Low-frequency Noise Reduction Research Articles

Novel bow-shaped local resonance acoustic metamaterials with ultra-wide low-frequency stopband: design, modeling, and testing

This paper proposes a novel bow-spring local resonance (LR) structure featuring an exceptionally wide low-frequency stopband. Unlike traditional methods reliant on heavy mass or stiffness adjustments, this structure effectively manipulates and amplifies the dynamic characteristics of negative stiffness solely by designing parameter values for the bow-spring set. Through finite element method analysis, an ultra-wide stopband ranging from 91 to 570 Hz is achieved within the LR structure. Further modification of the connection pattern with a perforated plate extends the upper edge to 686 Hz while reducing the lower edge to 76 Hz. Most notably, within the novel bow-spring LR structure, a stopband width of 610 Hz is attained, resulting in a gap-mid gap ratio of 160.1%. The numerical and experimental results demonstrate good agreement. These findings offer a new perspective and guidelines for developing LR structures with ultra-wide low-frequency stopbands, potentially finding applications in the field of low-frequency vibration and noise reduction.

Reverse optimization design of OAM sound barrier based on acoustic metamaterials

An optimized acoustic metamaterial (OAM) railway sound barrier has been designed to address the growing noise problem in railway construction. The low-frequency broadband noise attenuation is realized by an reverse design method relying on genetic algorithm-neural network (GA-NN). Simulation results show that the OAM can perfectly cover the target frequency band from 400 Hz to 1000 Hz with a bandgap ranging from 210 Hz to 1253 Hz. Compared with the initial acoustic metamaterial (IAM), the bandgap of the OAM is widened by 1295 Hz, and the sound transmission loss is improved by 15 dB. The noise reduction performance of the OAM is verified by the experiments of the fully-enclosed impedance tube and the simulation of the acoustic field modes. The acoustic experiments of the semi-enclosed OAM sound barriers show that the noise reduction of OAM sound barriers has a low-frequency broadband noise reduction effect. Compared with the existing sound barriers, the average noise reductions of OAM sound barriers in the target frequency band and bandgap range are improved to 15.9 dB and 7.8 dB, respectively.

Simulation and Experimental Study of the Suppression of Low-Frequency Flow Noise Signals by a Placoid-Scale Skin

This paper addresses the challenge of mitigating low-frequency flow noise signals in autonomous underwater vehicles through the optimization of a placoid-scale skin. Drawing inspiration from the bio-inspired surface features of cylindrical shell structures, an enhanced design of placoid-scale skin is developed using 3D printing technology. This improved structure effectively reduced boundary layer vortices and wake intensity, thereby contributing to the suppression of low-frequency flow noise signals. Experimental results demonstrate that the notable reduction in low-frequency flow noise within the frequency range of 0–500 Hz, with average noise reduction of approximately 5 dB observed at 150 Hz. This reduction is validated by a combination of numerical simulations and experimental testing, confirming the efficacy of the optimized placoid-scale skin in attenuating the low-frequency flow noise associated with uniformly advancing turbulent boundary layers underwater.

Ultra-thin ventilated metasurface pipeline coating for broadband noise reduction✰

This paper proposes a method of designing a metasurface muffling coating for pipeline ventilation and noise reduction, which realises low-frequency broadband noise reduction by laying ultra-thin labyrinth-channel acoustic absorbing metasurface structures on the inner wall surface of the pipeline. With the advantage of relatively loose size constraints in the length direction of the fluid conveying pipeline, the thickness dimensions required for low-frequency broadband sound absorption are transferred to the length direction, which significantly increases the effective ventilation area, and the ventilatable area is as high as 55.5 % of the overall structural section area. Since sound waves can be gradually attenuated in the pipeline as the fluid medium flows along the length of the pipeline rather than needing to be attenuated at the same time as in a flat plate acoustic structure, the performance requirements of the acoustic structure are substantially reduced, enabling a significant increase in the operating bandwidth. We have tried the arrangement method of coating layers with gradient thickness, which shows that the combination of different thicknesses of metasurface coating layers can broaden the muffling frequency band and improve the transmission loss of the muffling structure. The design is equally suitable for the noise reduction of liquid-filled piping systems, and the peak transmission loss can be as high as 54.6 dB, which represents a pleasing combination of low-frequency muffling and lightweight design, as well as excellent ventilation capability, and embraces a promising future for application.

