Low Frequency Broadband Noise Research Articles

Low-frequency broadband acoustic metamaterial absorber based on nested resonator and synergistic coupled weak resonances

Utilizing absorbers with sub-wavelength thickness for low-frequency broadband noise control is a challenging problem, and rapid on-demand design of structures based on target noise frequency has received continuous attention. In this work, nested cavities are introduced into the typical neck-embedded Helmholtz resonator (NEHR) to obtain the nested neck-embedded Helmholtz resonator (NNEHR), which exhibits better cavity partitioning volume fraction and covers wider sound absorption band. Both theoretical analysis and simulations demonstrate that NNEHR possesses superior acoustic properties: generating an additional absorption peak, lowering the position of the first absorption peak by 11 %, and increasing the absorption coefficient by 4.7 %. Instantaneous velocity fields and thermal-viscous energy dissipation density illustrate that the efficient sound absorption of NNEHR originates from resonant dissipation in the neck and surrounding areas. Furthermore, by combining the theoretical model with genetic algorithms, three sets of NNEHR metasurfaces are optimized. The optimized structures achieve average absorption coefficients of 0.831, 0.913, and 0.885 in the range of 250–550 Hz, 400–1000 Hz, and 500–2000 Hz, with a thickness of only 50 mm, reflecting sub-wavelength, low-frequency broadband sound absorption characteristics. The excellent absorption stems from synergistic coupled weak resonances and higher-order absorption peaks of constituent units, and the overdamping characteristic ensures the realization of the optimal structure with minimal thickness. The contribution of weak resonant units to the total sound absorption performance can be measured by the surface admittance normalized by the total incident area, and the essence of the coupling effect is the strong interaction between units with adjacent absorbing frequencies. The proposed structure exhibits good sound absorption performance and great adjustability, while the genetic algorithm optimization based on the theoretical model demonstrates high efficiency and robustness, providing a methodology for the design and on-demand optimization of low-frequency broadband structures.

Hydroacoustic analysis of a full-scale marine vessel: Prediction of the cavitation-induced underwater radiated noise using large eddy simulations

This numerical study provides insight into the mechanism of noise generation by a cavitating flow in the wake of a marine propeller under realistic operating conditions, which poses a significant threat to marine ecosystems. We examined a full-scale vessel with an entire hull and an isolated model-scale marine propeller (INSEAN E779A) with a maneuverable rudder under various highly turbulent inflow conditions that strongly affect the spectral characteristics of the radiated noise. Insight into the acoustic behavior was gained by employing a combination of the large eddy simulation (LES) treatment of turbulence and the Schnerr–Sauer volume of fluid cavitation model. The hydrodynamic solution was coupled with the Ffowcs Williams-Hawkings (FW-H) strategy for noise and vibration identification. We focused on the interactions between the characteristic cavitation patterns of marine propellers (sheet, tip, and hub cavities) and the dominant structures of the turbulent wake (tip, root, trailing edge, and hub vortices, as well as the distributed small-scale vorticity). The small-scale topological structures in the swirling wake of a propeller directly manifest in the radiated sound level and affect the intensity of multiple frequency ranges. Quantitative analysis of thrust, pressure fluctuations, and sound pressure levels (SPLs) demonstrates significant effects of blade loading, wake distribution, and cavitation development. The peak and average SPL distributions obtained through LES show lower dominant and higher average frequencies compared to those obtained by the FW-H method. The overall SPL obtained by LES were higher than those calculated using the FW-H acoustic analogy at all microphone locations. The overall noise was dominated by the low-frequency broadband noise, attributed to energetic helical vortices, and narrow-band peaks in the medium-high frequency range that originated from other sources, like cavitation structures.

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.

Anthropogenic noise disrupts early-life development in a fish with paternal care

Anthropogenic noise is a global pollutant but its potential impacts on early life-stages in fishes are largely unknown. Here, using controlled laboratory experiments, we tested for impacts of continuous or intermittent exposure to low-frequency broadband noise on early life-stages of the common goby (Pomatoschistus microps), a marine fish with exclusive paternal care. Neither continuous nor intermittent noise exposure had an effect on filial cannibalism, showing that males were capable and willing to care for their broods. However, broods reared in continuous noise covered a smaller area and contained fewer eggs than control broods. Moreover, although developmental rate was the same in all treatments, larvae reared by males in continuous noise had, on average, a smaller yolk sac at hatching than those reared in the intermittent noise and control treatments, while larvae body length did not differ. Thus, it appears that the increased consumption of the yolk sac reserve was not utilised for increased growth. This suggests that exposure to noise in early life-stages affects fitness-related traits of surviving offspring, given the crucial importance of the yolk sac reserve during the early life of pelagic larvae. More broadly, our findings highlight the wide-ranging impacts of anthropogenic noise on aquatic wildlife living in an increasingly noisy world.

