Low-frequency Performance Research Articles

Designing hybrid aerogel-3D printed absorbers for simultaneous low frequency and broadband noise control

Recent studies have demonstrated that granular aerogels with sub-50 μm particles surpass conventional acoustic materials like glass fibers and polyurethane foams in low-frequency sound absorption. However, incorporating such granular materials within practical structural solutions remains a challenge. In this study, we use additive manufacturing to overcome this challenge by designing a modular geometry that allows us to encapsulate such granular materials within a 3D printed scaffold. Using the 3D printed porous scaffold provides the added benefits of tunability and durability. Further, we introduce a design methodology and software tool to facilitate the efficient design of such hybrid sound absorbers. The proposed method uses the Johnson-Champoux-Allard model, Biot theory, and the transfer matrix method to model the acoustical behavior of hybrid absorber designs that use layered granular aerogels and 3D printed bulk absorbers. The tool can also be used to inversely characterize the acoustic bulk properties of such materials. We validate the design concept and tool by comparing the model predictions with experimental measurements. Finally, we outline strategies for designing hybrid absorbers that can provide application specific low-frequency and broadband absorption performance within specific frequency ranges, while considering engineering constraints on their total mass and depth.

Design and experiment of electromagnetic-hydraulic-rubber integrated vibration isolator

Low-frequency radiated noise, characterized by a distinctive “acoustic fingerprint” is generated when vibrations from marine machinery propagate through a ship’s hull into the ocean. This type of noise travels long distances with concentrated and stable energy, posing a significant threat to a ship’s acoustic stealth capabilities. Active-passive hybrid isolation is the primary method for reducing low-frequency radiated noise from ships; however, technical challenges remain, such as effectively integrating active and passive components, achieving high output force in a compact design, and addressing the poor linearity of actuator output at low frequencies. To address these issues, this paper presented an innovative electromagnetic-hydraulic-rubber integrated vibration isolator. Firstly, the study analyzed the dynamic characteristics of a two-degree-of-freedom isolation system and investigated the influence of active-passive hybrid vibration isolator parameters on control force and vibration reduction performance. Secondly, it was established that the magnetic circuit model and magnetic field strength expressions for the electromagnetic actuator using magnetic circuit analysis derive the analytical relationship between electromagnetic force, current amplitude, and frequency using the energy method. Subsequently, a mathematical model was developed for the rubber-hydraulic suspension component to examine its dynamic characteristics, hydraulic damping, and hydraulic force amplification transmission laws. Lastly, we organically combine the electromagnetic actuator with the rubber-hydraulic suspension and conduct a multi-physics joint simulation of the integrated vibration isolator using Comsol software to verify the effectiveness of the optimized design and vibration isolation control. Experimental research was carried out on the dynamic characteristics, output force properties, and fatigue characteristics of the integrated vibration isolator prototype. Results indicated that the established models and methods can achieve over 90% accuracy in predicting the performance of the electromagnetic actuator’s output force and exhibit good linearity within the 5–400 Hz range. The rubber-hydraulic suspension can achieve an amplification factor of up to 1.5 for the electromagnetic force while reducing the transmission of vibrations to the base. The research findings can enhance the low-frequency vibration isolation performance of marine machinery equipment and improve their acoustic stealth capabilities.

A nonlinear low frequency quasi zero stiffness vibration isolator using double-arc flexible beams

There is an increasing demand for integrated small vibration isolators in precision sensing instruments and medical equipment. Therefore, a quasi-zero stiffness (QZS) vibration isolator with double-arc flexible beams (D-AFB) is proposed. The QZS isolator consists of D-AFB units, the base, platform, fixture, guide rail, and linear bearing. The static performance of D-AFB is analyzed using the FEA method, and the QZS condition is obtained. The harmonic balance method (HBM) and the pseudo arc length method (PALM) are used to solve the dynamic equations of the nonlinear system, and the analytical solutions of displacement transmissibility and Force transmissibility are derived. In theory, the initial frequency is 77.6 % lower than that of the equivalent linear system. The QZS characteristic of D-AFB is verified by static experiment, and the low-frequency isolation performance of the QZS isolator is verified by dynamic experiment under constant amplitude and constant acceleration excitation. The D-AFB has a simple structure and can easily satisfy QZS conditions. So the D-AFB can be customized according to load requirements. Multiple vibration isolation loads can be obtained by adjusting the assembly of the D-AFB. The QZS isolator can adapt to load changes within 10 % and has an isolation effect on vibrations above 4 Hz. The idea of designing a QZS isolator with D-AFB as an array unit has a good research prospect and great application value in meeting the requirements of miniaturization and integration.

