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• Two proposed hybrid isolators employ inerters to enhance the low-frequency isolation performance of the QZS-VI. • The resonance peaks of the proposed isolators get rectified and moved to the left on the amplitude-frequency spectrum. • The type I QZS-IE isolator can utilize slighter mass blocks to achieve better low-frequency isolation. • The type II QZS-IE isolator merely straightens the resonance peak without other opposite effect. The pursuit of enhanced low-frequency vibration isolation motivates the development of quasi-zero-stiffness vibration isolators (QZS-VIs), yet with inherent limitations. The nonlinear hardening stiffness may distort the resonance peak rightward, narrowing the isolation bandwidth. Additionally, the high sensitivity to excitation variations can exacerbate the rightward distortion and jumping phenomenon. In this study, two types of hybrid isolators (QZS-IE isolators) are developed by integrating inerter elements (IEs) with traditional QZS-VI to enhance isolation performance. Their distinct mass block configurations yield varied enhancement behaviors. The unified dynamic equation for the two QZS-IE isolators is derived using harmonic balance and arc-length methods, validated by Runge-Kutta. The analyses of stability and periodicity clarify their response characteristics. Subsequently, the study investigates the effects of geometry and excitation parameters, comparing the enhancements of the two QZS-IE isolators. The results manifest that the introduction of inerter elements improves low-frequency vibration isolation, suppressing peak transmissibility and reducing isolation beginning frequency. The type I QZS-IE isolator outperforms the type II QZS-IE and traditional QZS isolators in low-frequency isolation performance and excitation sensitivity. However, the high-frequency isolation performance of the type I isolator is hindered by the ascending transmissibility curve post anti-resonant frequency. The type II QZS-IE isolator rectifies resonance peak distortion while avoiding adverse effects. The different configurations of the inerter elements and the related attenuating strategies offer a guideline for diversified vibration isolation objectives. [ABSTRACT FROM AUTHOR]

The excellent dielectric properties and tunable structural design of metal sulfides have attracted considerable interest in realizing electromagnetic wave (EMW) absorption. However, compared with traditional monometallic and bimetallic sulfides that are extensively studied, the unique physical characteristics of solid‐solution‐type sulfides in response to EMW have not been revealed yet. Herein, a unique method for preparing high‐purity solid‐solution‐type sulfides is proposed based on solid‐phase in situ exsolution of different metal ions from hybrid precursors. Utilizing CoAl‐LDH/MIL‐88A composite as a precursor, Fe0.8Co0.2S single‐phase nanoparticles are uniformly in situ formed on an amorphous substrate (denoted as CoAl), forming CoAl/Fe0.8Co0.2S heterostructure. Combing with density functional theory (DFT) calculations and wave absorption simulations, it is revealed that Fe0.8Co0.2S solid solution has stronger intracrystal polarization and electronic conductivity than traditional monometallic and bimetallic sulfides, which lead to higher dielectric properties in EM field. Therefore, CoAl/Fe0.8Co0.2S heterostructure exhibits significantly enhanced EMW absorption ability in the low‐frequency region (2–6 GHz) and can achieve frequency screening by selectively absorbing EMW of specific frequency. This work not only provides a unique method for preparing high‐purity solid‐solution‐type sulfides but also fundamentally reveals the physical essence of their excellent EMW absorption performance. [ABSTRACT FROM AUTHOR]

Real-time onboard health monitoring systems are critical for the railway industry to maintain high service quality and operational safety. However, the issue with power supplies for monitoring sensors persists, especially for freight trains that lack onboard power. Here, we propose a hybrid piezoelectric-triboelectric rotary generator (HPT-RG) for energy harvesting and vehicle speed sensing. The HPT-RG incorporates a rotational self-adaptive technique that softens the equivalent stiffness, enabling the piezoelectric non-resonant beam to surpass resonance limitations in a low-frequency region. The experiments demonstrate the feasibility of using the HPT-RG as an energy harvesting module to collect the rotational energy of the freight rail transport and power the wireless temperature sensors. To allow multiple monitoring in confined spaces on trains, a triboelectric sensing module is added to the HPT-RG to sense the operation speed and mileage of vehicles. Furthermore, the generator exhibits favorable mechanical durability under more than 600 h of official testing on the train bogie axle. The proposed HPT-RG is essential for creating a truly self-powered, maintenance-free, and zero-carbon onboard wireless monitoring system on freight railways. [ABSTRACT FROM AUTHOR]

Aiming at achieving low‐frequency and broadband sound absorption under the premise of light and thin layers, in this paper, polyvinyl butyral (PVB) nanofiber membranes were micro‐perforated and then combined sequentially to prepare multi‐layer micro‐perforated nanofiber membrane (MPNM) for acoustic noise reduction. It was demonstrated that the multi‐layer MPNM exhibited a high absorption (constantly over 50%) in the frequency of 480–2500 Hz. In addition, the established theoretical model of the sound absorbing coefficient can accurately predict the sound absorption performance of the structure with different layers, which can provide a theoretical foundation for the design of the structure of the nanofibrous membrane acoustic absorber. Based on the proposed acoustic model, the relationships between the absorption properties and the parameters were investigated, and it was found that the effective acoustic absorption frequency range and acoustic absorption coefficient curve of the multi‐layer MPNM were closely related to the size and arrangement of hole diameter, perforation rate, fiber membrane thickness, and cavity depth. Optimization of the structural parameters utilizing algorithms can achieve superior sound absorption performance, with an average absorption coefficient of 0.81 in the frequency of 100–2500 Hz. This study provides a theoretical and experimental basis for the development of low‐frequency sound‐absorbing materials and is of great significance for optimizing the acoustic performance of nanofiber membranes and expanding their applications in various acoustic engineering applications. [ABSTRACT FROM AUTHOR]

Introduction: The principle of passive sound control is based on the phenomenon of sound absorption by absorbers. The factors affecting sound absorption include porosity, pore size, pore opening size, thickness, and air flow resistance. Materials and Methods: In this study, the authors compared the optimization results of the effective parameters on sound absorption coefficient (AC) using the three optimization methods: Guided Local Search (GLS), Genetic Algorithm (GA) and Particle Swarm Optimization (PSO). The programming was done in MATLAB software. Thicknesses of 5, 10, 20, 30 and 40 mm were chosen for optimization at frequencies of 500 to 3000 Hz. Results: In frequencies above 2 kHz (thickness 5 to 40 mm), the three optimal methods had the same performance and estimated AC of 1. At low frequencies of 2 kHz and thicknesses of 30 and 40 mm, GA and PSO methods obtained an AC of 1. Conclusion: It seems that the GA and PSO optimization algorithm are suitable methods to optimize the AC of metal foam in low and high frequencies.

Highlights MXene-based structural microwave-absorbing materials achieve an impressive reflection loss of − 47.9 dB in the low-frequency S-band, without the presence of magnetic materials and with low density. The elucidation of the mechanism behind the shift of the absorption band from high to low frequencies is attributed to the coupling of controlled multilevel polarization with induced electric field interactions.

