Low-frequency Broadband Research Articles

Application of complex medical rehabilitation in patients in the late recovery period after acute cerebrovascular accident

Many observations have shown a close relationship between post-stroke spasticity and pain syndrome due to stroke; with regard to that, in 72% of patients, spasticity was accompanied by pain and only 1,5% of patients experienced pain in the absence of spasticity.Purpose of the study: to substantiate the inclusion of combined physiotherapy methods, i. e. low-frequency electrostatic massage and broadband modulation currents, and multimodal physiotherapy methods from the Alpha LED Oxy Light — Spa™ device, in the standard complex of medical rehabilitation to relieve pain and reduce spasticity in the late recovery period in patients who have undergone acute cerebrovascular accident. Material and methods: the paper presents the data on the inclusion of various physiotherapy methods in the rehabilitation program in the late recovery period in 120 patients with spasticity of the upper limb who underwent acute cerebrovascular accident. The dynamics of pain syndrome intensity were assessed according to the Numerical Pain Rating Scale and the Verbal Descriptor Scale, and the severity of spasticity in the upper limb was evaluated according to the Modified Ashworth Spasticity Scale. All patients were divided into 3 groups comparable in terms of clinical and functional characteristics: group 1 (40 patients), who, in addition to the standard treatment complex (exercise therapy, medical massage, mechanotherapy), were prescribed massage with a pulsed low-frequency electrostatic field from the HIVAMAT device and broadband modulation currents; group 2 (40 patients), whose therapy included physiotherapy methods from the Alpha LED Oxy Light — Spa™ device, against the background of standard therapy and medical rehabilitation; group 3 — 40 patients who received standard drug therapy and medical rehabilitation. Results. Inclusion of pulsed low-frequency electrostatic massage and broadband modulation currents in medical rehabilitation of patients with spasticity of the upper limb who have undergone acute cerebrovascular accident in the late recovery period contributes to the formation of analgesic and antispasmodic effects to a greater extent, compared to the Alpha LED Oxy Light — Spa™ multimodal effects, which is manifested in a significant reduction in pain and a decrease in spasticity.

Using the bispectrum to probe radio X-ray correlations in GRS 1915+105

ABSTRACT We present the results of bicoherence analysis on observations of GRS 1915+105 that exhibit quasi-periodic oscillations (QPOs). The bicoherence is a higher order statistic that can be used to probe the relation between the phases of a triplet of Fourier frequencies. Despite showing very similar power spectra, the observations exhibit different patterns in their bicoherence, indicating that the QPOs are phase coupled to the noise in different ways. We show that the bicoherence pattern exhibited correlates with the frequency of the QPO, the hardness ratio, as well as the radio properties of the source. In particular, we find that the nature of phase coupling between the QPO and the high and low frequency broad-band components is different between radio quiet, radio plateau, and radio steep conditions. We also investigate the phase lag behaviour of observations with QPO frequency above 2 Hz that show different bicoherence patterns and find statistically significant differences between them, indicating a change in the underlying physical mechanism. Finally, we present a scenario whereby the cooling of the jet electrons by soft photons from the accretion disc could explain the observed correlations between the bicoherence and radio properties.

Deep-subwavelength broadband sound absorbing metasurface based on the update finger coiling-up method

The bulky structure of traditional sound-absorbing materials limits its application to control low-frequency noise, and new deep-subwavelength low-frequency broadband acoustic materials have an urgent need. This paper proposes an update finger coiling-up method, which can improve the sound absorption coefficient and working bandwidth of the unit without increasing the thickness. In addition, the complexity of the structure can be simplified to facilitate manufacturing. The influence of the typical parameters of the unit on the sound absorption performance and the coupling performance of the unit are also studied. Finally, by combining 16 finger coiled-up units, a metasurface with continuous broadband sound absorption in the range of 375 Hz to 855 Hz is obtained, and the average sound absorption coefficient is above 0.85. In particular, the thickness of the metasurface is only 38 mm, which is 1/24 of the corresponding operating wavelength at 375 Hz. The proposed update finger coiling-up method provides a new idea for reducing the thickness of sound-absorbing materials, which will have good application prospects in noise control field.

