Low-frequency Noise Reduction Research Articles

Investigation of Fibrous Materials for Low Frequency and High-Frequency Passive Noise Reduction using Transfer Matrix Method

The concern about the adverse effects of noise pollution is enhancing day by day due to fast industrialization. Hence a wide variety of active and passive noise controllers made of natural and synthetic materials are widely used for various types of inevitable noise reduction. The different types of passive noise controllers fabricated using various techniques vary in their microstructure and surface morphology. In turn, they cause a difference in the noise-controlling mechanisms happening inside it, leading to its unique acoustic absorption characteristics. This article uses five different fibers of different fiber diameters to fabricate passive acoustic absorbers of different textures. The frequency response of their acoustic absorption coefficient is tested with the help of impedance tube apparatus using the transfer matrix method. Acoustic properties of coir fiber, basalt fiber, glass fiber, graphene fiber, and carbon fiber are investigated and compared. The present investigation focused on the influence of fiber diameter, the porosity, and the difference in the texture of these passive absorbers on the acoustic characteristics, frequency response of acoustic absorption coefficient, and the frequency corresponding to maximum absorption. The investigation also focuses on fabricating the passive absorbers for low-frequency noise reduction where only the active absorbers are available.

Research and design of broadband muffler based on second-order Helmholtz resonators

Noise is always a serious factor affecting people's quality of life. The most common sound-absorbing materials are porous materials, which work based on the principle that sound waves entering into the pores inside the material are subjected to air friction and viscous resistance, thus converting sound energy into heat. Porous materials have excellent performance of absorbing medium-frequency and high-frequency sound , but they are required to be thick enough to control the low-frequency sound waves with large wavelengths, which limits the application of porous materials in low-frequency noise control. In recent years, acoustic artificial structures have become a research hotspot, which can realize exotic effective acoustic parameters based on periodical structure or local resonance. Acoustic artificial structure provides a new material basis for noise control, in which Helmholtz resonator plays an important role because of its simple geometry. In this study, a broadband muffler is designed based on the second-order neck embedded Helmholtz resonator. In order to achieve low-frequency and broadband sound insulation with a limited number of units and structure length, the second-order resonator is chosen as a basic structure unit, which has a stronger low-frequency noise reduction capability and has one high-frequency transmission loss peak more than a conventional Helmholtz resonator. The acoustic characteristics and insulation performance of second-order resonators are analyzed through theoretical calculation, simulation calculation and experimental test. Then, based on the theoretical model and empirical rules, a broadband muffler composed of nine second-order resonators is designed by carefully adjusting the geometry parameters of each resonator. The three-dimensional printed resonators are installed on the side wall of a square standing wave tube for experimental measurement. In the experiment, the transmission loss curve of the muffler is measured by the two-load method. The result shows that the designed muffler has good sound insulation performances in a frequency range of 267–927 Hz, with the whole transmission loss above 20 dB and the maximum sound insulation up to 60 dB. The experimental result is consistent with the calculation result and simulation result. The muffler has simple structure and high practicability, which will have a wide application prospect in noise control engineering.

Sound Absorption Performance of Aluminum Silicate Fiber for Noise and Vibration Reduction of Distribution Transformer

In view of the low-frequency noise problem in urban substation, the sound absorption (SA) properties of aluminum silicate fibers (ASF) with different materials, unit weight, plate thickness and cavity thickness were tested in this paper. It was found that the high-purity ASF with larger unit weight, plate thickness and cavity thickness had larger low-frequency SA coefficient, which provided technical support for the development of new low-frequency noise reduction materials for substation.