Origami-inspire quasi-zero stiffness structure for flexible low-frequency vibration isolation

As an emerging technology, origami exhibits rich nonlinear static and dynamic properties. However, simulating creases remains a key challenge hindering practice application, especially for volumetric (deformable) origami. Inspired by the bistable behavior and axial-rotational coupling mechanism of triangulated cylindrical origami (TCO), this paper proposes a novel combined truss-spring structure with additional inertial mechanism (TCTS-RIM) for low-frequency vibration isolation. Unlike pure TCO, the TCTS-RIM uses elastic linkages to simulate all creases and modifies crease positions, enhancing physical realization in engineering applications and offering tunable stiffness and inertia on demand. Static analysis shows the modified truss-spring structure in TCTS-RIM has superior quasi-linear negative stiffness compared to pure TCO, which enables a very wide quasi-zero stiffness (QZS) range via linear spring compensation. The wide QZS range for different loads can be realized by setting appropriate structural parameters. From system dynamic equations, nonlinear inertia, nonlinear quadratic force, and nonlinear damping due to RIM are identified and discussed. The Alternating frequency–time harmonic balance method is used to obtain the displacement transmissibility, and its parametric influencing investigations verify that the proposed TCTS-RIM has excellent low-frequency vibration isolation performance and rich tunability. Comparative discussions show the wide QZS range significantly reduces the resonance frequency, broadens the isolation frequency band, and yields superior dynamic performance versus pure TCO isolators. Moreover, equivalent mass, anti-resonance and softening behavior induced by the rotational inertia mechanism further enhance low-frequency vibration isolation performance. An experimental prototype is built, and experimental results validate the correctness of the theoretical analyses. This work presents a simple mechanical implementation of volumetric origami, which may progress the engineering applications of origami and provide a viable approach to low-frequency vibration control.

Optimal Design and Performance Evaluation of HMDV Inertial Suspension Based on Generalized Skyhook Control

The hub motor driven vehicle (HMDV), due to the increase in unsprung mass and the influence of uneven electromagnetic radial forces from road excitations, results in the deterioration of body acceleration, significantly impacting ride comfort. This seriously impacts ride comfort. However, the inerter proves effective in reducing low-frequency vibrations in body acceleration, and skyhook control significantly enhances ride comfort. To fully exploit the benefits of the inerter, comprehensively elevate skyhook control performance, and effectively counteract the worsening of body acceleration caused by the hub motor, this study integrates skyhook control with an inertial suspension system. This integration markedly improves the suspension’s low-frequency isolation performance, thereby substantially enhancing vehicle ride comfort. Firstly, establish a quarter-vehicle dynamics suspension model based on the Switched Reluctance Motor (SRM) driven wheel motor system. Subsequently, confirm the constraints of two different generalized skyhook inertial suspension structures separately and optimize their parameters to determine the most suitable parameters for each structure. Finally, analyze the impact of these two suspension types on body acceleration to derive the optimal structure. Simulation results demonstrate that, compared to traditional suspensions, the optimized generalized skyhook inertial suspensions reduce the Root Mean Square (RMS) values of body acceleration and suspension working space by 18.8% and 22%, respectively. This reduction is particularly significant in the 0[Formula: see text]2[Formula: see text]Hz frequency range, effectively enhancing ride comfort. It also adequately validates the effective utilization of the advantages of inertial suspensions and skyhook control.