Quasi-zero-stiffness (QZS) isolators exhibit high-static and low-dynamic stiffness without sacrificing the load-bearing capacity and with the ability to shield the vibration in low-frequency ranges. Most of the current designs possess QZS property using a combination of positive stiffness element with negative stiffness element, increasing the complexity of the model. In the present research, a single-element mechanical metamaterial is tailored to obtain the QZS property. The building block consists of a single elastically-deformed beam, and the static results of the 3D-printed metastructure demonstrate QZS behavior. A mathematical model is developed for studying the nonlinear behavior of the proposed model, and the dynamic equation is solved using Harmonic-Balance method. Further, an experiment is performed to confirm the vibration isolation capability of the proposed model in low-frequency ranges. Stability analysis is performed mathematically using the perturbation analysis. The QZS mechanism obtained here solely depends on the flexible beam's geometrical configuration, hence making the design strategy material independent. [ABSTRACT FROM AUTHOR]

The proposed revisit to a classical problem in fluid–structure interaction is due to an interest in the analysis of the narrow resonances corresponding to a low-frequency fluid-borne wave, inspired by modeling and design of metamaterials. In this case, numerical implementations would greatly benefit from preliminary asymptotic predictions. The normal incidence of an acoustic wave is studied for a circular cylindrical shell governed by plane strain equations in elasticity. A novel high-order asymptotic procedure is established considering for the first time all the peculiarities of the low-frequency behavior of a thin fluid-loaded cylinder. The obtained results are exposed in the form suggested by the Resonance Scattering Theory. It is shown that the pressure scattered by rigid cylinder is the best choice for a background component. Simple explicit formulae for resonant frequencies, amplitudes, and widths are presented. They support various important observations, including comparison between widths and the error of the asymptotic expansion for frequencies. [ABSTRACT FROM AUTHOR]

Conventional brute-force attacks can now be detected and identified based on statistical analysis of logs and traffic data. However, they fail to detect low-frequency and distributed brute-force attack behaviors. To address different attack methods, new detection techniques have emerged. This study compares various machine learning algorithms and selects two methods, namely the clustering algorithm k-means and bdscan, as well as the decision tree algorithm for data learning. In one approach, normal user login data is integrated with enterprise email log data. The data is first statistically analyzed and filtered, followed by quantifying data characteristics using information entropy. Subsequently, machine learning algorithms are employed for classification, and the results are visualized for display. In another approach, labeled raw data is used to train a model using the decision tree algorithm. By comparing the two analysis results, a more accurate model can be obtained. These analytical methods can help enterprises strengthen email security and defend against low-frequency and distributed brute-force attacks. [ABSTRACT FROM AUTHOR]

Elastic diodes with nonreciprocity have the potential to enable unidirectional modulation of elastic waves. However, it is a challenge to achieve nonreciprocity at low frequencies (<100 Hz) using existing elastic diodes. This paper proposes a quasi-zero-stiffness (QZS) elastic diode to resolve such a tough issue and fulfill high-quality low-frequency nonreciprocity. The proposed elastic diode is invented by combining a QZS locally resonant metamaterial with a linear one, where the beneficial nonlinearity of the QZS metamaterial facilitates opening an amplitude-dependent band gap at very low frequencies. Firstly, the dispersion relation of the QZS metamaterial is derived theoretically based on the harmonic balance method (HBM). Then, the transmissibility of the QZS elastic diode in both the forward and backward directions is calculated through theoretical analyses and numerical simulations. Additionally, the influences of system parameters on the low-frequency nonreciprocal effect are discussed. The results indicate that considerable nonreciprocity is observed at a quite low frequency (e.g., 9 Hz), which is achieved by amplitude-dependent local resonance combined with interface reflection. Finally, a machine learning-based design optimization is introduced to evaluate and enhance the nonreciprocal effect of the QZS elastic diode. With the aid of machine learning (ML), the computational cost of predicting nonreciprocal effects during design optimization can be significantly reduced. Through design optimization, the nonreciprocal frequency bandwidth can be broadened while maintaining considerable isolation quality at low frequencies. [ABSTRACT FROM AUTHOR]

Noise pollution is growing nowadays and there is always a conflict between ventilation and noise insulation. The advances in acoustic metamaterials provide an effective solution for this conflict. In this paper, we propose a metastructure with protruded necks based on Helmholtz resonance. Sound transmission loss (STL) of 13 dB at low-frequency of 488 Hz is obtained in experiment with the ventilation of 20%, which is in good agreement with the simulation. It is revealed that introduction of protruded neck can reduce the resonant frequency. Sound absorption is the main mechanism for noise insulation at low frequency. Next, the effects of geometric parameters on sound insulation are discussed. A reduction in cross-sectional area of neck, an increase in neck length, and an increase in ventilation all lead to a decrease in both resonant frequencies and STL amplitudes. Finally, a two-layer structure is constructed by the proposed metastructure with different neck length in each unit. Both the amplitude and frequency range of STL are enlarged. These results exhibit a practical application for ventilated metastructure in sound insulation at low frequency. [ABSTRACT FROM AUTHOR]

In nature, lakes and water channels offer abundant underwater energy sources. However, effectively harnessing these green and sustainable underwater energy sources is challenging due to their low flow velocities. Here, we propose an underwater energy-harvesting system based on a cylindrical bluff body and a cantilever beam composed of a macro fiber composite (MFC), taking advantage of the MFC's low-frequency, lightweight, and high piezoelectric properties to achieve energy harvesting in low-frequency and low-speed water flows. When a water flow impacts the cylindrical bluff body, it generates vibration-enhanced and low-frequency vortices behind the bluff body. The optimized diameter of the bluff body and the distance between the bluff body and the MFC were determined using finite element analysis software, specifically COMSOL. According to the simulation results, an energy-harvesting system based on an MFC cantilever beam applied in a low-frequency and low-speed water flow was designed and prepared. When the diameter of the bluff body was 25 mm, and the distance between the bluff body and MFC was 10 mm and the maximum output voltage was 22.73 V; the power density could reach 0.55 mW/cm2 after matching the appropriate load. The simulation results and experimental findings of this study provide valuable references for designing and investigating energy-harvesting systems applied in low-frequency and low-speed water flows. [ABSTRACT FROM AUTHOR]