Band gap characteristics of bionic acoustic metamaterials based on spider web

For the purpose of low-frequency vibration isolation and noise reduction, combined with the nested mode of spider web, a new concave star bionic acoustic metamaterial was proposed in this paper. Based on the finite element method (FEM) and Bloch theory, the effects of vertex connection mode, nesting level (n), and rotation angle (α) on the band gap distribution, transmission spectrum, and isofrequency line of concave star structure were discussed. The results showed that the two-dimensional (2D) concave star structure had the lowest opening frequency of the first band gap when the vertices coincide and n = 3; when α = 45°, the three-dimensional (3D) concave star structure had the widest band gap distribution and the lowest first band gap in the frequency range within 1000 Hz. The correctness of the simulation results was verified by experiments. These findings can provide a new idea for low-frequency vibration isolation and noise reduction.

Geometric design and performance analysis of a foldcore sandwich acoustic metastructure for tunable low-frequency sound absorption

Acoustic metamaterial structures have received extensive attention for sound and vibration engineering applications from the scientific community in recent years. However, the real-life application of conventional acoustic metamaterial structures is frequently limited by fixed frequency bands and increased structural thicknesses in low-frequency noise reduction. In this study, we introduce an origami-based acoustic metamaterial structure that consists of a Miura-ori foldcore, along with a perforated and an unperforated panel. The proposed Miura-ori foldcore sandwich acoustic metastructure (MOF-SAM) exhibits adjustable low-frequency sound absorption capacities due to the foldability of the origami foldcore. Moreover, we employ numerical methods to investigate the sound absorption properties of the MOF-SAM, quantified by the sound absorption coefficient. The results indicate that the structure has a single absorption peak which is superior to that of acoustic structures composed of conventional honeycomb cores. The dissipation of acoustic energy is due to the structural vibrations of the metastructure and the losses in the folding process of the origami foldcore. The numerical results of this study show that the proposed sound absorption mechanism enables tunable low-frequency sound absorption. The geometric design and periodicity of the origami unit fragments offer multiple distinct absorption peaks and thus tunable acoustic performance. These findings of this study are expected to inspire novel designs for next-generation acoustic devices.

Membrane-type acoustic meta-material-based low-frequency noise reduction design and verification for transformers

In order to effectively reduce the radiated transformer noise, high sound insulation membrane-type acoustic meta-materials is designed based on CAE method and the main noise reduction frequency is typical 100Hz to make the maximum noise reduction of transformer. After a scale-reduced 500kV transformer semi an-echoic room noise tests comparison, the results show that the membrane-type acoustic meta-materials effectively suppressed the tonal noise of the transformer, providing a solid application foundation for the noise reduction design of transformer in the future.

Non-contact electromagnetic controlled metamaterial beams for low-frequency vibration suppression

To address the challenge of suppressing extremely-low frequency vibration and noise in precision instruments and equipment, a novel non-contact metamaterial beams is proposed in this paper. The resonators of the metamaterial beams integrate negative stiffness mechanism and electromagnetic damping tuning system. Based on the magnetic dipole theory and the electromagnetic induction law, the mechanical model of magnetic resonator is established. The band structure and transmission spectrum of the metamaterial beams are obtained by transfer matrix method. Besides, the results of numerical simulations are used to verify the accuracy of theoretical results. The result shows that the negative stiffness can be controlled by nonlinear magnetic force among the magnets. Based on this mechanism, the bandgap frequency can be reduced to a minimum of 50Hz. Then, the method of combining electromagnetic damping and negative impedance circuit is proposed to form a tuning system which can reduce the initial frequency of the bandgap to 4Hz. Interestingly, the proposed bandgap control mechanism achieves the extremely-low bandgap while broadens the bandwidth relatively, which overcomes the deficiency of traditional quasi-zero-stiffness metamaterials that reduce the bandgap frequency while causing the bandwidth to narrow. This study is expected to provide valuable ideas for the application of metamaterials in the field of low-frequency vibration and noise reduction.