Frequency stop-band optimization in micro-slit resonant metamaterials

An optimized unit cell design of a micro-slit resonant metamaterial is proposed to increase the size of the frequency stop-bands and to enhance the sound absorption at normal incidence. Micro-slit resonant metamaterials offer a compact and lightweight solution for low-frequency noise reduction, in contrast to traditional methods such as absorptive foams. A combination of numerical and semi-analytical solutions based on dispersion and absorption curves is presented. A novel algorithm allows for the decoupling of wave types from raw numerical data in the dispersion curves, without using the stiffness and mass matrices. A thorough optimization process of unit cell designs with genetic algorithms is performed. Focus is given to the first frequency stop-band, located in the frequency range of application of micro-slit resonant metamaterials. The process shows that relatively large resonators (with respect to the unit cell total area) produce a larger first frequency stop-band, whereas slit size, and resonator stiffness have a negligible effect. The optimized design increases the first frequency stop-band by 20% and the second stop-band by 25% compared to the literature standard. The absorption curves at normal incidence of acoustic waves are derived numerically for a rigid and elastic frame of the metamaterial backed by a cavity. These curves are validated by the JCAPL semi-analytical model. The optimized unit cell design shows a 9% increase in the first peak of absorption coefficient compared to the literature standard at a cavity depth of 30 mm, and an increase of 10% at a cavity of size 53 mm. Stop-band behavior does not influence sound absorption at normal incidence of acoustic waves in the frequency range of interest.

Development of a Slit-Type Soundproof Panel for a Reduction in Wind Load and Low-Frequency Noise with Helmholtz Resonators

With the rapid increase in automobiles, the importance of reducing low-frequency noise is being emphasized for a comfortable urban environment. Helmholtz resonators are widely used to attenuate low-frequency noise over a narrow range. In this study, a slit-type soundproof panel is designed to achieve low-frequency noise attenuation in the range of 500 Hz to 1000 Hz with the characteristics of a Helmholtz resonator and the ability to pass air through the slits on the panel surface for reducing wind load. The basic dimension of the soundproof panel is determined using the classical formula and numerical analysis using a commercial program, COMSOL Multiphysics, for transmission loss prediction. From the numerical study, it is identified that the transmission loss performance is improved compared to the basic design according to the shape change and configuration method of the Helmholtz resonator. Although the correlation according to the shape change and configuration method cannot be derived, it is confirmed that it can be used as an effective method for deriving a soundproof panel design that satisfies the basic performance.

Experimental study on influence of wall acoustic materials of 3D cavity for targeted energy transfer of a nonlinear membrane absorber

In view of applying the membrane nonlinear energy sink (NES) to reduce the low-frequency noise inside vehicle interior with acoustic absorption materials, a simplified acoustic cavity is used to model vehicle interior cavity. The system of one acoustic mode of the parallelepiped cavity and one membrane is established and the experimental set-up is built. The forced vibration responses in frequency-domain and time-domain are analyzed by using the set-up with and without the acoustic materials. Under certain excitation amplitude and frequency range, the strongly modulated responses (SMR) occurs and the plateau of suppressed resonance peaks appears. Meanwhile, the energy of the system is studied when the SMR exists. The targeted energy transfer (TET) phenomenon and the theoretical model of the system is verified by the experimental method. The wall acoustic materials of 3D cavity could affect the TET of the system, where the excitation amplitude for the beginning threshold of the desired working zone becomes larger and the excitation amplitude interval of TET becomes larger simultaneously. The ending threshold of the desired working zone of the NES is redefined. The numerical results are in good agreement with the experimental results. The application of membrane NES in 3D acoustic cavity proposed in this paper provides a new technical idea for vehicle interior low-frequency noise reduction in the future.

Dual-band perfect low-frequency acoustic absorption based on metaporous composite

We present a low-frequency dual-band (303 and 356 Hz) perfect acoustic absorption (PA) metaporous. The metaporous composite is constructed by introducing Helmholtz resonance and Fabry–Pérot resonance in porous materials. Our simulations show that the dual-band metaporous absorber has an ultra-thin thickness (λ/18 for 356 Hz). Using the coupling of the three dual-band PA absorbers, the low-frequency high-efficiency (>90%) broadband acoustic absorption is obtained in the frequency range of 332–562 Hz. Experiments confirm the dual-band perfect acoustic absorption and high-efficiency broadband acoustic absorption. The metaporous composite could have promising potential for low-frequency noise reduction.

