Control Of Low-frequency Noise Research Articles

Broadband Aircraft Noise Attenuation Across Low and High Frequencies Using Structured Materials

Controlling broadband noise throughout the spectrum, from low to high frequencies, is a significant challenge for the aerospace, transportation, and construction industries. Although recent research has focused on low-frequency noise control solutions using acoustic metamaterial designs, the narrow-band resonances of these materials remain a limitation. This paper presents a structured material system which addresses this limitation by proposing a parallel assembly of structured materials, enabling broadening of the resonance frequency band from low to high frequencies. The study involves an arrangement of structured metamaterials and Helmholtz Resonators embedded in a layer of glass wool. By meticulously grouping individual resonant frequencies, the proposed assembly effectively attenuates noise over the frequency range of interest. The present research contributes to the development of lasting solutions for aircraft noise across a broad frequency spectrum, showing promising potential for significant noise reduction within practical constraints.

Study on adjustable low-frequency sound absorbers based on digital shunt loudspeakers

Low-frequency noise is a critical frequency band that affects people’s physical and mental health. Traditional sound absorption technology is limited in effectiveness at low frequencies due to spatial constraints. Shunt loudspeakers, which consist of a loudspeaker connected to a shunt circuit, have emerged as a rapidly developing solution for low-frequency noise control. However, most shunt circuits are implemented using analog circuits, resulting in poor stability and accuracy due to the presence of parasitic resistance. As a result of this situation, this paper proposes an adjustable low-frequency sound absorber based on digital shunt loudspeakers, where the shunt circuit is realized through digital synthetic impedance based on FPGA technology. Additionally, the acoustic impedance can be easily adjusted by the digital shunt circuit without relying on cavity depth adjustments. Experimental results validate that the proposed sound absorber offers excellent performance in adjustable low-frequency sound absorption, thus confirming its theoretical basis.

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.

Research on the Performance of Sound Absorption Materials for Substations in Power Grid

As the environmental protection policies become increasingly strict, higher requirements will be proposed for low-frequency noise control problems, and research on control strategies for low-frequency noise in substations is imperative. In this work, principles of resistive acoustic absorption and resonance acoustic absorption materials were analyzed. Based on the measurement in acoustic impedance tubes, the sound absorption performances of two typical sound absorption materials, the glass wool and the micro-perforated panel, were tested. This study offers a new reference for power grid companies to select and combines suitable acoustic materials to realize the optimal application in substations based on noise reduction requirements of different frequencies.

Mechanism analysis and optimal design of sound-absorbing metastructure constructed by slit-embedded Helmholtz resonators

<sec>Low-frequency noise has always been a thorny problem in the field of noise control. In recent years, the development of sound-absorbing metastructures has provided new ideas for controlling low-frequency noise. In this work, we propose a low-frequency sound-absorbing metastructure constructed by Helmholtz resonators with embedded slit. Analytical and numerical models are established to analyze the sound absorption performance and mechanism of the proposed sound-absorbing metastructure, and optimization design is conducted to achieve low-frequency wideband absorption performance. The analytical modeling method and the performance of the proposed sound-absorbing metastructure are also experimentally verified. The main conclusions are summarized as follows.</sec><sec>1) By using transfer matrix method and finite element method, analytical and numerical models for calculating sound absorption coefficient are established. It is shown that analytical predictions are in good agreement with numerical calculations. It is demonstrated that a typical design of a 30-mm-thick single-cell metastructure can achieve a sound absorption coefficient of 0.88 at 404 Hz. Typical designs of two-cell parallel structure and the four-cell parallel structure (both with a thickness of 50 mm) can achieve two and four nearly perfect low-frequency sound absorption peaks in a frequency band of 200-400 Hz, respectively.</sec><sec>2) The low-frequency sound absorption mechanisms of the proposed metastructures are explained from four aspects: simplified equivalent model parameters, normalized acoustic impedance, complex-plane zero/pole distribution, and sound pressure cloud image and particle velocity field distribution. It is demonstrated that the main sound absorption mechanism is related to the thermal viscous loss of sound waves, caused by the inner wall of embedded slit.</sec><sec>3) The design for broadband low-frequency absorption performance is optimized by using differential evolution optimization algorithm. An optimized parallel-multi-cell coupled metastructure with multiple perfect sound absorption peaks below 500Hz is realized. For a thickness of 90 mm, the sound absorption coefficient curve of an optimized metastructure exhibits 8 almost perfect sound absorption peaks and an average sound absorption coefficient of 0.86 in a frequency range of 170－380 Hz.</sec><sec>4) Experimental samples are fabricated to test sound absorption. Experimental results are basically consistent with the analytical predictions. The results from analytical model, numerical calculations and experimental measurements are mutually verified.</sec><sec>In summary, the sound-absorbing metastructures with a thickness of sub-wavelength, proposed in this work, exhibit outstanding sound absorption performance at low frequencies. We demonstrate that they are suitable for low frequency broadband sound absorption below 500 Hz. Owing to their thin thickness and relatively simple construction, they have broad application prospects in practical noise control engineering.</sec>

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.