Performance optimization of a SERF atomic magnetometer based on flat-top light beam

We explore the impact of pumping beams with different transverse intensity profiles on the performance of the spin-exchange relaxation-free (SERF) atomic magnetometers (AMs). We conduct experiments comparing the traditional Gaussian optically-pumped AM with that utilizing the flat-top optically-pumped (FTOP) method. Our findings reveal that the FTOP-based approach outperforms the conventional method, exhibiting a larger response, a narrower magnetic resonance linewidth, and a superior low-frequency noise performance. Specifically, the use of FTOP method leads to a 16% enhancement in average sensitivity within 1 Hz–30 Hz frequency range. Our research emphasizes the significance of achieving transverse polarization uniformity in AMs, providing insights for future optimization efforts and sensitivity improvements in miniaturized magnetometers.

Low-frequency acoustic isolation performance of Helmholtz cavity acoustic metamaterials with bilateral single openings

Low-frequency acoustic isolation performance of Helmholtz cavity acoustic metamaterials with bilateral single openings

An investigation on synergistic resonances of membrane-type acoustic metamaterial with multiple masses

Membrane-type acoustic metamaterials (MAM) represent a class of materials that are lightweight, compact, structurally simple, and simultaneously offer flexibility and moldability, all while demonstrating exceptional sound attenuation properties in the low-frequency range. As a continuation, this study focuses on membrane-based acoustic metamaterials with synergistic resonance of multiple loaded mass blocks. By varying the unit shape, arrangement method, radius and material of the loaded mass blocks, the low-frequency sound insulation performance of membrane-based acoustic metamaterials with synergistic resonance of multiple loaded mass blocks was calculated using a numerical model that are experimentally validated. The results indicate that the peak transmiss loss (TL) frequencies can be tuned to specific values. The optimized membrane-type acoustic metamaterial could achieve a TL bandwidth up to 844 Hz (>5dB) and a TL bandwidth up to 376 Hz (>10 dB) within the low-frequency range. Acoustic-membrane analysis further revealed the vibration patterns of the structure under different frequency excitations, providing profound physical insights into the mechanisms of low-frequency sound insulation. Moreover, by altering the relative angles between the loaded mass blocks could further broaden the effective noise reduction bandwidth which can benefit the understanding on the optimized membrane-acoustic interactions.

Vibration and acoustic characteristics of acoustic black hole plates with variable elastic modulus

Acoustic black hole (ABH), as a new wave manipulation technique, shows excellent applications in vibration and noise reduction of structures. Nowadays, most ABHs use materials with a fixed elastic modulus, limiting their low-frequency performance. Herein ABH plates with variable elastic modulus (VM-ABH) is designed, and its vibration and acoustic radiation characteristics are investigated by using numerical analysis. The results show that the vibration response of VM-ABH has a decrease of 5–13.2[Formula: see text]dB relative to that of the uniform texture ABH (UT-ABH) in the frequency range of 10–5000[Formula: see text]Hz, and the degree of energy aggregation is significantly improved. Moreover, the sound pressure level was reduced by 3.6 dB. Meanwhile, by linearly varying the elastic modulus in the center region of the VM-ABH, the effects of gradient index and terminal elastic modulus on the damping characteristics and dynamic response are revealed. The research results provide new objects for the study of vibration and noise reduction of ABH.

Low-Frequency Sound Absorption Potential of Subwavelength Absorbers Based on Coupled Micro-Slit Panels

Due to space limitations during installation, reducing low-frequency noise has always been a challenging area. Sub-wavelength structures are typically favored in such scenarios for noise reduction. This paper explores the potential of micro-slit panels (MSP) for low-frequency sound absorption. To further optimize the panel thickness, coupled MSPs (CMSP) with a distance between two MSPs of less than 1 mm are proposed. Firstly, the low-frequency absorption performances of a single MSP based on two optimized schemes – the cavity-depth optimal scheme (COS) and the panel thickness optimal scheme (TOS) – are examined and compared with those of existing ultrathin metamaterials. The results demonstrate that MSP has significant potential for low frequency sound absorption, and COS allows for a smaller overall structural thickness but a larger panel thickness than TOS. Secondly, to reduce the panel thickness, the CMSP is developed and the theoretical model of its acoustic impedance is established and validated by experiments. Then, based on the theoretical model, the low-frequency absorption potential of CMSP is optimized using COS. The results show that both the overall thickness and the panel thickness of the CMSP absorber are reduced while maintaining better performance. Furthermore, the proposed absorber achieves a subwavelength scale since its total thickness can be as small as 0.138λ.