This work presents the study of effect of morphology of carbonyl iron, carbonyl iron-based, and e-waste-based mixtures on microwave absorption properties. Firstly, the flake shaped carbonyl iron particles and its mixture are prepared using conventional and efficient ball milling and blending method, respectively. It provides the absorption in lower frequency region. Further, the covered frequency range starts to shifts slowly towards higher frequency ranges due to the morphological changes. On the other hand, effect of complex morphology of e-waste-based mixtures are also studied. These heterogeneous mixtures are developed using the cutting mill and blending methods that can provide the absorption in higher frequency with very narrow bandwidth. In this manner, effect of morphology on single layer microwave absorption performance of developed heterogenous material is proposed. It is also explored that the developed samples cover a wider bandwidth by utilizing the multilayering technique. A three-layer absorber is designed using genetic algorithm and the results are also validated. The structural and morphological study of the developed heterogeneous materials has been done using X-ray diffraction (XRD) and Field Emission Scanning Electron Microscope (FESEM) analysis. Also, rigorous study has been done on magnetic properties and Cole-Cole plots to understand the magnetic and dielectric losses respectively occurring inside the materials. The results showed wide bandwidth absorption of 11.62 GHz (≤−10 dB bandwidth) in the lower frequency region ranges from 0.5 to 12.12 GHz (i.e., L, C, and X-band). The total thickness achieved is 1.85 mm which is very compact (<2 mm). Thus, a thin and wideband microwave absorber at lower frequency range using heterogeneous morphology of developed materials is also presented. The proposed microwave absorbing materials can be potentially utilized for the shielding applications. [Display omitted] • Study explores the impact of morphology on microwave absorption in heterogeneous mixtures. • Milled Carbonyl iron and its mixture exhibit absorption in lower frequency regions. • E-waste-based mixtures with complex morphology exhibit absorption in higher frequency regions. • Multilayering techniques extend bandwidth, with a three-layer absorber designed using genetic algorithms. • Wideband absorption (11.62 GHz) achieved in the lower frequency range (0.5–12.12 GHz) at a compact (<2 mm) thickness. [ABSTRACT FROM AUTHOR]

Highlights: MXene-based structural microwave-absorbing materials achieve an impressive reflection loss of − 47.9 dB in the low-frequency S-band, without the presence of magnetic materials and with low density. The elucidation of the mechanism behind the shift of the absorption band from high to low frequencies is attributed to the coupling of controlled multilevel polarization with induced electric field interactions. Two-dimensional carbon-based materials have shown promising electromagnetic wave absorption capabilities in mid- and high-frequency ranges, but face challenges in low-frequency absorption due to limited control over polarization response mechanisms and ambiguous resonance behavior. In this study, we propose a novel approach to enhance absorption efficiency in aligned three-dimensional (3D) MXene/CNF (cellulose nanofibers) cavities by modifying polarization properties and manipulating resonance response in the 3D MXene architecture. This controlled polarization mechanism results in a significant shift of the main absorption region from the X-band to the S-band, leading to a remarkable reflection loss value of − 47.9 dB in the low-frequency range. Furthermore, our findings revealed the importance of the oriented electromagnetic coupling in influencing electromagnetic response and microwave absorption properties. The present study inspired us to develop a generic strategy for low-frequency tuned absorption in the absence of magnetic element participation, while orientation-induced polarization and the derived magnetic resonance coupling are the key controlling factors of the method. [ABSTRACT FROM AUTHOR]

The working mechanism of an acoustic metamaterial (AM) for broadband elastic vibration suppression with non-independent local resonators is presented in this paper along with the general formulas for the effective mass (EM), dispersion relation, and transmission spectrum (TR) of this metamaterial unit. A kind of flexible metamaterial sticker that is lightweight and skillfully uses flexible materials is proposed based on a theoretical approach. The flexible metamaterial sticker has a surface density of only 2.22 kg/m² and an overall thickness of only 3 mm. It is made by depositing the flexible cylindrical supports in a square lattice pattern on the surface of the flexible plate. The finite element method (FEM) was used to systematically investigate the band structures, frequency response function (FRF), dynamic effective mass density (EMD), as well as the formation mechanisms of the flexural vibration bandgaps (FVBGs) of the metamaterial plates (composite structure after applying the metamaterial sticker). Additionally, a thorough analysis was conducted on the impacts of geometrical parameters (the rubber cylinder thickness, the flexible material plate thickness, the lattice constant, and the rubber cylinder radius) on the FVBGs. Finally, an overall vibration attenuation for the proposed metamaterials was estimated by using the spatial quadratic velocity and experiment. The findings confirmed that the AM caused multi-frequency negative EM, while the overall bandgap width was substantially wider than that of conventional metamaterials. Due to the numerous vibration modes of the flexible metamaterial, the suggested flexible lightweight metamaterial sticker can generate several observable local resonance FVBGs in the low-frequency range. Significantly broadening the bandwidth of FVBGs can be achieved by varying the rubber cylinder radius and thickness, as well as by adjusting the lattice constant and flexible material plate thickness. Within the FVBGs, the proposed lightweight flexible metamaterial sticker shows a good vibration-suppression performance, when compared with the traditional damping structure or metamaterials. [ABSTRACT FROM AUTHOR]

A theoretical model of the normal sound insulation of a rectangular locally resonant (LR) plate with simply supported boundary conditions is established. The accuracy of the model is verified through comparison with finite element simulation results. Considering a 1-m2 aluminum plate as the research object, a non-dominated genetic algorithm is introduced. The algorithm optimizes the lightweight sound insulation of an LR finite plate with a center frequency band of 20–800 Hz as the optimization objective of maximum average sound insulation and minimum mass. The optimization parameters are substrate thickness and local resonance parameters (including resonator target frequency (f obj), number of resonators, additional mass ratio, and resonator damping). Optimization results indicate that the average sound insulation fluctuates at approximately 2 dB due to changes in f obj. The results also provide an optimal solution to the algorithm. The optimal sound insulation value curve of LR plates corresponding to different surface densities of intermediate improved cases is fitted and compared with the sound insulation value of an equal-mass bare plate. Under lightweight optimization, the resonators are deemed capable of improving the local frequency band sound insulation. In contrast, the wideband average sound insulation can approximate but not surpass the insulation provided by the equal-mass bare plate. In practical applications, an LR plate that can improve the local frequency band sound insulation and ensure the overall lightweight sound insulation performance of the wide frequency band can be obtained. This can be achieved by analyzing the frequency spectrum of optimal points in the lightweight sound insulation optimization process. Relevant research can provide a basis for the evaluation and design of the low-frequency sound insulation of plates for transportation equipment, such as airplanes, high-speed trains, and cars. [ABSTRACT FROM AUTHOR]

Abstract The excellent dielectric properties and tunable structural design of metal sulfides have attracted considerable interest in realizing electromagnetic wave (EMW) absorption. However, compared with traditional monometallic and bimetallic sulfides that are extensively studied, the unique physical characteristics of solid‐solution‐type sulfides in response to EMW have not been revealed yet. Herein, a unique method for preparing high‐purity solid‐solution‐type sulfides is proposed based on solid‐phase in situ exsolution of different metal ions from hybrid precursors. Utilizing CoAl‐LDH/MIL‐88A composite as a precursor, Fe0.8Co0.2S single‐phase nanoparticles are uniformly in situ formed on an amorphous substrate (denoted as CoAl), forming CoAl/Fe0.8Co0.2S heterostructure. Combing with density functional theory (DFT) calculations and wave absorption simulations, it is revealed that Fe0.8Co0.2S solid solution has stronger intracrystal polarization and electronic conductivity than traditional monometallic and bimetallic sulfides, which lead to higher dielectric properties in EM field. Therefore, CoAl/Fe0.8Co0.2S heterostructure exhibits significantly enhanced EMW absorption ability in the low‐frequency region (2–6 GHz) and can achieve frequency screening by selectively absorbing EMW of specific frequency. This work not only provides a unique method for preparing high‐purity solid‐solution‐type sulfides but also fundamentally reveals the physical essence of their excellent EMW absorption performance.