Low frequency noise control using coupled loudspeaker and negative impedance circuit

It remains a technical challenge for low-frequency duct noise control devices to combine compact and broadband features at same time. A semi-active noise control approach is introduced to solve this problem. The device consists of a main pipe and two loudspeakers connected by a circuit containing a negative impedance network. Due to the effect of the negative impedance circuit, the virtual bypass is composed of the coupled loudspeaker and the circuit has sound transmission characteristics close to a physical duct in a certain frequency range. The effect of the negative impedance network and circuit on the performance of the silencer is studied by using the transfer matrix method. Through the design of the circuit, the sound transmission process of the virtual bypass can be realized. If the proper circuit network and parameters are selected, the virtual bypass HQ tube can achieve compact broadband noise control at low frequencies. A semi-active noise control device has 10dB noise reduction in the range of 100-300Hz.

Investigation on acoustical performances of micro-perforated panel with soft boundary backing cavity

Low-frequency noise control is still an issue due to its characteristic of having a long wavelength. Therefore, attenuation requires a bulky noise control component, including an acoustic absorber. Conventional absorber materials, such as micro-perforated panels (MPP), are a candidate to circumvent such a problem. However, the requirement of backing cavity depth causes the MPP to work under a quarter wavelength constraint. Eventually, the bulky characteristic of the absorber is persistent. Therefore, this study introduces a soft boundary condition to the MPP to enable the system to work at a lower frequency while the associated dimensions reduce. Open gaps between the backing cavity and rigid surface realize the soft boundary conditions and double-layer MPP is considered to test such a system. The experimental result shows that the soft boundary can enhance at low frequencies (<250 Hz) by 0.3-0.6 points. At the same time, the open gap can slightly increase the resulting absorption performance deficiency at mid-frequency.

Broadband low frequency noise attenuation using thin acoustic metamaterials for aircraft cabin noise mitigation

Broadband noise attenuation at low frequencies is a challenge for the aeronautical, ground transportation and construction industries. In the past few decades, various low frequency noise control solutions, based on acoustic metamaterials designs, have been presented in the literature. The proposed technologies showed promising acoustic performance and are considered as better solutions when compared to conventional sound insulation materials in application fields such as aerospace, where the available space for their integration is extremely limited. The noise attenuation of typical metamaterials is characterized by very narrow resonant frequency maxima which represent a good solution for tonal noise. However, in practical applications, the slight variations of the tonal noise frequencies may render a metamaterial ineffective. This paper presents a thin acoustic metamaterial design for improved broadband noise attenuation at low frequencies. The geometry is an assembly of structured materials arranged in parallel and embedded in a layer of fiberglass. The two structured materials are designed such that their resonant frequencies are optimally regrouped to create a resonant frequency band of maximum attenuation at low frequencies. A thermo-viscous acoustics approach was solved numerically with COMSOL Multiphysics in the frequency domain to predict the sound absorption coefficient and the normal incidence sound transmission loss of the proposed metamaterial design. The results obtained show a wide frequency band noise attenuation for this metamaterial at low frequencies.

Study of low-frequency noise suppression effect inside acoustic cavities by applying piezoelectric nonlinear energy sink

In this paper, the piezoelectric nonlinear energy sink (NES) is used to suppress the radiated noise from a thin plate, and the targeted energy transfer (TET) characteristics and mechanism of the coupled system of two piezoelectric NESs and a thin plate are analyzed, and the coupling model of the system with the acoustic cavity, rectangular thin plate and piezoelectric NES is established. The parameters of the piezoelectric NESs are adjusted according to the excitation amplitude, and then applied to the rectangular thin plate. The numerical results show that the piezoelectric NES causes the system to undergo strongly modulated response (SMR) and the resonance peak of radiation noise is effectively suppressed. It provides a new technique for passive control of low-frequency noise.

Machine learning assisted design of acoustic metamaterials with broadband sound-absorbing and superior mechanical performance

It remains challenging to design lightweight metamaterials with superior sound-absorbing and load-bearing capacities. One promising approach is using multiple coherently coupled resonators to serve as sound absorbers and reinforced grid cores of sandwich panels. However, with too many design parameters each requiring careful tuning to reach the stringent critical coupling condition, achieving an optimal design based on experience alone is challenging. An autoencoder-like neural network is constructed that, once well trained, significantly promote the inverse design process thanks to the highly efficient computational speed. Particularly, a probabilistic model is inserted into the network to tackle ill-posed inverse problems requiring man-made and probably unreal spectrum as an input, and can provide multiple ultra-thin (32 mm) and broadband designs. We have fabricated and tested the optimized metallic sandwich structures, showing broadband capacity of using solely nine resonators to achieve quasi-perfect sound absorption (over 0.9) from 399 to 675 Hz. The static compression and dynamic impact testing also verify the superior mechanical performance with high yield strength (21.2 MPa) and large normalized energy absorption (0.29) at lower relative density (13.5%). We believe this work is encouraging to accelerate the design of multifunctional absorbers targeted especially for low-frequency noise control in engineering.