Low-Frequency Broadband Piezoelectric Vibration Energy Harvester Based on Double L-shaped Beam Structures

Low-Frequency Broadband Piezoelectric Vibration Energy Harvester Based on Double L-shaped Beam Structures

A foldable underwater acoustic meta-structure with broadband sound absorption at low frequency

An underwater absorber consisting of a microperforated panel, foldable channel and rubber coating with perfect low-frequency sound absorption, broadband absorption and strong resistance to deformation is presented. The theoretical prediction and simulation analysis are in good agreement. It is demonstrated that sound energy is mainly dissipated in the rubber coating due to waveform conversion at the coupling boundary. A meta-structure with low-frequency and broadband absorption is realized by optimizing the structural parameters. Moreover, at a relatively regulated low-frequency wavelength, the spatial folded structure enables a deep subwavelength dimension. The proposed meta-structure has wide potential applications in underwater noise control.

Mechanism of the low-frequency wide-band within a nonlinear acoustic metamaterial

In this paper, the formation and broadening mechanism of the low-frequency bandgap are revealed by designing a type of nonlinear acoustic metamaterial (NAM) with gaps. The mechanism of the chaotic evolution through the harmonic coupling for the NAM structure is revealed in detail, which can effectively change spectral characteristics in the bifurcation sequence, and finally, lead to a low frequency and chaos. Then, a low-frequency broadband within the NAM structures with gaps is obtained by the finite element method (FEM) due to the mechanism of chaos. Most importantly, the width of the bandgap can be successfully enlarged by adjusting the small gap, because the energy of the elastic wave can be greatly decreased or redistributed with the system going from periodic motion to doubling bifurcation. This mechanism for achieving a low-frequency bandgap overcomes the shortcomings both in the traditional local resonance with too large additional mass and in the inertial amplification structures with narrow bandgaps. Therefore, this idea of achieving a low-frequency bandgap has great theoretical significance for controlling low-frequency sound waves.

Effects of slat track on the flow and acoustic field of high-lift devices

Most of the published works related to noise generation in high-lift devices only consider clean idealized geometries, whereas real aircraft wings contain important parts called the slat tracks that connect the slat and the main wing. To investigate the effects of slat tracks on the ambient flow and noise, simulations are performed for wings based on the 30P30N airfoil. Two configurations are considered, i.e., the 30P30N wing with and without a slat track. Improved detached-eddy simulation is carried out to compute the unsteady near-field flow. The far-field noise propagation is calculated using the Ffowcs Williams-Hawkings equation. The results show that the installation of the slat track has observable effects on the aerodynamic and aeroacoustic performances of the high-lift devices. The maximum lift coefficient of the high-lift device and the stall angle of attack is reduced. Low-frequency broadband noise radiated from the main wing is significantly increased and the directivity of the overall sound pressure level is changed due to the slat track. The far-field noise from the configuration with the slat track displays noticeable sensitivity to the angle of attack. The slat track has an amplification effect on the high-pressure fluctuation on the main wing, especially at a small angle of attack. This leads to nearly 10 dB in the far-field noise based on the discussed wing section. To reveal the relative noise mechanism, vortical structures near the slat track, large-scale spanwise flows above the main wing, and the vortex dynamics are investigated. A spiral vortex pair is captured as the most important flow structure. It is generated in the spanwise direction by the interaction of slat wakes with the boundary layer on the upper surface of the main wing. The pressure fluctuations on the main wing are highly correlated with this flow structure, which cannot be ignored for the study of noise in a real aircraft.