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.

Mitigating low-frequency broadband aircraft noise through a structured assembly of acoustic material and devices

Addressing the challenge of attenuating low-frequency broadband noise emerges as a critical concern within the fields of aeronautics, ground transportation, and construction industries, requires innovative solutions for enhanced acoustic control. Over the last few decades, the literature has seen an increase in low-frequency noise control solutions centered around acoustic metamaterial designs. These proposed technologies exhibit promising acoustic performance, especially proving superior to conventional sound insulation materials in constrained spaces, such as in aerospace applications. Despite the efficacy of typical metamaterials in attenuating tonal noise through narrow resonant frequency maxima, practical applications reveal some challenges, as even slight variations in tonal noise frequencies can compromise the overall effectiveness of such solutions. In response to this, the present paper introduces a novel thin acoustic metamaterial design aimed at improving broadband noise attenuation at low frequencies. This design uses carefully arranged structured metamaterials within a fiberglass layer to create optimal resonance frequency bands for maximum low-frequency noise attenuation. Performance assessment in the low-frequency domain employed COMSOL Multiphysics finite element methods, predicting sound absorption coefficient and transmission loss. Results confirm the effectiveness of the proposed metamaterial design, showcasing broad noise attenuation at low frequencies.

Theoretical and experimental investigation on the sound absorption performance of ultra-thin curled acoustic metasurface

A simplified model of the absorption coefficient of traditional Helmholtz resonators (THR) was established, and the influence of different geometric parameters on the absorption coefficient of THR was analyzed. To realize the low-frequency broadband acoustic structure design, an accurate theoretical model for the sound absorption coefficient of the curled acoustic metasurface (CAM) unit was established. Based on the complex frequency plane method (CFPM), the CAM units with perfect sound absorption at four discrete frequencies were designed. The low-frequency broadband acoustic metasurfaces in parallel under decoupled and coupled conditions were studied, and the thickness is only 12 mm. The high efficiency of sound absorption above 0.8 was achieved in the frequency range of 758 Hz–940 Hz. The experiment verifies the efficient sound absorption effect of the CAM unit and the broadband sound absorption effect under coupled conditions. The research in this paper has a certain potential applications for low-frequency broadband noise control technology.

Acoustic measurements in single-rotor/wing interaction at low disk loading and Reynolds number

The tiltrotor design is favored for urban air mobility (UAM) prototypes due to the combination of vertical takeoff and landing (VTOL) capability and efficient forward flight. With rising UAM air traffic at low altitudes, noise from these aircraft is a crucial design factor. Most tiltrotor noise research focuses on high disk loading and Reynolds number setups, leaving smaller aircraft configurations less explored. This study investigates aero-acoustic trends from rotor-wing interaction at low disk loading ([Formula: see text]100 N/m2) and Reynolds number (Re < 100,000). While prior literature suggests lowering disk loading and reducing rotor wake interference can mitigate rotor noise, such ideas lack empirical validation. The setup involves an anechoic chamber housing a two-blade rotor, along with flat and NACA 0012 airfoil wings. Microphones and a rotation stage capture acoustic data for analysis. Factors like flow recirculation, isolated rotor noise, rotor height, rotation direction/rate, and wing curvature are assessed for impact on noise signature. It is found that the deflected rotor wake in rotor-wing interaction significantly increases low-frequency broadband noise and overall sound pressure level (OASPL), compared to an isolated rotor. Dominant tonal noise diminishes based on the strength of the deflected rotor wake. These findings offer insights into reducing noise from rotor wake impingement on the wing.

Experimental Study of Aerodynamic Noise Source Measurement around an Automobile Door Mirror by Hot-film Sensor

In order to the evaluation and identification of the aerodynamic sound source generated from a door mirror of an automobile, the simple measurement technique using the film-type hot film sensor is proposed. In the wind tunnel test of the actual vehicle model, the feasibility on the qualitative evaluation of the aerodynamic sound sources with the hot film sensor is discussed experimentally. From the results of this experiment, author could explain that the low-frequency broadband noise is one of the features of its aerodynamic noise. The qualitative evaluation of the hot-film sensor indicated that the low-frequency aerodynamic noise is caused by fluctuations formed at the tips of the door mirrors. Although the location of the low-frequency broadband noise source was identified by evaluating the signal of the hot film sensor, the quantitatively evaluation of the strength of the aerodynamic sound source could not achieve.

