized waves.

Effect of gas bubble volume fraction on low-frequency acoustic characteristic of sandy sediment

Owing to the decomposition of organic material and other reasons, the actual marine sediment contains gas bubbles, and the existence of gas bubbles will significantly affect the low-frequency acoustic characteristics of sediment. Therefore, it is significant to investigate the effect of gas bubbles on the low-frequency sound velocity in the sediment. Considering the uncontrollable environmental factors of field experiment, an experiment platform for obtaining acoustic characteristics of a large-scale gas-bearing unsaturated sandy sediment is constructed in the indoor water tank. Considering the long wavelength of low-frequency acoustic wave and the multipath interference in water tank, the transmitted acoustic signals are received by hydrophones which are buried in the unsaturated sediment. The sound velocity data (79-142 m/s) in the gas-bearing unsaturated sediment are acquired by using a multi-hydrophone inversion method in the bounded space for the first time in a 300-3000 Hz range, and the sound velocity data (112-121 m/s) are also acquired by using a double-hydrophone method in the same frequency range. The refraction experiments at different horizontal distances between the source and the hydrophones are conducted, which verifies the reliability of sound velocity data acquired by using the multi-hydrophone inversion method and the double-hydrophone method. At the acoustic frequency well below the resonance frequency of the largest bubble in the sediment, the pore water and the gas bubbles are regarded as an effective uniform fluid based on effective medium theory. On this basis, the density and the bulk elastic modulus of pore water in the effective density fluid model are replaced by the effective density and the effective bulk modulus of the effective uniform fluid, then a corrected effective density fluid model is proposed in gas-bearing unsaturated sediment. The numerical analysis indicates that when the gas bubble volume fraction is small (1%), a small increase in the gas bubble will cause a significant decrease in the effective bulk elastic modulus of sediment, but the density of pore water is much greater than the density of gas bubbles, the presence of a small number of gas bubbles hardly changes the density of pore fluid and certainly does not change the density of sediment, which results in a significant decrease at a low-frequency sound velocity in the gas-bearing unsaturated sediment. Furthermore, with the increase of gas bubble volume fraction, the sound velocity predicted by the corrected model gradually decreases, and the decreasing trend gradually becomes gentle. The corrected model reveals the effect of gas bubbles on the low-frequency acoustic characteristic of sediment. By analyzing the sensitivity of the predicted sound velocity to parameters of the model, the gas bubble volume fractions (1.07%-2.81%) of different areas are acquired by inversion according to the measured sound velocity distribution and the corrected model. In the future, it will provide a new method of obtaining the volume fraction and the distribution of gas bubbles in the sediment.

Visualizing detecting low-frequency underwater acoustic signals by means of optical diffraction.

A novel and simple technique based on the light diffraction effect for visualization of low-frequency underwater acoustic waves (LFUAWs) in real time has been developed in this paper. A cylindrical object has been put on the surface of the water. A low-frequency underwater longitudinal wave can be generated into a water surface transversal capillary wave around the cylinder by our technique. Modulating the phase of a laser beam reflected from a water surface by surface acoustic waves (SAWs) realizes the acousto-optic effect. Then, a steady and visible diffraction pattern is experimentally observed. A physical model of the SAW is established to verify the feasibility of our technique. An analytical expression of wavelength, wave amplitude, and excitation frequency has been derived to study the physical properties of LFUAWs, and it explains the experimental phenomenon very well. As a result, the technique is effective, easy, and practical for visualizing LFUAWs and has significance for applications.

Numerical and experimental analysis of the modulation of fiber Bragg gratings by low-frequency complex acoustic waves

Abstract The acoustically modulated bandwidth of fiber Bragg gratings is simulated and experimentally demonstrated. Finite element and transfer matrix methods are used to investigate a superposition of flexural and longitudinal acoustic waves and the induced complex strain in the grating, for the first time. The results show that the longitudinal strain is suitable to generate a bandwidth-voltage response. The modeling is useful for the design and characterization of acousto-optic modulators, which is attractive for the fast control of several photonic devices.

Precise rainbow trapping for low-frequency acoustic waves with micro Mie resonance-based structures

We have realized the acoustic rainbow trapping in the low frequency region (200–500 Hz) through micro Mie resonance-based structures. The structure has eight channels with a high refractive index obtained by coiling space, that can excite strong interactions with incident waves and support various orders of multipoles due to the Mie resonances of the microstructure. By utilizing the structure, the precise spatial modulation of the acoustic wave is demonstrated both theoretically and experimentally. The effect of trapping broadband acoustic waves and spatially separating different frequency components are ascribed to the monopolar Mie resonances of the structures. The trapping frequency is derived and the trapping positions can be tuned arbitrarily. With enhanced wave-structure interactions and tailored frequency responses, such micro structures show precise spectral-spatial control of acoustic waves and open a diverse venue for high performance acoustic wave detection, sensing, filtering, and a nondestructive test.