A Low-Power CMOS Bandgap Voltage Reference for Supply Voltages Down to 0.5 V

A voltage reference is strictly required for sensor interfaces that need to perform nonratiometric data acquisition. In this work, a voltage reference capable of working with supply voltages down to 0.5 V is presented. The voltage reference was based on a classic CMOS bandgap core, properly modified to be compatible with low-threshold or zero-threshold MOSFETs. The advantages of the proposed circuit are illustrated with theoretical analysis and supported by numerical simulations. The core was combined with a recently proposed switched capacitor, inverter-like integrator implementing offset cancellation and low-frequency noise reduction techniques. Experimental results performed on a prototype designed and fabricated using a commercial 0.18 μm CMOS process are presented. The prototype produces a reference voltage of 220 mV with a temperature sensitivity of 45 ppm/°C across a 10–50 °C temperature range. The proposed voltage reference can be used to source currents up to 100 μA with a quiescent current consumption of only 630 nA.

Lightweight high-stiffness single-phase foam metamaterials with fluted resonant cavities for low frequency bandgaps

In this paper, a lightweight three-dimensional single phase phonon crystal structure with high stiffness is proposed. Its band gap is as low as 250 Hz, which has both bearing capacity and low frequency vibration and noise reduction ability. The band gap characteristics, vibration modes and transmission characteristics of the structure are analysed by finite element simulation. The results show that the structure can open a complete band gap when the normalized frequency is less than 0.02, and the coupling between the elastic wave propagation and the local resonances of the resonant element is the key to open the band gap. Geometric parameters can be used to control the band gap position, while material parameters do not affect the relative position and bandwidth of the band gap.

Research on Design and Mechanism of Sound Insulating Structure Based on Acoustic Metamaterials

In view of the difficult problems of low-frequency noise reduction, based on the clamping boundary-type acoustic metamaterial theory with negative effective mass density, a structure combining membrane-type and clamping boundary-type is designed, which can realize good sound insulation performance in the low-frequency region, and the theoretical calculation and numerical simulation are in good agreement. At the same time, the transmission property of the structure at zero refractive index and impedance matching is studied. The research suggests that the structure has good low-frequency broadband sound insulation performance, and can achieve high transmission performance of sound waves through structural regulation.

Low-frequency noise control using layered granular aerogel and limp porous media

The acoustic absorption of granular aerogel layers with a granule sizes in the range of 2 to 40 μm is dominated by narrow-banded, high absorption regions in the low-frequency range and by reduced absorption values at higher frequencies. In this paper, we investigate the possibility of developing new, low-frequency noise reduction materials by layering granular aerogels with traditional porous sound absorbing materials such as glass fibers. The acoustic behavior of the layered configurations is predicted using the arbitrary coefficient method, wherein the granular aerogel layers are modeled as an equivalent poro-elastic material while the fibrous media and membrane are modeled as limp media. The analytical predictions are verified using experimental measurements conducted using the normal incidence, two-microphone impedance tube method. Our results show that layered configurations including granular aerogels, fibrous materials, and limp membranes provide enhanced sound absorption properties that can be tuned for specific noise control applications over a broad frequency range.

Broadband low-frequency noise reduction using Helmholtz resonator-based metamaterial

Achieving broadband noise attenuation at low frequencies is still a significant challenge. Helmholtz resonators offer good low-frequency noise attenuation but are effective only over a narrow band; the cavity volume required at these frequencies is also larger. This article proposes a new broadband acoustic metamaterial (AMM) absorber, which uses polyurethane (PU) foam embedded with small-size resonators tuned to different frequencies. The AMM design is achieved in three phases: (1) develop a transfer-matrix-based one-dimensionalmodel for a resonator with intruded neck; (2) use this model to develop a novel band broadeningmethod, to select appropriate resonators tuned to different frequencies; and (3) construct a unit cell metamaterial embedded with an array of resonators into PU foam. A small-size resonator tuned to 415 Hz is modified, by varying the intrusion lengths of the neck, to achieve natural frequencies ranging from 210 to 415 Hz. Using the band broadening methodology, 1 unit cell metamaterial is constructed; its effectiveness is demonstrated by testing in an acoustic impedance tube. The broadband attenuation characteristics of the constructed unit cell metamaterial are shown to match well with the predicted results. To demonstrate further the effectiveness of the idea, a metamaterial is formed using 4 periodic unit cells and is tested in a twin room reverberation chamber. The transmission loss is shown to improve significantly, at low frequencies, due to the inclusion of the resonators.