Measurement of Compound Sound Sources with Adaptive Spatial Radiation for Low-Frequency Active Noise Control Applications

The proposed compound sound sources for low-frequency noise control applications are composed of dipole sources. Their spatial radiation, which is critical in the modal field of small, closed spaces, is intended to be controlled with independent driving signals of each dipole. The need for small and efficient low-frequency elementary monopole sources led to the proposed vented sub-woofer loudspeaker design with low force factor (low-Bl) drivers. The investigated sources are set up in quadrupole configurations and measured in terms of polar near field response patterns to verify the theoretical predictions. The measurement results consist of the validation of the proposed compound sound sources on the implementation of active noise control problems in the low-frequency range. Also, their small size and modular construction make them interesting for use in other applications.

Low-frequency broadband absorber with coherent coupling based on perforated panel and space-coiling channels

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

A versatile dynamic noise control framework based on computer simulation and modeling

Abstract This article attempts to effectively reduce the impact of active noise pollution on human life, and to make up for the traditional passive noise control technique. In low-frequency noise control, there are some shortcomings. The making of active noise control (ANC) technique, in low-frequency noise reduction, can achieve very good results. This article proposes a versatile dynamic noise control framework based on computer simulation and modeling. The research is mainly focused on the principle and application of versatile dynamic noise control framework. To accomplish this, a research method combining theoretical analysis, software simulation, and hardware realization is adopted. The derivation process of the adaptive algorithm (LMS algorithm, filter-XLMS algorithm, etc.) is introduced in detail, and the influencing factors of algorithm performance, a variable step size normalization algorithm based on relative error is proposed. Perform simulation calculations on various algorithms in MATLAB, analyze parameters such as step factor, filter order, etc., and the degree of influence on the algorithm’s convergence speed and steady-state performance. Common command set software is used, the path adaptive identification is realized, and the program design of the versatile dynamic noise control framework is used. After completion of software and hardware debugging on the experimental platform of generalized comfort, the experimental equipment layout is completed. Using the additive random noise method, the adaptive offline modeling of the first path of the versatile dynamic noise control framework is realized. Finally, utilizing the experimental platform of generalized comfort, the adaptive ANC experiment of the single-channel filtered least mean square algorithm is conducted, then the experimental data are analyzed, and at last, the actual application effect of the versatile dynamic noise control framework is verified.

Active Low-Frequency Noise Control Implementing Genetic Algorithm on Mode Coupling of a Compound Source

A system for low-frequency noise control in small, enclosed sound fields is proposed, using compound sound sources optimized by a genetic algorithm (GA). It is the integration of the developed low-Bl driver compound sources with a GA computer program in the Python language, aiming to control the modal field. The lack of appropriate free space in small rooms is critical for positioning the secondary sound sources; therefore, the proposed system has been designed to adapt to any available position. Two quadrupole topologies of the secondary compound source are applied and examined in a room. The convergence of the algorithm to the optimal solutions is attained through parametric configuration. The spatial radiation of the compound source at a single fixed position is adapted to couple with the modal noise field and attenuate it. The experimental results indicate that the proposed system can successfully control resonances of different low frequencies down to 50 Hz at multiple positions. The tonal noise attenuation reaches up to 32 dB at 100 Hz, confirming the applicability of the small subwoofer loudspeaker configurations for low-frequency control. This new method offers a practical and effective alternative to the typical abatement techniques that use distributed monopole sources in limited spaces.

Design and analysis of experimental adaptive feedback system for active noise control (ANC) in a duct

The limitations of passive noise control methods impose a need for new technical solutions to solve the problem of reducing low-frequency noise, which is considered to be a dominant component of noise disturbance. In recent years, the subject of intensive research are the active noise control systems, which have aroused considerable interest and represent a promising solution to the problem of low-frequency noise control. This paper proposes a robust methodology for simplified design and analysis of an experimental active noise control system for real-time control of acoustic environment in a duct. The proposed feedback control model is based on using the LMS algorithm, combined with FxLMS algorithm for estimation and neutralization of the secondary path in the electro-acoustic system. The study shows the potential of the FPGA module and the Real-time module of cRIO from National Instruments, combined with the LabView software environment when applied in adaptive system for active noise control. The reliability and validity of the developed active noise control system is tested for a frequency range of 100 to 1000 [Hz], by measuring the amplitude-time domain in [V] and sound level in [dB]. The comparison of the experimental results shows great efficiency of the system at lower frequency range from 200 to 400 [Hz], where a maximum reduction in sound level achieved at a frequency of 200 [Hz] is 14 [dB] or 17 [%]. A significant sound level reduction is also achieved at both 300 [Hz] and 400 [Hz] which is 12 % or 10 [dB] in both cases. Given the analysis of the challenges and opportunities of the developed active noise control system, recommendations for advancements and future work are proposed.