Experimental determination of the viscous flow permeability of porous materials by measuring reflected low frequency acoustic waves

An acoustic reflectivity method is proposed for measuring the permeability or flow resistivity of air-saturated porous materials. In this method, a simplified expression of the reflection coefficient is derived in the Darcy's regime (low frequency range), which does not depend on frequency and porosity. Numerical simulations show that the reflection coefficient of a porous material can be approximated by its simplified expression obtained from its Taylor development to the first order. This approximation is good especially for resistive materials (of low permeability) and for the lower frequencies. The permeability is reconstructed by solving the inverse problem using waves reflected by plastic foam samples, at different frequency bandwidths in the Darcy regime. The proposed method has the advantage of being simple compared to the conventional methods that use experimental reflected data, and is complementary to the transmissivity method, which is more adapted to low resistive materials (high permeability).

Oblique shock waves in a two electron temperature superthermally magnetized plasma

A study is presented for the oblique propagation of low-frequency ion acoustic (IA) shock waves in a magnetized plasma consisting of cold ions and two temperature superthermally distributed electrons. A nonlinear Korteweg de-Vries-Burger (KdV-Burger) equation is obtained by using the reductive perturbation method (RPM) which governs the dynamics of the IA shock wave. Using the solution of KdV-Burger equation, the characteristics of the IA shock wave have been studied for various plasma parameters. The combined effects of the cold to hot electron temperature ratio ( $\sigma$ ), the density ratio of hot electrons to ions ( $f$ ), the superthermality of cold and hot electrons ( $\kappa_{c}, \kappa_{h}$ ), the strength of the magnetic field ( $\omega_{ci}$ ), and the obliqueness ( $\theta$ ), significantly influence the profile of the shock wave. The findings in the present study could be important for the electrostatic wave structures in the Saturn’s magnetosphere, where two temperature electrons exist with a kappa distribution.

Please Explain the Basic Concept of Ultrasound Elastography, and How Can It Help Differentiate Benign From Malignant Breast Lesions?

the strain source and then visualizes the strain induced in tissue with the use of either an external or internal source. In harmonic elastography, low-frequency acoustic waves are transmitted within the tissue. The phase and amplitude of the propagating waves are visualized using color Doppler imaging [3, 4]. In transient elastography, an ultrasound scanner with a high frame rate (i.e., 10,000 frames per second) is used to generate and then track the propagation of shear waves in tissue [5]. Shear waves are tissue movements perpendicular to the direction of the ultrasound wave transmission. In terms of breast elastography, malignant masses tend to be more irregular, are heterogeneous, and typically appear larger at elastography than at gray-scale imaging [6, 7]. In terms of quantitative analysis, malignant lesions generally also exhibit maximum stiffness greater than 80–100 kPa [7, 8]. Invasive cancers with a high histologic grade, large tumor size, nodal involvement, and vascular invasion have also been shown to be signifi cantly correlated with high mean stiffness at shear-wave elastography. Elastography may be useful in improving the specificity of ultrasound evaluation of BI-RADS categories 3 and 4A lesions, including complicated cysts, by avoiding unnecessary biopsy of BI-RADS category 4A masses (i.e., masses for which suspicion of malignancy was low) without a significant loss in sensitivity. Several classification systems in volving elastography, most of which are based on the color pattern, have been proposed. For example, a bull’s-eye artifact has also been described as a characteristic of benign breast cysts, which may appear as a round or oval lesion with a stiff rim associated with two soft spots, one located centrally and the other posteriorly. Ultrasound elastographic features

An open-structure sound insulator against low-frequency and wide-band acoustic waves

To block sound, i.e., the vibration of air, most insulators are based on sealed structures and prevent the flow of the air. In this research, an acoustic metamaterial adopting side structures, loops, and labyrinths, arranged along a main tube, is presented. By combining the accurately designed side structures, an extremely wide forbidden band with a low cut-off frequency of 80 Hz is produced, which demonstrates a powerful low-frequency and wide-band sound insulation ability. Moreover, by virtue of the bypass arrangement, the metamaterial is based on an open structure, and thus air flow is allowed while acoustic waves can be insulated.