AbstractBackgroundMethodsResults and conclusion\nKEY POINTSWe report a case on the efficiency of low-frequency repetitive transcranial magnetic stimulation (rTMS) on cigarette consumption cessation.The patient was 29 years old, He received intensive psychiatric treatments along with ten years of history of depressive and obsessive-compulsive disorders.The first four years of treatment were marked by good outcomes. However, the last 6 years were marked by persisting depressive symptoms associating with psychotic features and sexual obsessions.The patient received 20 sessions of rTMS over two weeks, 10 times per week at 1 Hz frequency, each session lasted 20 min with 200 pulses and 100% of motor threshold. In the meanwhile, the patient continued his full drug-based treatment according to the reported prescription.After 20 sessions, the patient reported a decreased severity of his depressive symptoms with a BDI (Beck Depression Inventory) score of 6 but without significant improvement of OCD with a YBOCS score of 32. However, the patient reported great improvement in his tobacco craving and a diminishing number of consumed cigarettes in the second week of the rTMS protocol of treatment. This ended with tobacco cessation within a month. However, this occurred without any medication or psychological support for treating tobacco dependence.The patient 29 years old has received intensive psychiatric treatments along with ten years of history of depressive and obsessive-compulsive disorders.The last 6 years were marked by persisting depressive symptoms associated with psychotic features and alternatives or adjuncts to conventional anxiolytics.The patient received rTMS in meanwhile, the patient continued his full drug-based treatment according to the reported prescription.An improvement in tobacco craving that ended with tobacco cessation within a month without significant improvement in OCDHowever, this occurred without any medication or psychological support for treating tobacco dependence.The patient 29 years old has received intensive psychiatric treatments along with ten years of history of depressive and obsessive-compulsive disorders.The last 6 years were marked by persisting depressive symptoms associated with psychotic features and alternatives or adjuncts to conventional anxiolytics.The patient received rTMS in meanwhile, the patient continued his full drug-based treatment according to the reported prescription.An improvement in tobacco craving that ended with tobacco cessation within a month without significant improvement in OCDHowever, this occurred without any medication or psychological support for treating tobacco dependence. [ABSTRACT FROM AUTHOR]

The three-dimensional (3D) low-frequency lightning mapping system (LF-LMS) in north China has been updated. The lightning electric field derivative (dE/dt) sensor and continuous acquisition mode has been newly designed to ensure a capability of entire lightning processes detection, especially weak discharges during lightning the initiation process. The twice cross-correlation delay estimation and the grid iteration nested optimization location algorithm are used to realize the 3D location of the discharge channel, and the location resolution and calculation speed are balanced consequently. The location results of the rocket-triggered lightning demonstrated that the system achieved a high-resolution mapping of lightning discharge channels, which coincided well with the optical images. The horizontal and vertical location error for rocket triggered lightning was less than 40 m in both horizontal and vertical. Intracloud (IC) lightning flashes were observed to be initiated by three different discharge processes, initial breakdown pulse (IBP), narrow bipolar event (NBE), and initial E-change (IEC). The corresponding initial height was 10.5 km, 6.9 km, and 9.2 km, respectively. The upward negative leader was initially located, followed by scatter radiation sources and negative recoil leaders in the lower negative charge region for all cases. The electric field characteristics of the IEC and subsequent IBPs indicated that they are different discharge processes with the same current direction. The IEC process might correspond to the discharge process with continuous current and less noticeable current changes. [ABSTRACT FROM AUTHOR]

AbstractThe paper explores the increasing concern over environmental vibrations resulting from machine operation and rail transit. Notably, low-frequency vibrations generate surface waves with characteristics like significant energy carrying capacity, long-distance propagation, and slow attenuation. The primary challenge in ground vibration attenuation lies in addressing low-frequency vibrations. The study employs the finite element method to investigate the vibration attenuation of a three-component local resonance pile in three-dimensional space. The analysis delves into the impact of the unit cell structure’s geometry size on bandgaps, and the influences of the period number on the Attenuation Zones (AZs) are also discussed. Furthermore, a comparison is made regarding the vibration attenuation performance of a two-component local resonance pile. The findings reveal that the appropriate design of a three-component local resonance pile can generate low-frequency complete bandgap. The concrete radius, rubber layer thickness, and buried depth of the pile significantly affect Rayleigh wave bandgap. Frequency domain analysis indicates that an increase in period number expands AZs into the bandgap range. Time domain analysis demonstrates a noticeable decrease in ground acceleration amplitude within the bandgap range. The research results presented in this paper offer a novel approach to address the low-frequency vibration attenuation. [ABSTRACT FROM AUTHOR]

Radiative efficiency (RE) is a key climate metric used in the evaluation of a greenhouse gases global warming potential. However, experimental methods are commonly limited to measurements in the spectral region greater than ∼500 cm−1. In this study, density functional theory with the wB97X-D/def2TZVPPD level of theory was employed to calculate the infrared absorption spectra and the contribution of the 0–500 cm−1 spectral region to the total RE for the nearly 800 greenhouse gases, 20 different classes of primarily halogenated compounds, included in the World Meteorological Organization (WMO) Scientific Assessment of Ozone Depletion Annex [1]. In general, the results indicate that the contribution of the low-frequency regime to the total overall RE is not significant, < 2%, with a few exceptions, e.g. several compounds in the hydrocarbon class. [ABSTRACT FROM AUTHOR]

Purpose: One of the key challenges in the research of phononic crystals is achieving small-size control of large wavelengths, which means obtaining low-frequency band gaps with relatively small lattice dimensions. Previous studies have mostly been unsatisfactory in this regard. To obtain lower starting frequencies and more satisfactory band gap widths, this work presents a novel design for a phononic crystal structure. Design Approach: The proposed phononic crystal consists of a silicon rubber connecting plate, an epoxy resin substrate, and tungsten metal cone scatterers. Through finite element method (FEM) calculations and analysis, we have successfully achieved an ultrawide band gap. To delve further into the origin of the ultra-wideband gap in a newly conceived phononic crystal, the vibrational modes of this crystal were carefully studied. Findings: This work has successfully achieved an ultrawide band gap with a width ranging from 122.47 to 4360.2 in the case of a lattice constant of a = 8.5 mm. It was found that the low-frequency ultra-wideband gap cannot be obtained without the presence of silicone rubber. Furthermore, an equivalent spring model was developed, and the accuracy of this model was successfully validated through meticulous calculations. At last, It is found that d1, d4, h1, and h3 have the most pronounced effect on the ultrawide bandgap, and the intrinsic reason is the fact that they determine the geometric structure of the silicone rubber connection plate. Research Limitations/implications: Due to the chosen research method of finite element analysis, the study results may vary depending on the different mesh discretizations, but this type of error is small and can be ignored. Practical Implications: This work provides a new design solution for phononic crystal miniaturization. Originality/value: Compared with previous reports, the new phonon crystals designed in this paper have smaller size, lower starting frequency, and wider band gap. [ABSTRACT FROM AUTHOR]