Noise reduction through a waveguide structure consisting of expansion chambers with a geometrical defect

Noise control helps to make working environments safer and keep operations in line with health and safety standards. Exhaust noise is the main component of noise pollution in urban environments. In this paper, we focus on noise control by improving acoustic attenuation performance using a one-dimensional waveguide structure composed of simple periodic expansion chambers with a geometric defect. This defect is located at the center of the periodic structure and results from a modification in the length of the central chamber of the system. The objective is to study the properties of acoustic transmission and transmission loss and to examine the effect of defects in a periodic acoustic structure. The system’s spatial periodicity enables us to design wide band gaps where sound waves cannot propagate. This characteristic is very important for reducing noise in our environment. The effect of the cross-sectional ratio on the band gap behavior was also examined in this work. In addition, we have shown that the presence of a defect in a regular structure leads to a perturbation of the structure’s spatial periodicity. This leads to the creation of defect modes or resonance modes in the band gaps. We also controlled the number and amplitude of defect peaks within the band gap by varying the length of the defect. The results of this work are of interest for various applications, such as the creation of wide acoustic bands, low-frequency noise reduction, and acoustic wave filtering.

Microgravity Decoupling in Torsion Pendulum for Enhanced Micro-Newton Thrust Measurement

To enhance the accuracy of micro-Newton thrust measurements via a torsion pendulum, addressing microgravity coupling effects caused by platform tilt and pendulum mass eccentricity is crucial. This study focuses on analyzing and minimizing these effects by alleviating reference surface tilt and calibrating the center of mass during thrust measurements. The study introduced analysis techniques and compensation measures. It first examined the impact of reference tilt and center of mass eccentricity on the stiffness and compliance of the torsion pendulum by reconstructing its dynamic model. Simscape Multibody was initially employed for numerical analysis to assess the dynamic coupling effects of the tilted pendulum. The results showed the influence of reference tilt on the stiffness and compliance of the torsion pendulum through simulation. An inverted pendulum was developed to amplify the platform’s tilt angle for microgravity drag-free control. Center of mass calibration can identify the gravity coupling caused by the center of mass position. Based on the displacement signal from the capacitive sensor located at the end of the inverted pendulum, which represents the platform’s tilt angle, the pendulum’s vibration at 0.1 mHz was reduced from 5.7 μm/Hz1/2 to 0.28 μm/Hz1/2 by adjusting the voltage of piezoelectric actuator. Finally, a new two-stage torsion pendulum structure was proposed to decouple the tilt coupling buried in both pitch and roll angle. The study utilized theoretical models, numerical analysis, and experimental testing to validate the analysis methods and compensation measures for microgravity coupling effects in torsion pendulums. This led to a reduction in low-frequency noise caused by ground vibrations and thermal strains, ultimately improving the micro-Newton thrust measurement accuracy of the torsion pendulum through the platform’s drag-free control.

Low-Frequency Discontinuous Noise Diagnosis and Reduction of Air Conditioner Outdoor Unit

Abnormal discontinuous low-frequency noise occurs in the outdoor unit when the air conditioner is operating at 86~96 Hz. This noise has the characteristics of low frequency, strong penetration, long transmission distance, and poor sound quality, which can cause serious noise pollution. In order to reduce the low-frequency noise of the outdoor unit of the air conditioner, the discontinuous noise frequency is first located by testing the time-domain and frequency-domain maps of the noise. It is then compared with the frequency of fan rotation noise and structural radiation noise. It is found that the discontinuous noise is caused by the coupling of fan rotation noise and structural radiation noise. It is found that low-frequency noise improvement is significant when the frequency difference between two noise peaks is greater than 8 Hz or when a single noise peak is less than 25 dB(A) by studying the methods of frequency modulation and amplitude modulation of noise peaks. By adjusting the fan speed from 930 rpm to 940 rpm, the problem of 94 Hz noise is solved. Then, through testing the natural frequencies of the pipe and shell, it is found that the natural frequencies of the pipe are close to the excitation frequencies. The structural radiation noise is mainly caused by the pipe. Modal analysis is conducted on the pipe. When the normal boundary with the condenser and the normal boundary with the valve of the pipe is constrained by springs, the simulated values are closer to the measured values. Finally, structural optimization design is carried out on the pipe. When the U-tube of the exhaust pipe is shortened by 30 mm, a 30 g mass block is added to the cold inlet pipe, and a 97 g mass block is added to the lower part of the valve pipe of the suction pipe. The discontinuous noise of the optimized prototype is significantly improved.