Study on the acoustic performance of bending metasurface with ultra-thin broadband quasi-perfect sound absorption

In order to achieve low-frequency broadband noise control under subwavelength thickness, this paper established a theoretical model for calculating the sound absorption coefficient of curled acoustic metasurface (CAM) composed of embedded hole, equal width curled channel, and gradient width curled channel, which is based on the thermal viscosity equivalent model and impedance equivalent model. Considering the over resistance effect caused by multi-unit composite structure, a multi-parameter control strategy was used to design bending acoustic metasurface (BAM) units for perfect sound absorption at four discrete frequencies of 546 Hz, 563 Hz, 585 Hz, and 601 Hz, which is based on the complex frequency plane method. and the thicknesses of these unites are only (1.2 cm). The broadband and efficient sound absorption effect of the four parallel composite BAM units was theoretically studied. The simulation and experiment verified that the composite BAM not only achieved perfect sound absorption at 573 Hz with a subwavelength thickness of 12 mm, but also had an efficient sound absorption(α > 0.8) frequency band of 512Hz-621Hz, with a bandwidth up to 109 Hz. The research in this paper provides a certain theoretical basis for the design of low-frequency broadband compact acoustic structures, and has certain reference value for low-frequency broadband noise control.

Influence of Boundary Impedance of 3D Cavity on Targeted Energy Transfer between a Damped Acoustic Mode and a Nonlinear Membrane Absorber

In order to apply the nonlinear energy sink (NES) to reduce the low-frequency noise inside a 3D acoustic cavity with an impedance boundary, a two degrees-of-freedom (DOF) dynamic model of the coupled system of one damped acoustic mode of a regular 3D acoustic cavity and a nonlinear membrane absorber as the NES is established. The damping coefficient of the acoustic mode is obtained based on the finite element analysis method and the relationship of the boundary impedance and the absorption coefficient of the cavity. Based on the set-up of the system, the damping coefficient of the acoustic mode is determined. The frequency responses and the targeted energy transfer (TET) phenomenon of the coupled system are analyzed, and the theoretical and numerical results of the frequency responses of the system are in good agreement with the experimental ones. The effects of the wall impedance of the cavity on the optimal TET are discussed. With the increase in the impedance ratio of the wall, the amplitude of the acoustic displacement plateau decreases, and the frequency bandwidth of the plateau and the two thresholds of the optimal TET interval of the excitation increase. It provides a comprehensive theoretical model and experimental basis for the application of NES in the actual complex structure and provides a reliable design method and control strategy for controlling vehicle interior low-frequency broadband noise.

Realizing the perfect sound absorption and broadening effective band using porous material and micro-perforated plate

The noise attenuation ability of a single material or structure, especially for low-frequency noise, is limited by its thickness. Aiming to achieve high-efficiency noise attenuation at low frequencies, this paper proposes the methods of porous material filling and micro-perforated plate (MPP) embedding to design a perfect sound absorber at different frequencies using the under-loss Helmholtz resonator (HR). Based on the transfer matrix method, the theoretical calculation models of the sound absorption coefficients of the HR, Helmholtz resonator with porous material (HRP), and Helmholtz resonator with micro-perforated plates (HRM) are constructed. Based on the theoretical models, the under-loss absorber HR1 with the peak absorption at 243 Hz, and the HRP and HRM with perfect absorption at 212 Hz and 157 Hz are designed, respectively. The impedance analysis and complex frequency plane method are used to analyze the sound absorption mechanisms of the HR1, HRP, and HRM. The accuracy of the theoretical model is verified by the finite element method. Finally, the three acoustic absorbers are manufactured using 3D printing technology, and the absorption coefficients are evaluated experimentally. The experimental results show that the HR1 has a high working frequency at 245 Hz and a narrow bandwidth of high-efficiency sound absorption ([Formula: see text]), which is only 12 Hz. The working frequency of the HRP is 214 Hz, and its high-efficiency sound absorption bandwidth is 54 Hz. The HRP has the lowest working frequency at 157 Hz and the widest high-efficiency sound absorption bandwidth of 58 Hz among the three absorbers. The research results presented in this paper provide a reference for the realization of low-frequency broadband noise attenuation designs and have certain application potential in noise control.