The low-frequency ultrawideband synthetic aperture radar (UWB SAR) has the ability of the foliage-penetrating and high-resolution imaging, which can detect the foliage-concealed target. However, due to low-frequency UWB SAR characteristics and forest detection environments, there are usually some nontarget strong scattering points in the low-frequency UWB SAR images, which may increase the difficulty of the foliage-concealed target change detection. To solve the problem of the weak antijamming ability of the foliage-concealed target change detection, a foliage-concealed target change detection scheme based on the convolutional neural network in the low-frequency UWB SAR images is proposed, which combines the traditional image difference method and deep-learning method. First, a relatively low inspection threshold is set for the target change detection based on the image difference method, which can obtain a lot of the position information of the detection point. Moreover, for the target characteristics in the foliage-concealed scenarios, the corresponding data extraction and enhancement techniques are used to effectively extract the detection point samples from the detection image and reference image, which can prevent the overfitting of the model training caused by the sample scarcity. Finally, the samples of the detection points are input to the target classification network with the double input and single output for the classification training and testing. The experimental results tested on the CARABAS-II SAR dataset demonstrate the correctness and effectiveness of the proposed scheme, which has the better change detection performance and anti-interference capability.

It is well known that abnormal reward processing is a characteristic feature of various psychopathologies including schizophrenia (SZ). Reduced reward anticipation has been suggested as a core symptom of SZ. The present study aims to evaluate the event-related oscillations (EROs) delta, theta, alpha, beta, and gamma in patients with SZ during the Monetary Incentive Delay (MID) task, which elicits the neural activity of reward processing. Twenty-one patients with SZ and twenty-two demographically matched healthy controls were included in the study. EROs were compared between groups and correlation analyses were conducted to determine a possible relationship between clinical scores and ERO values. Compared with healthy controls, the SZ group had reduced (1) delta and theta amplitudes in the reward condition (2) total beta and non-incentive cue-related beta amplitudes, and (3) incentive cue-related frontal gamma amplitudes. These reductions can be interpreted as impaired dopaminergic neurotransmission and disrupted cognitive functioning in the reward processing of SZ. In contrast, SZ patients showed higher incentive cue-related theta and occipital gamma amplitudes compared to controls. These increments may reflect negative symptoms in SZ. Moreover, theta amplitudes showed a negative correlation with Calgary Depression Scale for Schizophrenia scores and a positive correlation with attentional impulsivity. This is the first study showing the impairments of SZ patients in EROs from delta to gamma frequency bands compared with healthy controls during reward anticipation. Being the first comprehensive study, our results can be interpreted as providing evidence for disrupted brain dynamics in the reward processing of SZ studied by EROs. It may become possible to help patients' wellness by improving our understanding of reward processing in schizophrenia and developing innovative rehabilitation treatments based on these findings. [ABSTRACT FROM AUTHOR]

Efficient broadband absorption of low-frequency sound via ultra-thin structure remains challenging due to the narrow-band property generated by the dispersive nature of resonance. In this study, we investigate the absorption mechanism of a component composed of a perforated panel and space-coiling channels through the coupling effect, acoustic impedance matching, and complex frequency analysis. In addition, the influence of geometrical parameters, resonance frequency intervals, and number of components in the coupled system on the band is investigated. Accordingly, the strategy for developing absorbers is to design individual components in the under-damped state by adjusting the geometrical parameters, then put together multiple components with different channel lengths in parallel. On the basis of this strategy, a low-frequency and broadband absorber is theoretically proposed and experimentally demonstrated, which can achieve broadband absorption from 250 Hz to 450 Hz. The design strategy has potential applications in low-frequency noise control engineering, such as plants, automotive and aerospace industries. [ABSTRACT FROM AUTHOR]

Remote sensing images are frequently contaminated by clouds that often degrade the performance of subsequent applications. Cloud removal, therefore, is a standard step in remote sensing image preprocessing, and single-image-based thin cloud removal is a well-established area of research. Existing single-image-based thin cloud removal methods however, lack the capacity for simultaneous executions of efficient long-range modeling and physical attribute consideration. To extend this work and fill this gap, a novel blind single-image-based thin cloud removal method, called cloud perception integrated fast Fourier convolutional network (CP-FFCN), was designed and implemented. The CP-FFCN consists of two modules: the cloud perception module (CPM) and a fast Fourier convolution (FFC)-conducted reconstruction module (FFCN). The CPM uses a frequency spatial attention mechanism to realize long-range modeling of clouds, globally detect them in the cloudy image. It helps the CP-FFCN remove the clouds without external prior knowledge of the cloud distribution. The reconstruction module was designed with an FFC-conducted U-Net architecture to recover the clean images from cloudy scenarios, guided by the locations of clouds as detected by the CPM. In addition, the FFC blocks deployed in the encoder and decoder components in the U-Net architecture selectively learn the attributes of clouds and fogs from the frequency spectrograms to remove the clouds and reconstruct the underlying ground objects. The CP-FFCN selectively learns the frequency features for adequate cloud separation and at the same time efficiently models the long-range information for comprehensive scenario reconstruction with the help of these two modules. We adopted the Google Earth data and Landsat-8 imagery to train the CP-FFCN model and evaluate it on simulated and naturally occurring cloudy scenarios. The visual outcomes illustrate that the proposed CP-FFCN successfully removes thin and small-scale thick clouds with complex ground object scenarios, without external cloud masks and additional reference data. The quantitative analyses further demonstrate the higher effectiveness of the CP-FFCN when compared with several other state-of-the-art thin cloud removal methods, yielding a PSNR value over 39.24 and a SSIM value over 0.98 on the Landsat 8 images. [ABSTRACT FROM AUTHOR]

In nature, lakes and water channels offer abundant underwater energy sources. However, effectively harnessing these green and sustainable underwater energy sources is challenging due to their low flow velocities. Here, we propose an underwater energy-harvesting system based on a cylindrical bluff body and a cantilever beam composed of a macro fiber composite (MFC), taking advantage of the MFC’s low-frequency, lightweight, and high piezoelectric properties to achieve energy harvesting in low-frequency and low-speed water flows. When a water flow impacts the cylindrical bluff body, it generates vibration-enhanced and low-frequency vortices behind the bluff body. The optimized diameter of the bluff body and the distance between the bluff body and the MFC were determined using finite element analysis software, specifically COMSOL. According to the simulation results, an energy-harvesting system based on an MFC cantilever beam applied in a low-frequency and low-speed water flow was designed and prepared. When the diameter of the bluff body was 25 mm, and the distance between the bluff body and MFC was 10 mm and the maximum output voltage was 22.73 V; the power density could reach 0.55 mW/cm2 after matching the appropriate load. The simulation results and experimental findings of this study provide valuable references for designing and investigating energy-harvesting systems applied in low-frequency and low-speed water flows.