Mesoscopically ordered Fe3O4/C nano-composite for superior broadband electromagnetic wave absorption

Due to the seriousness of the electromagnetic pollution, it is urgent to develop high-efficiency broadband electromagnetic wave absorption (EMA) materials. As a traditional EMA material, the question of expanding absorption bandwidth for Fe3O4/C composite need to be addressed. Based on this consideration, rationally designed Fe3O4/C nano-composite with the regular mesoscopic (1–100 nm) arrangement structure has practical significance. Benefited from this unique structure, the obtained Fe3O4/C mesoscopic ordered nano-composite exhibits broadband absorbing characteristics in 2–12 GHz. The effective absorption bandwidth value of the material is 6.40 GHz at low frequency (covered C band) and the minimum reflection loss is −50.47 dB. The superior broadband is derived from the good whole-test-frequency impedance matching and the strong low-frequency magnetic loss, which is achieved by the regularly mesoscopic arrangement of the magnetic nanoparticles. This study will provide certain inspirations and guidance for the design of low frequency broadband EMA materials in the future.

Characteristics and consequences of vortex wandering around a ship hull at drift: a numerical study

The present paper investigates the major trailing vortex next to the KVLCC2 hull at 30°static drift and ReLoa≈2.6⋅106. Numerical simulations of a single-phase flow with the SST-based IDDES model are conducted with OpenFOAM. Proper orthogonal decomposition reveals the transverse displacement at the vortex centre. Spectral analysis indicates both the presence of a low-frequency broadband wandering motion and the existence of a distinct amount of turbulence at the centre by a broad inertial subrange. Tracking the instantaneous vortex centre in time reveals that the amplitude in relation to the core size is comparable to the far-field wingtip vortex studied by Devenport and colleagues in 1996. A new observation is the significant bending of the vortex axis. This reveals difficulties for the use of vortex planes being corrected for wandering. Following the correction is restricted to the vortex centre. One consequence of wandering is that about two thirds of resolved TKE can be attributed to it.

Acoustic performance of cavities with gradient changes of cavity numbers under low lattice constant in a soft elastic medium

Air cavities with particular arrangements embedded in elastic layers are generally used as anechoic coatings. In this paper, an idea of cavity numbers with gradient changes is introduced in a soft elastic medium that comprises periodical cylindrical cavities to enhance the low-frequency absorption capacity effectively. The presented model provides a sound absorption coefficient that is higher than 0.5 in the frequency ranges of 75 to 95 Hz and 380 to 720 Hz. Thus, validating the idea can effectively enhance the low-frequency absorption capacity compared with those in previous literature. Analytical results based on effective medium approximation theory are compared with those obtained numerically using the finite element method. These results indicate that the invalidity of the analytical model is due to the coupling between cavities under low lattice constant. Power density dissipation and displacement vectors are plotted to study sound absorption mechanism, thereby showing that the dissipated energy is mainly resulted from the multiple-scattering effect. The scattering between cavities induces the transverse wave caused by the wave mode conversion dissipates in the elastic medium and results in sound absorption. Simulation results show that the absorption peaks can be effectively manipulated by tuning the gradient profile of the cavity numbers. The proposed model is then verified by the corresponding experiment. Results obtained numerically and experimentally are agreed well, and its feasibility is verified. The presented model would offer a new approach for the metamaterial design on low-frequency broadband sound absorption and could have potential applications on the design of anechoic coatings.

A band gap optimization scheme for two-dimensional locally resonant phononic crystal with square spiral rings

This paper investigates the band gap (BG) characteristic of a two-dimensional square spiral locally resonant phononic crystal, and a BG optimization scheme is proposed. First, the influence of three structural parameters (the side length of scatterer, the thickness of elastic beams and the number of square rings) on BGs are studied. Meanwhile, the BG formation mechanism is analyzed by studying the displacement fields of the eigenmodes at the BG boundaries. Second, a 7-level numerical experiment of 3-parameter is designed. The finite element method is employed to analysis the start frequency of the first BG and the total width of the first two BGs, which are expressed as regression equations through response surface method. Finally, the optimal structural parameters with low-frequency broadband characteristic are calculated by interior point method. The numerical results are in good agreement with the calculation results, which show the effectiveness of the optimization scheme.