Research on low frequency noise reduction device for substation equipment

Aiming at the pain point that it is difficult to control the low-frequency noise of the substation equipment, a new noise reduction method is proposed. It can reduce the noise propagation of the equipment without changing the structure of the equipment. The discrete element model of particle dampers for substation equipment is established. The energy consumption value of the particle system is used as an evaluation index. It optimizes parameters such as filling rate and particle size and determines the parameters of the scheme with the best damping effect. Through the combination of simulation and test, the test prototype was built to verify the effect. The test data showed that the total noise extremes of the first and second-floor measurement points were reduced by 14.55 dB and 6.99 dB, respectively, after installing the particle damper. This solution adds a new means of low-frequency noise control for substation equipment.

Active Noise Control of Air Conditioning System Noise

A HVAC (Heat Ventilation Air Condition) system of the passenger seat in the autonomous bus is used for heating and cooling of the seat. The compressor of the HVAC system generates the operating noise, and the noise makes passengers annoyance. The frequency spectrum of the compressor noise composes of operating frequency and its harmonics components. The frequency band of operating frequencies is low, and that of harmonic frequencies is high. Active noise control (ANC) has been used for the reduction of low-frequency noise while a passive method using absorption material has been used for the attenuation of high-frequency harmonic noise. However, some part of harmonic components at low-frequency band among harmonic components is not able to reduce by using absorption materials. In this paper, a new ANC method is proposed for the reduction of the noise level of low-frequency harmonic components based on the harmonic filtered x- LMS (HFX-LMS) algorithm. The novel method is successfully applied to the noise control of the compressor of the HVAC system of the passenger seat in the autonomous bus.

Study on acoustic performance of new spiral sound absorbing and insulating metamaterial

Middle and low frequency band sound waves have strong penetration and diffraction abilities during the propagation process, making it difficult to attenuate. The control of middle and low frequency broadband sound waves has become a challenging topic. Therefore, it is necessary to develop new materials or structures with middle and low frequency sound absorption, insulation, and noise reduction functions. The special properties of acoustic metamaterial provide new ideas for the development of sound absorption and insulation. In order to effectively control the noise in the lower frequency band, a new spatial spiral acoustic metamaterial was designed. First, the mechanism of band gap characteristics of a new spatial spiral acoustic metamaterial is analyzed. Then, a new spatial spiral acoustic metamaterial is completed through the finite element software COMSOL Multiphysics and the calculation and analysis of sound absorption and insulation performance in the 100–2500 Hz frequency band are completed. With the help of 3D printing, the preparation of metamaterial is completed, and the comparative experimental study of sound absorption and insulation performance of spiral acoustic metamaterial is carried out to verify the accuracy of the calculation method.

Sound insulation performance and modal analysis of asymmetrical insulating laminated glass

Windows are commonly the primary way for noise to enter the building environment. Most windows achieve higher sound insulation targets by simply increasing glass thickness or laminate. However, the glass thickness cannot be increased freely due to the window frame’s limitation. This research selects eleven kinds of asymmetric insulating laminated glass (glass + polyvinyl butyral + glass + air + glass) within 27 mm, which are the typical thicknesses of window frames. The three pieces of glass have equal thicknesses. The half-space acoustics method investigated the acoustic properties and modal behavior of the glass with finite thickness. The results show that when PVB or three glass increases simultaneously, and the sound insulation of each glass can be improved by 0.9∼3.4 dB or 3.7∼21.8 dB at first resonant frequencies, respectively. The glass with the best sound insulation is 7 mm glass + 0.76 mm polyvinyl butyral + 7 mm glass + 5 mm air + 7 mm glass (7 + 0.76 + 7 + 5A + 7 for short). Its sound transmission loss (TL) is 38.5 dB from 125 to 4000 Hz and 16.8 dB in the first resonant frequency (200 Hz). In addition, its first resonant frequency is the highest among eleven kinds of glass, and the number of natural frequencies in the low frequency range is only 9. These findings can provide a basis for designing asymmetric insulating laminated glass to improve the low frequency noise reduction effect effectively.

Propagation mechanism of low-frequency elastic waves and vibrations in a new tetragonal hybrid metamaterial

In order to achieve low-frequency vibration and noise reduction, artificial metamaterial with great potential in this field have been widely found. Based on previous studies, a new tetragonal hybrid metamaterial structure is proposed in this paper. It is exciting that the combination of inclusions with different material properties and sizes can affect the generation of band gaps. Then, based on the selected frequency, the propagation characteristics of waves in the structure were qualitatively analyzed using group velocity and phase velocity, and the potential for vibration reduction and noise reduction of the proposed structure was verified through finite element simulation of vibration in finite period structures. The results show that the proposed structure can open multiple broadband gaps, and the frequency range of the band gap can be flexibly adjusted according to needs. This study provides a new way for the design of low-frequency phononic metamaterial systems.