This paper presents the design of a novel hybrid tri-stable piezoelectric energy harvester (HTPEH) that improves the energy harvesting efficiency of piezoelectric energy harvesters operating at low frequencies. The proposed HTPEH is an extension of a conventional tri-stable cantilever piezoelectric energy harvester with an elastic base (TPEH + EB), and it incorporates vertical and rotational elastic amplifiers that are installed between the mass block at the left end of a cantilever beam and the left side of a U-shaped frame to enhance the system's nonlinear characteristics. By utilizing Hamilton's principle, a HTPEH's nonlinear electromechanical equation is formulated, and the harmonic balance method is used to obtain analytical solutions for the displacement amplitude, voltage amplitude, and power amplitude. The study investigates the effects of various parameters, including the elastic amplifier-to-base stiffness ratio and elastic amplifier-to-piezoelectric cantilever beam mass ratio on the energy harvesting performance of the HTPEH. The results indicate that the displacement and voltage amplitude of the HTPEH exhibit two peaks as the external excitation frequency increases, and adjusting the relative mass and stiffness between the elastic amplifier and the piezoelectric cantilever beam can alter the frequency band interval where the two response peaks of the system are located. This allows the HTPEH to enter inter-well motion at low-level external excitation, resulting in high output power. The HTPEH outperforms the TPEH + EB in terms of energy harvesting efficiency under low-frequency environmental excitation.

A sandwiched piezoelectric accelerometer is developed and optimized for acquiring low-frequency, wide-band seismic data. The proposed accelerometer addresses the challenges of capturing seismic signals in the low-frequency range while maintaining a broad frequency response through the design of multi-stage charge amplifiers and a sandwiched structure. The device's design, fabrication process, and performance evaluation are discussed in detail. Experimental results demonstrate its performance in amplitude and phase response characteristics. [ABSTRACT FROM AUTHOR]

We present the theoretical investigation on the flexural wave propagation and vibration attenuation characteristics in a Euler–Bernoulli beam-type metastructure with double-sides inertial amplified resonators. Based on Bloch theory, the dispersion relation and propagation characteristics of flexural waves are calculated by the spectral element method. The influences of system parameters on the flexural wave attenuation of the proposed beam-type metastructures are analyzed. Results show that broadband low-frequency flexural wave attenuation can be achieved by the proposed double-sides beam-type metastructures. The tunability of flexural wave band gap and vibration suppression performance significantly enhances. In addition, the low-frequency flexural wave band gap can be effectively modulated and optimized by changing the system parameters. [ABSTRACT FROM AUTHOR]

Interferometric synthetic aperture radar (InSAR) technology can be used to produce a high spatial resolution digital elevation model (DEM) on a global scale. However, above-ground vegetation strongly biases the DEM accuracy in forested areas by shifting the scattering phase center below the top of the canopy and above the underlying topography. Such a bias is related to the radar wavelength, forest structure, dielectric property, radar incidence angle, and terrain slope. In this paper, based on the strong penetration characteristics of the low-frequency (P-band) SAR signal, the time–frequency (TF) analysis is introduced to increase the InSAR observation space, where the decomposed sublook images can also be utilized to perform interferometry. By analyzing and modeling the sublook interferograms, a comprehensive dual-baseline framework for correcting the InSAR phase shifting is proposed to generate accurate digital terrain, surface, and canopy height models (DTM, DSM, and CHM) over forest areas. The proposed method is validated using P-band InSAR datasets acquired above the boreal coniferous forest in Remningstorp, southern Sweden, and the rainforest in Lopé, Gabon, Africa. For the boreal forest, the root-mean-square errors (RMSEs) in terms of the DTM, CHM, and DSM range from 2 to 4 m, while for the tropical rainforest with complicated topography, the RMSEs of the three elevation models range from 5 to 8 m compared with light detection and ranging (LiDAR) references. The high consistency between the InSAR-derived DTM/DSM and references demonstrates the effectiveness and stability of the proposed method and represents an improvement of 49–70% compared to the raw InSAR DEM. [ABSTRACT FROM AUTHOR]

Phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6) both grossly underestimate the magnitude of low-frequency Sahel rainfall variability; but unlike CMIP5, CMIP6 mean historical precipitation does not even correlate with observed multi-decadal variability. We demarcate realms of simulated physical processes that may induce differences between these ensembles and prevent both from explaining observations. We partition all influences on simulated Sahelian precipitation variability into (1) teleconnections from sea surface temperature (SST); (2) atmospheric and (3) oceanic variability internal to the climate system; (4) the SST response to external radiative forcing; and (5) the "fast" (not mediated by SST) precipitation response to radiative forcing. In a vast improvement from previous ensembles, the mean spectral power of Sahel rainfall in CMIP6 atmosphere-only simulations is consistent with observed low-frequency variance. Low-frequency variability is dominated by teleconnections from observed global SST, and the fast response only hurts the performance of simulated precipitation. We estimate that the strength of simulated teleconnections is consistent with observations using the previously-established North Atlantic Relative Index (NARI) to approximate the role of global SST, and apply this relationship to the coupled ensembles to infer that both fail to explain low-frequency historical Sahel rainfall variability mostly because they cannot explain the observed combination of forced and internal variability in North Atlantic SST. Yet differences between CMIP5 and CMIP6 in mean Sahel precipitation and its correlation with observations do not derive from differences in NARI, but from the fast response or the role of other SST patterns. [ABSTRACT FROM AUTHOR]

Objective: The spatiotemporal gait changes in advanced Parkinson's disease (PD) remain a treatment challenge and have variable responses to L-dopa and subthalamic deep brain stimulation (STN-DBS). The purpose of this study was to determine whether low-frequency STN-DBS (LFS; 60Hz) elicits a differential response to high-frequency STN-DBS (HFS; 180Hz) in spatiotemporal gait kinematics. Methods: Advanced PD subjects with chronic STN-DBS were evaluated in both theOFF andONmedication states with LFS and HFS stimulation. Randomization of electrode contact pairs and frequency conditions was conducted. Instrumented Stand and Walk assessments were carried out for every stimulation/medication condition. LM-ANOVA was employed for analysis. Results: Twenty-two PD subjects participated in the study, with a mean age (SD) of 63.9 years. Significant interactions between frequency (both LFS and HFS) and electrode contact pairs (particularly ventrally located contacts) were observed for both spatial (foot elevation, toe-off angle, stride length) and temporal (foot speed, stance, single limb support (SLS) and foot swing) gait parameters. A synergistic effect was also demonstrated with L-dopa and both HFS and LFS for right SLS, left stance, left foot swing, right toe-off angle, and left arm range of motion. HFS produced significant improvement in trunk and lumbar range of motion compared to LFS. Conclusion: The study provides evidence of synergismof L-dopa and STN-DBS on lower limb spatial and temporal measures in advanced PD. HFS and LFS STN-DBS produced equivalent effects among all other tested lower limb gait features. HFS produced significant trunk and lumbar kinematic improvements. [ABSTRACT FROM AUTHOR]