Band-gap dynamics and programming for low-frequency broadband elastic metamaterial

Local resonant elastic metamaterials are broadly studied as sound barriers, vibration isolators and energy harvesting walls. For real applications of these devices in the environment with low frequency and broadband sound waves, band-gap frequency and width of the elastic metamaterial should be programmed. This paper proposes an effective dynamic method to estimate the band-gap edge frequencies of the local resonant elastic metamaterials with four representative resonators. Particularly, uniform resonator, tapered resonator, double-decked resonator and necked resonator are examined. According to Bloch’s theorem, periodic boundary conditions are introduced to transform vibration study of the elastic metamaterial into vibration analysis of its unit cell. Wave equations of the axial vibrations of the unit cells are established. The frequency functions are deduced for predicting the lower and upper edge frequencies of the band-gap. Finite element analyses with Bloch periodic conditions are performed, and the band structures and eigenmodes of the four configurations verify the mathematical model and frequency functions. The resonator dimension is programmed for realizing the maximum normalized band-gap width with respect to the lower edge frequency and unit cell volume considering space limitation. The elastic metamaterial with the necked resonators is demonstrated to have the most superior low-frequency and broadband performances.

Reflective Metasurfaces with Multiple Elastic Mode Conversions for Broadband Underwater Sound Absorption

Unlike their electromagnetic and acoustic counterparts, elastic waves involve different wave modes. The interplay and the coupling among them increase the complexity of the problem while also offering a larger space for wave manipulation. Elastic bulk wave conversion in an elastic metamaterial has recently shown great promise in medical ultrasound and nondestructive testing. Unlike the transmission-type conversion, however, reflective elastic mode conversion has been explored less in terms of analysis and design, despite the enormous possibilities that it might offer for energy trapping and dissipation. In this work, we develop a theoretical framework for constructing elastic anisotropic metasurfaces that can enable reflective longitudinal-to-transverse (L-to-T) and transverse-to-longitudinal (T-to-L) wave conversions. We capitalize on the mechanism of multiple reflective mode conversion to achieve broadband, subwavelength, and near perfect sound absorption in the underwater environment. The reflective scattering properties of the metasurfaces are systematically exploited for incident longitudinal or transverse waves. The conversion mechanism is rooted in reflective Fabry-Perot (FP) resonance, whose occurrence conditions and features are predicted for prescribed effective parameters of the metasurface. We then establish an inverse-design framework for conceiving an underwater coating system formed by a viscoelastic rubber layer and the metasurface. A series of metasurfaces allowing for customized mode conversions are realized for delivering broadband low-frequency and high-efficiency underwater sound absorption. Specifically, an ultrathin rubber-metasurface layer in which the metasurface with a thickness of approximately \ensuremath{\lambda}/70 can lead to nearly 100% sound absorption. Furthermore, we demonstrate that a persistently high absorption (over 80%) can be obtained in a rather robust manner within a wide range of wave incidence angle from \ensuremath{-}60\ifmmode^\circ\else\textdegree\fi{} to 60\ifmmode^\circ\else\textdegree\fi{}. More importantly, high-efficiency acoustic absorption exceeding 75% can be readily achieved through multiple mode conversions within the ultrabroadband range featuring a relative bandwidth of 119%. We reveal the combined FP resonance mechanism of underwater sound absorption, i.e., the FP resonance of the metaconverter, which determines the L-to-T and T-to-L conversion ratio, and the FP resonance of the rubber-metasurface layers, which enhances the wave attenuation inside the rubber. The proposed reflective multiple mode-conversion mechanism and metasurface design methodology open a route towards a class of elastic-wave-based devices with promising potential for underwater applications.