Low-frequency forbidden band characteristics of metamaterials based on combined resonant units

With a view to increasing the width of the local resonant band gaps and obtaining damping at low-frequency, a metamaterial structure with multiple resonant units and an adjustable band gap is proposed. The single-unit structure of the metamaterial consists of a main system and three resonant units. Simultaneously, the energy band structure and frequency response characteristics of the metamaterial are investigated with the finite element method and the dispersion equation. Further, the effects of natural frequency, mass of resonant units, mass ratio, and stiffness ratio on the energy band structure are demonstrated. Also, the effect of damping on the band gap in the metamaterial structure is examined. The low-frequency vibration isolation performance of the structure is verified on experimental platform. In conclusion, the designed metamaterial structure provides three adjustable band gaps in the low-frequency range with good vibration damping.

Research progress of piezoelectric polymer PVDF in sound absorption technology

Owing to its impedance properties and piezoelectric effect, the piezoelectric polymer polyvinylidene fluoride (PVDF) finds application in the field of noise control. Traditional sound-absorbing materials have gradually fallen short of the demands of sound-absorbing functionality and environmental adaptability in today’s increasingly acute noise pollution environment. When PVDF is added to sound absorption, materials or structures have an additional loss path for the conversion of sound energy to electric energy. This helps to increase the low-frequency sound absorption band and improve low-frequency sound absorption performance. Through the action of viscosity, friction, relaxation, piezoelectric effect, and dielectric loss, the sound energy in the passive sound-absorbing material containing PVDF is transformed into energy dissipation in the form of heat energy, electric energy, and so on under the matching of PVDF low impedance. In active control, secondary sound and force sources can be supplied by PVDF-based transducers and actuators to reduce noise. This paper first provides an overview of the piezoelectric characteristics of β-phase PVDF and its optimization techniques. It then goes on to discuss the use of PVDF in passive sound-absorbing materials, active controls, and hybrid sound absorption systems. Finally, it summarizes the benefits, issues that still need to be resolved, and future directions for these three noise reduction techniques.

Identifying heavy stellar black holes at cosmological distances with next-generation gravitational-wave observatories

ABSTRACT We investigate the detectability of single-event coalescing black hole binaries with total mass of $100\!-\!600{\, {\rm {M}}_{\odot }}$ at cosmological distances (5 ≲ z ≲ 20) with the next generation of terrestrial gravitational wave observatories, specifically Einstein Telescope and Cosmic Explorer. Our ability to observe these binaries is limited by the low-frequency performance of the detectors. Higher order multipoles of the gravitational wave signal are observable in these systems, and detection of such multipoles serves to both extend the mass range over which black hole binaries are observable and improve the recovery of their individual masses and redshift. For high-redshift systems of $\sim 200 {\, {\rm {M}}_{\odot }}$ we will be able to confidently infer that the redshift is at least z = 12, and for systems of $\sim 400 {\, {\rm {M}}_{\odot }}$ we can infer a minimum redshift of at least z = 8. We discuss the impact that these observations will have in narrowing uncertainties on the existence of the pair-instability mass gap, and their implications on the formation of the first stellar black holes that could be seeds for the growth of supermassive black holes powering high-z quasars.