Parametric study of a Schamel equation for low-frequency dust acoustic waves in dusty electronegative plasmas

In this paper, our attention is first concentrated on obliquely propagating properties of low-frequency (ω ≪ ωcd) “fast” and “slow” dust acoustic waves, in the linear regime, in dusty electronegative plasmas with Maxwellian electrons, kappa distributed positive ions, negative ions (following the combination of kappa-Schamel distribution), and negatively charged dust particles. So, an explicit expression for dispersion relation is derived by linearizing a set of dust-fluid equations. The results show that wave frequency ω in long and short-wavelengths limit is conspicuously affected by physical parameters, namely, positive to negative temperature ion ratio (βp), trapping parameter of negative ions (μ), magnitude of the magnetic field B0 (via ωcd), superthermal index (κn,κp), and positive ion to dust density ratio (δp). The signature of the penultimate parameter (i.e., κn) on wave frequency reveals that the frequency gap between the modes reduces (escalates) for k<kcr (k>kcr), where kcr is critical wave number. Alternatively, for weakly nonlinear analysis, reductive perturbation theory has been used to construct 1D and 3D Schamel Korteweg-de Vries (S-KdV) equations, whose nonlinearity coefficient prescribes only compressive soliton for all parameter values of interest. The survey manifests that deviation of ions from Maxwellian behavior leads intrinsic properties of solitary waves to be evolved in opposite trend. Additionally, at lower proportion of trapped negative ions, solitary wave amplitude mitigates, whilst the trapping parameter has no effect on both spatial width and the linear wave. The results are discussed in the context of the Earth's mesosphere of dusty electronegative plasma.

Low frequency acoustic characteristics of periodic honeycomb cellular cores: The effect of relative density and strain fields

Aluminum honeycomb cores, widely used in lightweight composite sandwich structures, are inherently weak and highly susceptible to buckling, permanent deformation and damage. This damage can detrimentally affect the structural integrity of sandwich structures and should be detected and repaired. Detecting damage to honeycomb cores is difficult as cores are sandwiched between composite sheets and not accessible. Typically, for such scenario a non-destructive testing method such as ultrasound is appealing. However, with the periodic and porous structure of honeycomb cores, at ultrasound frequencies they are dispersive and exhibit high transmission loss; thus they are incompatible with standard ultrasound techniques. Conversely, at sub-ultrasound frequencies honeycomb cores are less dispersive. Therefore low frequency elastic waves can potentially be used to non-destructively inspect honeycomb cores. To enable low frequency non-destructive testing of honeycombs, it is instrumental first to characterize their sub-ultrasound wave propagation and dispersion characteristics. Accordingly, this work, using finite element analysis, provides insights into the propagation characteristics of low frequency acoustic elastic waves in aluminum honeycomb cores in terms of phase velocity; dispersion and directionality characteristics; and frequency band gaps. Also, the effect of relative density and defects in the form of deformation on sub-ultrasound wave characteristics are explored.

Theoretical and experimental study on the acoustic wave energy after the nonlinear interaction of acoustic waves in aqueous media

Based on the Burgers equation and Manley-Rowe equation, the derivation about nonlinear interaction of the acoustic waves has been done in this paper. After nonlinear interaction among the low-frequency weak waves and the pump wave, the analytical solutions of acoustic waves’ amplitude in the field are deduced. The relationship between normalized energy of high-frequency and the change of acoustic energy before and after the nonlinear interaction of the acoustic waves is analyzed. The experimental results about the changes of the acoustic energy are presented. The study shows that new frequencies are generated and the energies of the low-frequency are modulated in a long term by the pump waves, which leads the energies of the low-frequency acoustic waves to change in the pulse trend in the process of the nonlinear interaction of the acoustic waves. The increase and decrease of the energies of the low-frequency are observed under certain typical conditions, which lays a foundation for practical engineering applications.

Space-time coherence of low-frequency acoustic waves in a shallow-water area with fluctuating parameters

The space-time correlations of narrowband acoustic signals propagating in shallow water are theoretically investigated. It is assumed that acoustic field fluctuations are caused by the combined effect of wind waves and random internal waves. The acoustic field is represented as a superposition of two components: one of them is characterized by relatively small space-time scales and is formed by wind waves, whereas the other is characterized by large scales and is caused by both random internal waves and wind waves. A numerical simulation is performed for space and time correlation functions of tone signals under typical conditions of the Barents Sea. It is demonstrated that the results of numerical calculations are in good agreement with experimental data.

Infrasound from underwater sources

Atmospheric low-frequency sound, i.e., infrasound, from underwater events has not been considered thus far, due to the high impedance contrast of the water-air interface making it almost fully reflective. Here, we report for the first time on atmospheric infrasound from a large underwater earthquake (Mw 8.1) near the Macquarie Ridge, which was recorded at 1325 km from the epicenter. Seismic waves coupled to hydroacoustic waves at the ocean floor, after which the energy entered the Sound Fixing and Ranging channel and was detected on a hydrophone array. The energy was diffracted by a seamount and an oceanic ridge, which acted as a secondary source, into the water column followed by coupling into the atmosphere. The latter results from evanescent wave coupling and the attendant anomalous transparency of the sea surface for very low frequency acoustic waves.