Acoustic metamaterial inverse design based on machine learning using Gauss–Bayesian models

The emergence of acoustic metamaterials (AMs) provides distinct response to acoustic waves unattainable in nature. However, conventional AMs design has relied on extensive experimentation and a trial-and-error approach requiring significant computational resource. Here, we introduce an inverse-design method based on machine learning using Gauss–Bayesian (GB) models and implement it on low-frequency broadband sound attenuating devices. By utilizing the superiority of GB model on multi-parameter design and optimization efficiency, we successively customize ventilated microperforated sound-absorbing structure, sound absorbers with coherently coupled weak resonances (CCWRs) and ventilated meta-silencers with acoustic consecutive Fano resonances (ACFRs). Under the judiciously adaptive acquisition function and Matérn kernel, the designed GB frameworks with small samples are available to seek the optimal solution established on above physical effects by the cycle of training and prediction with high accuracy and efficiency. Good agreements between numerical and experimental results further verify the effectiveness of the proposed method. With the advantage of low cost, high efficiency and flexibility, our GB-based machine learning method paves the way for tremendous opportunities in future design of various metamaterial-based applications, as well as far-reaching implications for acoustics-related fields.

A compact multifunctional metastructure for Low-frequency broadband sound absorption and crash energy dissipation

Sound absorption structures, as an effective approach to reducing sound radiation, have always received extensive attention in the engineering of vibration and noise reduction. In this work, a novel compact multifunctional metastructure is proposed for both low-frequency broadband sound absorption and excellent crash energy dissipation. Cavity resonators with internally extended tubes (CRIET) with hexagonal honeycomb configuration were selected as unit components. An optimization method was proposed to against degradation from impedance coupling effect and contribute to flatter and higher sound absorption coefficient (SAC) curves. The as-designed metastructure has been demonstrated experimentally to have quasi-perfect broadband sound absorption in the target range of 600–1000 Hz with an average SAC above 0.9 and a deep-subwavelength thickness of 30 mm. Compression experiment showed that the CRIET metastructure with hexagonal honeycomb configuration had the same unique excellent load-bearing and crash energy dissipation performance as an intact honeycomb structure. This study provides inspirations and methods for the compact and multifunctional design of broadband sound absorption structures.

Broadband Low-Frequency Sound Absorption via a Hexagonal Acoustic Metamaterial in the Honeycomb Structure

We proposed a low-frequency absorber comprised of a hexagonal honeycomb structure is presented and demonstrated. The acoustic metamaterial of the hexagonal honeycomb structure with different folding numbers n is investigated via finite element simulations to solve COMSOL multi-physics commercial software. This technique is design to expose the influence of thickness and eccentric hexagonal designs to improve acoustic performance. A hexagonal honeycomb structure made of a soft polymer is constructed. The effect of the folding number on the absorption peak is studied, and the results indicate that a more significant folding number can dramatically increase the absorption peak frequency with the absorber decreasing thickness. The impact of structural parameters for honeycomb structure designs is demonstrated to produce a low-frequency sound absorber. To achieve unitary sound absorption at low frequencies results via adjusting the inner and outer thicknesses with the folding number n. we show influenced the hexagonal folding number and adjusting the internal parameters of the acoustic metamaterial to paving the path for creating an ideal sound absorber

Periodic and aperiodic 3-D composite metastructures with ultrawide bandgap for vibration and noise control