Characterization of DC performance and low-frequency noise of an array of nMOS Forksheets from 300 K to 4 K

The DC and low-frequency noise performance of an array of 800 parallel Forksheet MOSFETs were investigated by performing measurements over a wide temperature range from 300 K to 4 K. The array structure allowed to measure a representative average performance of the devices and provided a large effective area for 1/f noise analysis. Results showed an improvement in the saturation drain current when going from room temperature to cryogenic temperatures, with the subthreshold swing saturating around 100 K and the threshold voltage shifting by approximately 150 mV, following similar trends observed in Silicon cryogenic electronics. Additionally, the study confirms that the noise at cryogenic temperatures does not follow the commonly assumed linear scaling with temperature. This deviation from the linear scaling has been associated with the presence of tail states at the interface in bulk and silicon-on-insulator (SOI) devices. These results suggest that the excess 1/f noise in this advanced device architecture is not related to the device architecture but rather to the microscopic material properties of semiconductor/dielectric interfaces.

A high-performance infrared photoacoustic sensor with improved low-frequency response microphone

Infrared photoacoustic gas sensors with the advantages of good selectivity, low interference, fast response speed, good stability, high accuracy, good explosion-proof performance, high signal-to-noise ratio, and long service life, have developed rapidly in recent years. In order to solve the problem of poor low-frequency response performance of the MEMS piezoelectric thin film microphone sensor prepared with AlN and Mo thin films in infrared photoacoustic gas sensors, this article analyzes its principle and tests an improved solution. The internal stress distribution was changed through hot tempering experiments, significantly improving its structure. The article qualitatively and quantitatively analyzes the inherent mechanism of this phenomenon. The experimental results confirm that the improved scheme improves its response performance under low-frequency signals, with good stability and repeatability.

Low-frequency bandgap characteristics and vibration attenuation performance of metamaterial-tailored concrete-filled steel tube columns

Low-frequency vibrations, such as earthquakes and wind loads, may negatively affect the proper operation of buildings and bridges and even lead to structural failure. Even though, in the past two decades, metamaterial-based structures have been utilized in structure vibration suppression, they are mainly used in the mitigation of high-frequency vibrations, and it has been an open challenge to suppress low-frequency vibrations below 100Hz and ensure adequate bearing capacity in engineering structures. In this work, we present metamaterial composite column structures with built-in local resonators to achieve the low-frequency vibration mitigation effect. Compared to ordinary concrete-filled steel tube (CFST) columns, the metamaterial columns (meta-columns) can utilize the rubber shear modulus to create a low-frequency flexural bandgap in 30–50 Hz. Dynamic stiffness is introduced in the Timoshenko beam and Bloch's theorem, the metamaterial analytical model is established, and the bandgap generation mechanism and vibration reduction effect are analyzed in conjunction with numerical simulations. Then, the bandgap effect and vibration attenuation law of the meta-columns are investigated by shaker tests at different acceleration amplitudes. Guided by theory and simulations, we have successfully fabricated several meta-composite columns with low-frequency bandgaps, which attenuate up to half of the vibrational energy near the bandgap center. The bandgap ranges derived from the theoretical and numerical models show a high degree of consistency with the experimental results, with deviations generally within 10 %, all indicating that a bandgap exists near the resonator self-oscillation frequency. Additionally, the bandgap is significantly affected by the number of unit cells and resonators, especially the meta-column with fewer resonators fails to form a bandgap where the top response is smaller than the bottom response. As the number of resonators and metamaterial units increases, the meta-columns exhibit wider bandgap and stronger vibration reduction performance.

Proportional-retarded control of a quasi-zero-stiffness vibration isolator

This paper explores the control design and performance analysis of an active quasi-zero-stiffness vibration isolator utilizing the proportional-retarded (PR) control architecture. The PR architecture replaces the conventional “derivative” term in the proportional-derivative (or displacement-velocity) controller with the novel “retarded (time-delayed)” term, which is a new and effective way of active vibration suppression. Comprehensive gain-delay conjoint regulation strategies are developed for the strengthened isolation performance. For impact excitations, a complete stability analysis is conducted near the equilibrium position. Furthermore, a parameterized dominant pole placement algorithm is proposed to optimize the transient response of the PR-actuated isolator. Focusing on the rectangular pulse and long-duration step impacts, the algorithm adjusts the dominant poles to optimize the transient performance metrics, including the maximum displacement overshoot, steady-state displacement overshoot, and settling time. Regarding harmonic excitations, the delay-coupled effect is scrutinized for better low-frequency isolation performance, including the resonance frequency, nonlinear steady-state amplitude, multiple steady-state bands and nonlinearity elimination, equivalent active damping and stiffness, and absolute displacement transmissibility. In the case of random excitations, we demonstrate the superiority of the PR structure over the passive system.