This paper proposes a ventilation barrier for low-frequency sound insulation with a double-opening Helmholtz's periodic structure. The energy band characteristics of the unit structure and the sound transmission loss of the ventilation barrier are calculated using the finite element method. The research results show that the ventilation barrier exhibits excellent sound insulation in the range of 323–803 Hz, and the sound insulation band is consistent with the bandgap of the unit structure. In order to reveal the sound insulation mechanism, we have performed a modal analysis of the unit structure. Further, the effects of different dimensional parameters on the structural bandgap are analyzed by changing the structure. The results show that extending the length of the structure inlet or outlet can effectively reduce the start frequency of the low-frequency bandgap, thus achieving the regulation of the sound insulation band. The proposed low-frequency sound insulation ventilation barrier has a simple structure with great application prospects. In addition, the proposed ventilation barrier provides a new idea for noise control in low-frequency ventilation environments. [ABSTRACT FROM AUTHOR]

Variable-cross-section beams have better mass and strength distribution compared with constant cross-section beams, which can optimize the harvesting power of piezoelectric vibration energy harvesters, which are widely used in self-supplied and low-power electronic devices, providing more convenience and innovation for the development of micromechanical intelligence and portable mobile devices. This paper proposes a piezoelectric energy harvester with a tristable-exponential-decay cross section, which optimizes the strain distribution of the cantilever beam through exponential decay changes to improve the harvesting efficiency of the harvester in low-frequency environments. First, the nonlinear magnetic force is obtained based on the magnetic dipole, and the dynamic model is established by using the Euler–Bernoulli beam theory and Lagrangian equation. The influence of the structural parameters of the harvester on the system dynamics and output characteristics is analyzed in the two dimensions of time and frequency, and the influence of the exponential decay coefficient on the system dynamic response and output power is deeply studied. The research shows that the exponential decay section can reduce the first natural frequency of the cantilever beam; by changing the amplitude, frequency, d and dg of the excitation acceleration, the switching of the monostable, tristable and bistable states of the system can be realized. With a decrease in the exponential decay coefficient, under a low-frequency excitation of 0–7 Hz, the output power of the cantilever beam per unit volume is significantly improved, and under a 4 Hz low-frequency excitation, the acquisition output power per unit volume of the cantilever beam is increased by 7 times, thus realizing low-frequency, high-efficiency energy harvesting. [ABSTRACT FROM AUTHOR]

The advantage of mechanical metamaterials is that the properties can be improved at the macro level through reasonable artificial design at the micro level. Mechanical metamaterials are applied to solve the problems of vibration and noise in engineering applications. In this paper, a creative chiral star-shaped compositive mechanical metamaterial (CSCMM) is proposed by combining the chiral lattice structure and star-shaped honeycomb. The generation mechanism of the bandgap and the impaction of structural parameters are studied to obtain the low-frequency bandgap by single-phase materials. On this basis, the influence of star angle on bandgap generation and the variation of structure vibration mode during bandgap generation are studied. In addition, the phase and group velocities of elastic waves in chiral star-shaped compositive mechanical metamaterial are studied. Finally, the frequency response function (FRF) of the elastic wave propagating in the structure is calculated, and the result is consistent with the band diagram. Therefore, the design method of compositive mechanical metamaterial can effectively realize the improvement of property and reduce the attenuation of elastic waves, and it provides a new design idea for the vibration and noise reduction design of metamaterial. [ABSTRACT FROM AUTHOR]

The effect of high contrast on the multiscale behaviour of elastic laminates is studied. Mathematical modelling in this area is of significant interest for a variety of modern applications, including but not limited to advanced sandwich structures and photovoltaic panels. As an example, the antiplane shear of a symmetric, three-layered plate is considered. The problem parameters expressing relative thickness, stiffness and density are assumed to be independent. The high contrast may generally support extra length and time scales corresponding to degenerated boundary layers and propagating long-wave low-frequency vibration modes. The main focus is on the relation between these two phenomena. The developed multiparametric approach demonstrates that those do not always appear simultaneously. The associated explicit estimates on contrast parameters are established. In addition, the recent asymptotic extension of the classical Saint-Venant's principle is adapted for calculating the contribution of the degenerate boundary layer or long-wave low-frequency propagation mode. The peculiarity of the limiting absorption principle in application to layered media is also addressed. [ABSTRACT FROM AUTHOR]

A high-precision equipment is very sensitive to vibration, even micro vibration. How to isolate the low-frequency broadband multidirectional vibration remains a challenge. As the main component of piezoelectric smart vibration isolation model, the piezoelectric stack consists of a piezoelectric actuator, a piezoelectric sensor, and a rubber layer. The numerical results obtained by the finite element method agree well with theoretical solutions. The vibration attenuates rapidly under displacement and velocity feedback control with negative gains. As the control gain decreases, the vibration isolation band is extended to lower frequency. A piezoelectric smart multidirectional vibration isolation platform model is further proposed by inclined installation of two piezoelectric stacks. A spring-like structure is designed to exert a preload pressure on these piezoelectric stacks. After optimization on the control gain, the platform can isolate vibration from 0 to 3000 Hz in multiple directions. [ABSTRACT FROM AUTHOR]

Methods of computer technology are widely used in modern acoustics. They are used to visualize the structures of wave fields, when the geometry of the boundaries and the physical properties of the region where the waves propagate are known. When using these methods, the location of the emitters and receivers is usually controlled. At present, known methods based on the use of information recorded only by pressure transducers have reached the limit. Vector-phase characteristics methods have previously been used to study the spatial distribution of the main characteristics of sound and infrasonic fields. But to study the structure of the acoustic field in the speakers of finite length, they have not been used so far. This is the relevance of the chosen topic. In this work the research of acoustic characteristics of a catenoid loudspeaker at low frequencies and a vector - phase field of a loudspeaker by numerical methods is given. The following tasks are solved: compiling an analytical review of research methods of sound fields; description of the mathematical apparatus of wave propagation in the catenoid mouthpiece; simulation of acoustic characteristics in the vector - phase field of the loudspeaker at low frequencies in ANSYS. In loudspeakers, nonlinear distortions associated with the heterogeneity of the magnetic field in the gap of the magnetic circuit and, in violation of Hooke's law, are not so significant. The defining moment in the development of the theory of wave radiation by loudspeakers was the publication of Webster's work, in which the wave equation is given. Although the membrane performs a simple harmonic oscillation. The problem of amplifying the action and direction of the acoustic signal was solved, during which a catenoid horn was chosen. The propagation of the wave is no longer sinusoidal, and contains in addition to the main harmonics of higher orders. That is, nonlinear effects inside the speaker are detected. Previously, the design of sound generators did not provide structural elements that reduce or eliminate completely nonlinear effects in the speaker, which reduced the efficiency of generating the sound of the fundamental tone. The solution of the problem of modeling the process of propagation of the second harmonic along the horn is considered. In order to calculate the acoustic characteristics in the cavity of the catenoid loudspeaker, it is necessary in ANSYS CAD. [ABSTRACT FROM AUTHOR]