Periodic composite structures as acoustic metamaterials have caught enormous research interest for the inherent peculiar dynamics characteristics, effective medium properties, and fantastic mechanical features to control vibration and noises at deep subwavelength scales. A 3-D composite metastructure design endowed with ultrawide three-dimensional bandgap is highly desirable for low frequency broadband vibration and noise control. In that context, the present study proposes two types of 3-D composite mechanical metastructure unit cell designs that are capable enough to induce low frequency ultrawide bandgaps by principle of mode separation. A 3-D polymeric casing is designed and spherical/cylindrical steel masses are embedded to enhance the dynamical characteristics and mechanical properties of the resonant systems. By a numerical study on wave dispersion, the presence of ultrawide bandgap and governing physical mechanism resulting in such broadband bandgap are discussed. An asymptotic parametric study is performed to investigate the effect of metastructure geometric parameters on the reported bandgaps. We performed numerical and experimental frequency response study on the periodic and aperiodic arrangements of the composite metastructures to envisage wave attenuation inside the bandgaps. Both periodic and aperiodic arrays of metastructures yield vibration attenuation over ultrawide frequency range that is promising for low frequency broadband vibration and noise control applications. The proposed composite metastructure design morphology and manufacturing/fabrication processes are also explained for practical design and applications. The proposed mechanical metastructure designs, our modelling technique, research methodology, and the reported findings may contribute to the design and application of metadevices where low frequency broadband vibration and noise control are desirable.

The Low-Frequency Broadband Mechanism of Nonlinear Elastic Metamaterials With Gaps

The Low-Frequency Broadband Mechanism of Nonlinear Elastic Metamaterials With Gaps

Large stroke tri-stable vibration energy harvester: Modelling and experimental validation

An important issue in vibration energy harvesting is to increase the effective operation bandwidth. One solution is to develop multi-stable energy harvesters which can execute inter-well motion under certain conditions. However, almost all the n-stable harvesters are based on the cantilever-beam-magnets configuration if n > 2, namely, to arrange several fixed magnets near the free end of a piezoelectric cantilever beam with tip magnet, which usually has relatively small stroke due to the deformation limitation of the beam. In this paper, we propose a new tri-stable mechanism using four mechanical springs with well-designed configuration, which has large stroke due to the structural property. Thereafter a tri-stable electromagnetic energy harvester is constructed based on the proposed tri-stable mechanism. The dynamic response and electrical output characteristics are theoretically analyzed by numerical simulations using the dimensionless electromechanical coupled equations. It is shown that the inter-well oscillation can be activated in broadband low-frequency range, which generates considerable induced current. Experiments are also conducted to validate the theoretical model and to demonstrate the low-frequency broadband energy harvesting capability. The output power reaches the level of several hundred milliwatts, which is much larger than that of the previous tri-stable harvesters at milliwatt level.

Band gap mechanism and vibration attenuation characteristics of the quasi-one-dimensional tetra-chiral metamaterial

In this paper, the quasi-one-dimensional tetra-chiral metamaterial is proposed for low-frequency broadband vibration attenuation, inspired by the 3D chiral compression-torsion coupling metamaterials. Different from the traditional metamaterials utilizing translational local resonators to induce reaction force to prevent vibration transmission, this paper can realize rotation and translation on a resonator by introducing chiral structure, which is more beneficial for vibration energy dissipation. The band gap mechanism and vibration attenuation characteristics of the proposed metamaterial are studied based on the discrete mass-spring model. The dependences of the band gap characteristics on the parameters are studied in detail as the bigger the chiral angle, the bigger the mass ratio, the smaller the stiffness ratio, the smaller the dimensionless rotational inertia, the wider the band gap. The analytical boundary equations for band gaps are derived to provide guidance for desired band gap design, and multiple and ultra-wide band gaps can be obtained by tuning the structural parameters. Through the investigation of eigenstate modes, the mode reversals at dispersion curves intersection and veering are observed, and the interchange mechanism for the boundary equations of dispersion curves is unveiled. The vibration transmission in finite unit cells validates the frequency ranges with high vibration attenuation predicted by band structure. The movement energy analysis and wave packet propagation in finite unit cells also demonstrate the vibration attenuation characteristics of the proposed metamaterial. This study can provide new ideas and theoretical guidance for metamaterial design.