Inverse design of laminated plate-type acoustic metamaterials for sound insulation based on deep learning

The laminated plate-type acoustic metamaterial (LPAM) has designable low-frequency sound insulation performance and ultrathin and ultralight characteristics. However, the complexity of LPAM design increases significantly as the number of layers increases. An inverse design method is proposed to determine the topology and design parameters of LPAM based on deep learning. To train the deep learning model, the finite element and analytic models are combined to generate sufficient training data. The experimental validation of the finite element model and the acoustic impedance model are carried out to reduce the deviation between the experiments and simulations. The LPAM design system contains the pre-processing, inverse design and post-processing modules. The pre-processing module can produce candidate targets based on the required sound-insulation design target. For each candidate target, the inverse design module utilizes convolutional neural networks to autonomously design topology and parameters of LPAM. Finally, the pre-processing and post-processing modules are combined to choose the optional LPAMs meeting the sound-insulation target. To verify effectiveness of the LPAM design system, typical cases, involving the sound insulation targets of one and two sound insulation peaks, and uniform sound insulation in a broadband from 50 Hz to 600 Hz, are designed successfully and efficiently.

Tunable Ultralow-Frequency Bandgaps Based on Locally Resonant Plate with Quasi-Zero-Stiffness Resonators

In order to suppress the transverse vibration of a plate, a quasi-zero-stiffness (QZS) resonator with tunable ultralow frequency bandgaps was introduced and analyzed. The resonator was designed by introducing the quasi-zero-stiffness systems into mass-in-mass resonators. The plane wave expansion method was employed to derive the bandgap characteristics of the locally resonant (LR) plate with QZS resonators, and corresponding simulations were carried out by finite element method (FEM). The results show that an LR plate with a QZS resonator can provide two bandgaps, and the ranges of the bandgaps agree well with the vibration attenuation bands calculated by FEM. Owing to the introduction of the QZS system, the bandgaps can be easily transferred to a lower frequency or even an ultralow frequency. The damping of the QZS resonators can effectively broaden the vibration attenuation bands. In addition, the differentiated design of the bandgap frequencies can be realized to obtain broadband low-frequency transverse wave suppression performance. Finally, a mechanical structure design scheme was proposed in order to achieve flexible adjustment of the bandgap frequency, which significantly increases the engineering applicability of QZS resonators.

A Lightweight Method for Detecting and Correcting Errors in Low-Frequency Measurements for In-Orbit Demonstrators.

In the pursuit of enhancing the technological maturity of innovative magnetic sensing techniques, opportunities presented by in-orbit platforms (IOD/IOV experiments) provide a means to evaluate their in-flight capabilities. The Magnetic Experiments for the Laser Interferometer Space Antenna (MELISA) represent a set of in-flight demonstrators designed to characterize the low-frequency noise performance of a magnetic measurement system within a challenging space environment. In Low Earth Orbit (LEO) satellites, electronic circuits are exposed to high levels of radiation coming from energetic particles trapped by the Earth's magnetic field, solar flares, and galactic cosmic rays. A significant effect is the accidental bit-flipping in memory registers. This work presents an analysis of memory data redundancy resources using auxiliary second flash memory and exposes recovery options to retain critical data utilizing a duplicated data structure. A new and lightweight technique, CCM (Cross-Checking and Mirroring), is proposed to verify the proper performance of these techniques. Four alternative algorithms included in the original version of the MELISA software (Version v0.0) are presented. All the versions have been validated and evaluated according to various merit indicators. The evaluations showed similar performances for the proposed techniques, and they are valid for situations in which the flash memory suffers from more than one bit-flip. The overhead due to the introduction of additional instructions to the main code is negligible, even in the target experiment based on an 8-bit microcontroller.