Acoustic metamaterial for highly efficient low-frequency impedance modulation by extensible design

We propose a novel class of acoustic metamaterial for low-frequency broadband sound absorption with high efficiency. The design principle is physically decoupled into low-frequency mitigation and working frequency widening. The embedded tubes with phase shift are employed to alleviate low-frequency noise, achieving a quasi-perfect absorption (α>0.9) at 240 Hz with the peak value of 0.998. The spider-web-like configuration brings the high-efficiency acoustic impedance modulation via manipulating the coherent coupling effect, and allows for constantly broadening the effective working frequency in lower frequency domain by taming the resonant characteristics and the scale ratio of original and extended units. The theoretical and numerical methods are greatly validated by measured data. We further demonstrate the broadband potentials of the metamaterial, with a continuous quasi-perfect absorption from 301 Hz to 534 Hz (i.e., 1/22 of the operating wavelength) achieved from analytical results. The underlying physics is explained via coherent coupling, acoustic impedance matching, and system damping in detail. All in all, our work provides a viable and effective scheme in overcoming extremely low-frequency broadband noise for lightweight structures.

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

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

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

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

Adaptive active noise feedforward compensation for exhaust ducts using a FIR Youla parametrization

The adaptive feedforward control strategy can effectively attenuate the low-frequency broad-band noise with unknown and variable characteristics. However, most feedforward active noise control systems have the internal acoustic feedback coupling between the secondary noise source and the measurement of the image of the reference noise, which may lead to instability of the system. In addition, there are potential fields of applications in which the broad-band noise source may be mixed with the narrow-band noise components and the delay of the secondary path may be larger than the delay of the primary path. In order to deal with these limitations, in this paper an adaptive Youla(Q) parameterized feedforward active noise control system is introduced. Firstly, a central controller is designed to stabilize the internal acoustic feedback loop. Then the central controller will be augmented into a Youla(Q) parameterized FIR-type adaptive controller with the recursive least squares (RLS) adaptive algorithm, and the Q parameter can be adjusted online to the desired value to attenuate the unknown and variable noise. Finally, the proposed adaptive Youla(Q) parameterized feedforward active noise control approach is successfully applied to the noise attenuation of a “U”-shaped industrial exhaust duct.

An improved spectral method and experimental tests for the low-frequency broadband noise of marine propellers

An improved spectral method and experimental tests for the low-frequency broadband noise of marine propellers

SeMSA: a compact super absorber optimised for broadband, low-frequency noise attenuation

The attenuation of low-frequency broadband noise in a light, small form-factor is an intractable challenge. In this paper, a new technology is presented which employs the highly efficient visco-thermal loss mechanism of a micro-perforated plate (MPP) and successfully lowers its frequency response by combining it with decorated membrane resonators (DMR). Absorption comes from the membranes but primarily from the MPP, as the motion of the two membranes causes a pressure differential across the MPP creating airflow through the perforations. This combination of DMR and MPP has led to the Segmented Membrane Sound Absorber (SeMSA) design, which is extremely effective at low-frequency broadband sound absorption and which can achieve this at deep sub-wavelength thicknesses. The technology is compared to other absorbers to be found in the literature and the SeMSA outperforms them all in either the 20–1000 Hz or 20–1200 Hz range for depths of up to 120 mm. This was verified through analytical, finite element and experimental analyses.

Low frequency broadband noise radiated by highly skewed propeller ingesting inflow turbulence

Low frequency broadband noise radiated by highly skewed propeller caused by inflow turbulence is numerically studied. In the first place, the broadband thrust spectrum acting on the blade surface is computed based on the strip theory. Once the thrust is obtained, pressure spectrum of noise resulting from the broadband force (force sources) and vibrations of blades (velocity sources) are computed separately. When computing noise radiated by the force sources, the blade surfaces are simplified as distributed dipole sources. When computing noise radiated by the velocity sources, three steps are applied as followed: Firstly, a simplified propeller-shafting system is established. The impedance function of each exciting node on the blade surfaces is formed based on the FEM model. In the second step, the random vibrations of the blades are calculated by multiplying the impedance function matrix with the thrust spectrum matrix. Finally, noise radiated by velocity sources is computed with the solutions of wave equations. On the basis of the numerical results, the radiated noise power of force sources and velocity sources are compared. The relationships of the modal characteristics and the radiated noise are discussed. Furthermore, the influences of inflow condition on the radiated noise level are discussed. The results and analysis may be helpful for the low-frequency vibration and noise reduction of the propeller.

Maze-like acoustic metamaterial for low-frequency broadband noise suppression

We propose the maze-like acoustic metamaterial (MLAMM) unit suitable for regulating the transformer noise. The influence of the length and width of the acoustic waveguide on the sound transmission loss (TL) is studied, and the TL and resonance frequencies of the MLAMM unit are calculated. The visco-thermal effect weakens the resonance transmission peak of the MLAMM unit. Finally, experiments verify that the proposed MLAMM unit has low-frequency broadband sound insulation performance, and the TL is about 25 dB in the frequency band of 100 Hz–500 Hz. The research provides a reasonable and effective solution for noise suppression of transformers.