This study evaluated a synergistic antimicrobial treatment using a combination of low frequency and a low-intensity ultrasound (LFU) and a food-grade antioxidant, propyl gallate (PG), against a model gram-positive (Listeria innocua) and the gram-negative bacteria (Escherichia coli O157:H7). Bacterial inactivation kinetic measurements were complemented by characterization of biophysical changes in liposomes, changes in bacterial membrane permeability, morphological changes in bacterial cells, and intracellular oxidative stress upon treatment with PG, LFU, and a combination of PG + LFU. Combination of PG + LFU significantly (>4 log CFU/mL, P

This study evaluated a synergistic antimicrobial treatment using a combination of low frequency and a low-intensity ultrasound (LFU) and a food-grade antioxidant, propyl gallate (PG), against a model gram-positive (Listeria innocua) and the gram-negative bacteria (Escherichia coli O157:H7). Bacterial inactivation kinetic measurements were complemented by characterization of biophysical changes in liposomes, changes in bacterial membrane permeability, morphological changes in bacterial cells, and intracellular oxidative stress upon treatment with PG, LFU, and a combination of PG + LFU. Combination of PG + LFU significantly (>4 log CFU/mL, P

The three-dimensional (3D) low-frequency lightning mapping system (LF-LMS) in north China has been updated. The lightning electric field derivative (dE/dt) sensor and continuous acquisition mode has been newly designed to ensure a capability of entire lightning processes detection, especially weak discharges during lightning the initiation process. The twice cross-correlation delay estimation and the grid iteration nested optimization location algorithm are used to realize the 3D location of the discharge channel, and the location resolution and calculation speed are balanced consequently. The location results of the rocket-triggered lightning demonstrated that the system achieved a high-resolution mapping of lightning discharge channels, which coincided well with the optical images. The horizontal and vertical location error for rocket triggered lightning was less than 40 m in both horizontal and vertical. Intracloud (IC) lightning flashes were observed to be initiated by three different discharge processes, initial breakdown pulse (IBP), narrow bipolar event (NBE), and initial E-change (IEC). The corresponding initial height was 10.5 km, 6.9 km, and 9.2 km, respectively. The upward negative leader was initially located, followed by scatter radiation sources and negative recoil leaders in the lower negative charge region for all cases. The electric field characteristics of the IEC and subsequent IBPs indicated that they are different discharge processes with the same current direction. The IEC process might correspond to the discharge process with continuous current and less noticeable current changes.

ObjectiveThe spatiotemporal gait changes in advanced Parkinson's disease (PD) remain a treatment challenge and have variable responses to L-dopa and subthalamic deep brain stimulation (STN-DBS). The purpose of this study was to determine whether low-frequency STN-DBS (LFS; 60 Hz) elicits a differential response to high-frequency STN-DBS (HFS; 180 Hz) in spatiotemporal gait kinematics.MethodsAdvanced PD subjects with chronic STN-DBS were evaluated in both the OFF and ON medication states with LFS and HFS stimulation. Randomization of electrode contact pairs and frequency conditions was conducted. Instrumented Stand and Walk assessments were carried out for every stimulation/medication condition. LM-ANOVA was employed for analysis.ResultsTwenty-two PD subjects participated in the study, with a mean age (SD) of 63.9 years. Significant interactions between frequency (both LFS and HFS) and electrode contact pairs (particularly ventrally located contacts) were observed for both spatial (foot elevation, toe-off angle, stride length) and temporal (foot speed, stance, single limb support (SLS) and foot swing) gait parameters. A synergistic effect was also demonstrated with L-dopa and both HFS and LFS for right SLS, left stance, left foot swing, right toe-off angle, and left arm range of motion. HFS produced significant improvement in trunk and lumbar range of motion compared to LFS.ConclusionThe study provides evidence of synergism of L-dopa and STN-DBS on lower limb spatial and temporal measures in advanced PD. HFS and LFS STN-DBS produced equivalent effects among all other tested lower limb gait features. HFS produced significant trunk and lumbar kinematic improvements.

[Display omitted] Mechanical energy harvesting transforms various forms of mechanical energy, including ocean waves, wind, and human motions, into electrical energy, providing a viable solution to address the depletion of fossil fuels and environmental problems. However, one major obstacle for the direct conversion of mechanical energy into electricity is the low frequency of the majority of mechanical energy sources (≤5 Hz), resulting in low energy conversion efficiency, output power and output current. Over recent years, a numerous innovative technologies have been reported to enable improved energy harvesting utilizing various mechanisms. This review aims to present an in-depth analysis of the research progress in low-frequency energy harvesting technologies that rely on triboelectric, electrochemical, piezoelectric, and dielectric elastomer effects. The discussion commences with an overview of the difficulties associated with low-frequency energy harvesting. The critical aspects that impact the low-frequency performance of mechanical energy harvesters, including working mechanisms, environmental factors, and device compositions, are elucidated, while the advantages and disadvantages of different mechanisms in low-frequency operation are compared and summarized. Moreover, this review expounds on the strategies that can improve the low-frequency energy harvesting performance through the modulations of material compositions, structures, and devices. It also showcases the applications of mechanical energy harvesters in energy harvesting via waves, wind, and human motions. Finally, the recommended choices of mechanical energy harvesters with different mechanisms for various applications are offered, which can assist in the design and fabrication process. [ABSTRACT FROM AUTHOR]

Aiming at the insulation of low-frequency acoustic sound, a 100 mm crescent disc asymmetric membrane-type acoustic metamaterial structure was designed in this paper, which was composed of aluminum material as the frame and iron material as the mass attached to the surface of flexible ethylene-vinyl acetate copolymer film. The finite element method was adopted to calculate its transmission loss and displacement field. The asymmetric structure, the structure parameters and the mass block's eccentricity together with the vibrational modes analysis were investigated in this study for a better sound insulation performance. The results show that, compared with the symmetric membrane-type acoustic metamaterials, the design of the asymmetry in a single cell makes the low-frequency sound insulation band widened by 23 Hz. Meanwhile, more vibrational modes are generated which illustrates that the coexistence of Lorentz resonance and Fano resonance promotes a better sound insulation performance of the crescent disc asymmetric structure. The large-size asymmetric membrane-type acoustic metamaterial structure designed in this paper can reduce the sound insulation frequency to 10 Hz with a wide low-frequency sound insulation performance within 10 ~ 500 Hz. It provides a new method for improving the low-frequency sound insulation effect of sound barriers in terms of structural optimization design. [ABSTRACT FROM AUTHOR]