Reduction of Low-Frequency Vessel Noise in Monterey Bay National Marine Sanctuary During the COVID-19 Pandemic

Low-frequency sound from large vessels is a major, global source of ocean noise that can interfere with acoustic communication for a variety of marine animals. Changes in vessel activity provide opportunities to quantify relationships between vessel traffic levels and soundscape conditions in biologically important habitats. Using continuous deep-sea (890 m) recordings acquired ∼20 km (closest point of approach) from offshore shipping lanes, we observed reduction of low-frequency noise within Monterey Bay National Marine Sanctuary (California, United States) associated with changes in vessel traffic during the onset of the COVID-19 pandemic. Acoustic modeling shows that the recording site receives low-frequency vessel noise primarily from the regional shipping lanes rather than via the Sound Fixing and Ranging (SOFAR) channel. Monthly geometric means and percentiles of spectrum levels in the one-third octave band centered at 63 Hz during 2020 were compared with those from the same months of 2018–2019. Spectrum levels were persistently and significantly lower during February through July 2020, although a partial rebound in ambient noise levels was indicated by July. Mean spectrum levels during 2020 were more than 1 dB re 1 μPa2 Hz–1 below those of a previous year during 4 months. The lowest spectrum levels, in June 2020, were as much as 1.9 (mean) and 2.4 (25% exceedance level) dB re 1 μPa2 Hz–1 below levels of previous years. Spectrum levels during 2020 were significantly correlated with large-vessel total gross tonnage derived from economic data, summed across all California ports (r = 0.81, p < 0.05; adjusted r2 = 0.58). They were more highly correlated with regional presence of large vessels, quantified from Automatic Identification System (AIS) vessel tracking data weighted according to vessel speed and modeled acoustic transmission loss (r = 0.92, p < 0.01; adjusted r2 = 0.81). Within the 3-year study period, February–June 2020 exhibited persistently quiet low-frequency noise and anomalously low statewide port activity and regional large-vessel presence. The results illustrate the ephemeral nature of noise pollution by documenting how it responds rapidly to changes in offshore large-vessel traffic, and how this anthropogenic imprint reaches habitat remote from major ports and shipping lanes.

Bridge Resistance Compensation for Noise Reduction in a Self-Balanced PHMR Sensor.

Advanced microelectromechanical system (MEMS) magnetic field sensor applications demand ultra-high detectivity down to the low magnetic fields. To enhance the detection limit of the magnetic sensor, a resistance compensator integrated self-balanced bridge type sensor was devised for low-frequency noise reduction in the frequency range of 0.5 Hz to 200 Hz. The self-balanced bridge sensor was a NiFe (10 nm)/IrMn (10 nm) bilayer structure in the framework of planar Hall magnetoresistance (PHMR) technology. The proposed resistance compensator integrated with a self-bridge sensor architecture presented a compact and cheaper alternative to marketable MEMS MR sensors, adjusting the offset voltage compensation at the wafer level, and led to substantial improvement in the sensor noise level. Moreover, the sensor noise components of electronic and magnetic origin were identified by measuring the sensor noise spectral density as a function of temperature and operating power. The lowest achievable noise in this device architecture was estimated at ~3.34 at 100 Hz.