Low-frequency duct noise control using coupled loudspeakers.

A duct noise control device is introduced based on a Herschel-Quincke (HQ) tube and electro-mechanical coupling. The device consists of the main duct segment and one set of connected loudspeakers, which functions as a side-by tube in a traditional HQ tube. The acoustic waves imposed on the upstream loudspeaker can be transmitted to the other loudspeaker via the connecting circuit immediately, which represents a fast track when compared with the wave transmission via the fluid medium in the main duct. It is the core noise attenuation mechanism in this silencer. The transfer matrix method is used to investigate the silencer performance. A periodic silencer array is also developed to broaden the bandwidth and increase the magnitude of noise attenuation. The results predicted by the plane wave theory fit well with the simulation using a three-dimensional finite element method. A simplified experiment was conducted to verify the silencing effect of the silencer and the accuracy of theoretical prediction method. Contrasting with a traditional reactive silencer, low-frequency noise attenuation can be achieved in a more compact and flexible way, which can be seen as an improvement in low frequency noise control.

Numerical modelling of acoustic metamaterial made of periodic Helmholtz resonator containing a damping material in the cavity

Acoustic metamaterials are frequently used in many fields such as aerospace, buildings and ground transportation industries for low frequency noise control applications. Different solutions based on membrane or Helmholtz resonators have been investigated in the past few years. In the present study, a numerical design of acoustic metamaterial made of Helmholtz resonator with a membrane in its cavity is presented. The resonator with a neck extended into the cavity is periodically embedded within a porous material. The membrane inside the resonator cavity is modelled as a linear isotropic elastic material with free and fixed boundary conditions. The transmission loss (TL) of the proposed metamaterial design, predicted by finite element method presents multiple resonant peaks while only one peak is obtained with a conventional resonator. Two TL resonance peaks are observed when the membrane circumferential boundary is free inside the resonator cavity. For fixed boundary conditions, more than two resonance peaks are obtained. The proposed metamaterial design can therefore be used in many industrial applications for low frequency noise attenuation.

Technical aspects of physical implementation of an active noise control system: challenges and opportunities

Active noise control systems have become a subject of intensive worldwide research that have aroused considerable interest as being a promising solution to the problem of low-frequency noise control. Advanced real-time signal processing technologies offer opportunities that have been adopted in many active noise control industry applications, as well as in the modern urban environment where noise reduction is gaining priority status. Depending on the application, the concept of active sound control can be implemented using different control strategies. The development and application of such systems requires in-depth knowledge and theoretical analysis in the field of digital signal processing, sensor technology, adaptive control, understanding in hardware solutions for acquisition and data processing, as well as software capabilities for modeling, visualization and control of signals. Also, the construction of such a system must give an overview of the choice and characteristics of its components in terms of the impact of the elements that make it up on its accomplishment. This paper provides an insight into the engineering aspects for implementation of an active noise control system in a duct, emphasizing all technical requirements that need to be analyzed and might further affect the systems overall performance.

A multifunctional ultra-thin acoustic membrane with self-healing properties for adaptive low-frequency noise control

This paper proposes a novel multifunctional ultra-thin membrane based on a Polyborosiloxane-based gel with stimuli-responsive sound absorption and sound transmission loss (STL) and characterised by excellent self-healing properties. This adaptive behaviour is the result of a dynamically activated phase transition in the membrane’s polymeric network which is given by the interaction with the travelling sound pressure wave. The presence and the extent of such phase transition in the material was investigated via oscillatory rheological measurements showing the possibility to control the dynamic response by modifying the Boron content within the polymer. Acoustic analyses conducted at different stimuli responses showed high and dynamic absorption (95%) at the absorption coefficient peaks and an adaptive shift to lower frequencies while sound amplitudes were increased. An average STL up to 27 dB in the frequency range between 500 to 1000 Hz was observed and an increased STL above 2 dB was measured as the excitation amplitude was increased. Results demonstrated that the new membrane can be used to develop deep subwavelength absorbers with unique properties (1/54 wavelength in absorption and 1/618 in STL) able to tune their performance in response to an external stimulus while autonomously regaining their properties in case of damage thanks to their self-healing ability.