Noise isolation in building HVAC system using recyclable plastic bottles

The paper proposes an improvement of the acoustic environment in a sustainable building by utilizing differently sized discarded bottles. The HVAC (heating, ventilation and air conditioning) ducts in buildings serve as conduits for purified air, but they also generate noise as a result of turbulent flow caused by internal irregularities within the ducts. This study involves the modification in the design of ducts by utilizing the discarded or recyclable bottles. By incorporating recyclable bottles along the periphery of the duct, there is decrease in the generation of flow noise. The work presents a comprehensive investigation that employs numerical, experimental, and theoretical methods to assess the sound transmission loss (STL) within the redesigned air duct. The four-microphone impedance tube technique is utilized to measure the STL. The findings reveal that connecting bottles along the length of the air duct at specific intervals leads to a significant reduction in low-frequency noise. Additionally, increasing the number of bottles arranged radially along the air duct leads to a further enhancement and widening of the bandwidth of sound transmission loss (STL).

Impression difference to vehicle interior sound between active and passive driving situation

In near future, autonomous vehicles are expected to be more popular due to the technology development. This transition from active to passive driving condition may change the driver’s impression to vehicle interior sound. And the interior sound improvement for autonomous vehicles should be done according to the difference. In this study, we investigated whether driver’s impression to interior sound changes or not according to the driving situation through subjective evaluation test using a driving simulator. In the test, an original driving simulator was prepared to reproduce identical sounds and conditions at both active and passive driving conditions. As the presented sounds, sound pressure level (SPL) decreased or increased sound at low, mid, or high frequency bands were reproduced with the original sound. Participants drove the simulator under various driving conditions including acceleration and deceleration and evaluated discomfort about the sound. As the result, low or high frequency SPL decreased sounds were evaluated to be more discomfort at passive condition than that at active condition. Especially this tendency became significant at the acceleration and deceleration condition. From these results, high or low frequency noise reduction was found to be important in the future autonomous vehicle development.

Flow and acoustic fields investigation of noise reduction by micro vortex generators in supersonic nozzles

The role of micro-vortex generators (MVGs) in supersonic jet noise reduction is investigated. Studies are performed to understand the noise reduction mechanisms of these devices. MVGs are implemented on a scale model representative of GE-F404 nozzles. Configurations consisting of single and dual arrays of MVGs were tested for pressure ratios relevant to takeoff operating conditions. A combination of laboratory measurements and large-eddy simulations are used. Particle image velocimetry reveals that MVGs placed inside the nozzle form oblique shocks that interact with the jet plume's shock cell structure and greatly modify the shock-cell structure issued from the nozzle throat. This modification can weaken the jet plume shock-cell structure and even reduce the jet core velocities. Convective velocities estimated from the high-speed Schlieren imaging showed a significant reduction in magnitude caused by MVGs. When compared to the baseline design, the MVG nozzles showed noise reduction up to 10 dB in the upstream direction and near 5 dB in the peak downstream radiation angle at cold jet conditions. Near-field acoustic measurements showed a significant reduction of low-frequency turbulent mixing noise. Scaling analysis showed that MVGs are capable of delivering the same noise reduction performance across different model scales.

MSSA-based adaptive low-frequency noise reduction using spectrum overlap measure

A multistage singular spectrum analysis based method is presented to extract the useful component from the residue stage by stage. In every stage of the decomposition, the sum of the signal-dominated SSA components is treated as the denoised signal. The signal-to-noise ratios of the denoised signals first increase and then decrease. A measurement called spectrum overlap factor (SOF) is proposed to estimate the optimal stage which achieves the highest SNR. First, the factor is calculated to measure the spectrum overlap degree between the residue and the denoised signal in every stage. Then, the curve of the SOF with respect to the number of stages is analyzed. Further, the minimum of the SOFs, which indicates the less spectrum overlap, allows for the estimation of the optimal stage. The proposed strategy avoids inappropriate parameter selection effectively since the estimation of the optimal stage is automatic. Besides, simulation results show that the proposed method has satisfactory denoising performance in different test scenarios.