A Damping Optimization Method Based on the Operational Mode Analysis for Low-Frequency Noise Reduction

A Damping Optimization Method Based on the Operational Mode Analysis for Low-Frequency Noise Reduction

Design and experimental investigation of broadband quasi-perfect composite loaded sound absorber at low frequencies

We designed a composite loaded sound absorber (CLSA) composed of a sub-wavelength Helmholtz resonator and porous material, which was loaded on the side branches of a rectangular tube. Based on the transfer matrix method, an equivalent acoustic model was established to calculate the acoustic performance of the absorber. According to the equivalent acoustic model and critical coupling theory, a loaded absorber with thickness of 51 mm and perfect absorption at 156 Hz was achieved using the complex frequency plane analysis method (CFPM). The finite element simulation and acoustic experiment results verified the perfect sound absorption effect of the absorber at low frequencies. To overcome the defects of single frequency and narrow band of the perfect absorber, we designed nine different porous perfect absorber units that could achieve perfect sound absorption between 160 Hz and 312 Hz. Different numbers of porous perfect absorbers were placed on the side branch of the rectangular tube, and a broadband quasi-perfect sound absorption of more than 0.9 between 160 Hz and 285 Hz was realized by optimization. The experimental results further verified the quasi-perfect sound absorption effect of the CLSA. The theoretical model of low-frequency broadband quasi-perfect sound absorber and the design of a CLSA in this study provide a new possibility for the reduction of low or medium frequency noise in the tube.

APPROACHES TO CLASSIFICATION OF REDUCTION METHODS OF LOW FREQUENCY NOISE AND VIBRATION OF POWER PLANTS

The importance of the problem of low-frequency noise and vibration reduction now may be considered as urgent. Increased influence of low-frequency noise and vibration may cause both human health problems and equipment damage. Power plants (internal combustion engines, compressors, heat-exchanges etc.) are one of the main low-frequency noise and vibration sources. The principles of classification of methods of power plants low-frequency noise and vibration reduction are suggested. Author is proposing energetic approach, according of which all the methods and arrangements of reduction may be classified as passive (adaptive and non-adaptive), active and hybrid passive-active. The classification is illustrated by the different examples, including constructions of mufflers and dampers, some of which are developed by author.

1J3-1 Reduction of Low-Frequency Noise in Cross Sectional Ultra-sound Property Micro Imaging by Frequency-Resolved Spatial Averaging.

1J3-1 Reduction of Low-Frequency Noise in Cross Sectional Ultra-sound Property Micro Imaging by Frequency-Resolved Spatial Averaging.

On articulated plates with micro-slits to tackle low-frequency noise

In recent years, new concepts of acoustic absorbers dedicated to the reduction of low-frequency noise have been developed. Among them, liners with moving parts, such as membrane-based liners, have been an object of particular interest. In the present paper, we propose a liner concept based on a cantilever beam made of articulated plates with micro-slits. Compared to membrane technologies, these micro-slits introduce a small leakage from the backing cavity that reduces the high compressibility effects occurring at very low frequencies in a small cavity. An acoustic liner including an ensemble of such articulated plates has been fabricated and characterized for grazing acoustic incidence in absence and in presence of flow. Measurements in an impedance tube at normal incidence have also been performed, and perfect absorption is obtained at a frequency where the liner thickness corresponds to 1/16th of the acoustic wavelength. A new and simple model is proposed to predict the attenuation of this type of acoustic treatment. The results are in good agreement with the measurements, indicating a correct identification of the physical phenomena here at stake.

Broadening low-frequency bandgaps in locally resonant piezoelectric metamaterials by negative capacitance

This paper combines negative capacitance (NC) with inductance (L) to enlarge low-frequency bandgap width in locally resonant piezoelectric metamaterials. The studied metamaterials are obtained by directly bonding patches on the surfaces of host structures, then connecting patches to shunts. Shunts with NC and L in series and in parallel are both studied. Analytical expressions of the bandgap ranges are derived, which reveal that the bandgap size is increased not simply because the NC enhancing the material’s electro-mechanical coupling factor, but in a more complicated way. Parametric studies are performed to analytically investigate the tuning properties of the LR bandgap by NC. Results demonstrate that by modifying NC value, the LR bandgap size can be significantly increased. Numerical simulations are done to verify the effects of the broadened bandgap on vibration transmission and reveal the limitations of the used analytical model. Practical implementation of the shunts are also discussed, recommendations on choosing the shunt configurations and NC values are given. This paper gives a theoretical guideline on designing piezoelectric metamaterials with bandgap effects at desired frequency ranges for practical applications like low-frequency vibration and noise reduction or isolation.