Noise Control Applications Research Articles

Development of a plasma electroacoustic actuator for active noise control applications

Conventional loudspeakers generate sound through the vibration of a diaphragm, attached to a rigid frame through elastic suspensions. Although such construction is satisfactory for sound diffusion in steady environments, it is likely to fail in harsh conditions, which is often the case for active noise control applications. Plasma-based actuators appear to be a promising alternative since they do not involve any fragile moving parts. In this paper, a positive corona discharge actuator in a wire-to-mesh geometry is proposed in the perspective of active noise control applications, as it is capable of generating sufficient sound pressure levels with limited signal distortion. The study introduces analytical and numerical models aiming at characterizing the sound field generated by the corona discharge actuator. The numerical simulation can facilitate the designing of such transducers. The acoustic power of the experimental prototype is increased through the optimization of emitter wires arrangement. The comparison of analytical model and numerical simulation with the experiment is presented. The analytical model successively describes the low frequency sound pressure field, while the numerical simulation is valid in the broader frequency range.

Bridging-Coupling Phenomenon in Linear Elastic Metamaterials by Exploiting Locally Resonant Metachain Isomers

Elastic and acoustic metamaterials are man-made structures designed to control and manipulate waves through band gaps. The generalized bandwidth of metamaterials with locally resonant (LR) band gaps is usually narrow, limiting their applications in noise and vibration control. On the other hand, nonlinear metamaterials with dipolar resonances can present a broad chaotic band with low amplitude resonances due to bridging coupling, which is only observed under sufficiently large excitation amplitudes. In this work, we show that bridging-coupling phenomenon can also be observed in linear metamaterials with dipolar resonance, producing a special pass band with low amplitude resonances at low frequencies without excitation amplitude dependence. Such phenomenon emerges in a modified resonant metachain with zigzag interconnections and is based on the isomerism of traditional metamaterials. Although this modified isomer preserves the static stiffness and mass density (i.e., static performance) of the original metamaterial, its dispersive behavior presents interesting properties (i.e., branches with negative mass features and overlapping of opposite group velocity), resulting in an overall negative effective mass between the LR band gaps that is responsible for the remote interaction and the enhanced wave performance. In addition to the theoretical discussions, the bridging coupling is experimentally observed in a three-dimensional printed elastic metamaterial rod supporting longitudinal waves. The design can be extended to elastic bending, acoustic and electromechanical media, as well as to systems with periodicity in two and three dimensions (i.e., plates and solids). Therefore, the findings of this work open avenues to investigate the bridging-coupling phenomenon in linear systems as well as to explore the properties of metamaterial isomers.

Optimizing the bandwidth of plate-type acoustic metamaterials

Plate-type acoustic metamaterials (PAM) consist of a thin film with multiple periodically attached masses. Although these metamaterials can be very lightweight and thin, the resulting sound transmission loss at low frequencies can be much larger than the corresponding mass-law. This is a result of anti-resonances at which the sound transmission through the PAM is strongly reduced. One general challenge, however, is that the anti-resonances are only very narrowband. This makes the application of PAM to noise control problems with broadband noise sources or changing tonal sources difficult. In this contribution, different design strategies to improve the bandwidth of PAM for low-frequency noise control applications (multiple masses per unit cell or stacking multiple PAM layers) are evaluated using optimizations. An efficient modal based model is employed to represent the PAM using their eigenfrequencies and modal masses. The model is validated using simulations and experimental measurements. The optimization results show that it is possible to significantly improve the bandwidth of PAM using the investigated design strategies. In fact, it is shown that the same bandwidths can be achieved either using multiple masses or multiple PAM layers. This allows for some flexibility in the design of suitable noise control treatments with PAM.

Attenuation of low-frequency sound in U-shaped duct with membrane coupled acoustic resonator: Modeling and analysis

This study proposes a new solution to control low-frequency noise in acoustic duct. The control device is in the shape of a Helmholtz resonator combined with a tensioned membrane in the air cavity, referred to as a membrane resonator. The membrane resonator is connected to the main waveguide through two necks. A theoretical model is developed to evaluate its acoustic performance when attached to a U-shaped duct as the host structure. Numerical simulation shows that the proposed resonator generates effective transmission loss in the very low-frequency region, as a result of the vibro-acoustic coupling between the integrated membrane and the two-end Helmholtz resonator. Since the attenuation band is deep sub-wavelength, a lumped-mass model is developed to capture the main physics involved in the acoustical-mechanical coupling. With the developed models, the tuning range of the proposed system is further investigated. Several tuning mechanisms are demonstrated by changing the membrane tension, thickness, material type and combining multiple resonators. The transmission loss has fairly large variation range and the desired frequency band is fully controllable. The proposed resonator has simple geometry and mechanical design, whilst the deep sub-wavelength characteristics and the offered tunability are promising. It has the potential to be used solely or as the basic building block in a wide range of noise control applications.

Acoustic simulation of cavities with porous materials using an adaptive model order reduction technique

Porous materials are often used in the context of passive noise control applications due to their low cost and good sound absorption properties. As the most commonly used predictive tool, the finite element (FE) method is often applied in the design phase to obtain detailed information on the performance of complex vibro-acoustic systems with porous damping treatments. In this case, however, its use inevitably leads to very large, complex-valued and frequency-dependent numerical models, which makes original full-order model evaluation intractable due to time and memory limitations. In this paper, an adaptive model order reduction strategy is presented to reduce the number of degrees of freedom involved in the associated problems such that the required computational cost can be largely alleviated while a desired high accuracy can be achieved in a controllable way. This technique consists of making use of Taylor expansion to the multiple frequency-dependent scalar functions coming from the complex material behavior, and then performing the structure-preserving second-order Arnoldi algorithm based on a robust error indicator to solve the underlying FE model in the frequency domain. A further improvement in terms of the approximation accuracy of the constructed compact reduced-order model is also proposed to incorporate all available moment-matching information. Two widely used model types for porous materials are investigated, i.e. the equivalent fluid description and the Biot theory. Mono-physical poroelastic-poroelastic coupling and multi-physical acoustic-porous coupling are considered in order to demonstrate the simplicity, versatility and efficiency of the approach.

Sound absorption theory for micro-perforated panel with petal-shaped perforations.

Micro-perforated panel (MPP) absorbers with circular perforations are used in many noise control applications due to their attractive wide-brand sound absorption performance. Different from a common MPP with circular perforations, a unique type of MPP absorber with petal-shaped perforations is proposed. The sound absorption theory for the MPP with petal-shaped perforations is developed by accurately considering the fluid velocity in the petal-shaped perforation hole. This theory can account for the effect of altered perforation morphology (from circular to petal) on sound absorption. Finite element simulations are performed to validate the proposed theory, with good agreement achieved. The sound absorption of MPP with petal-shaped perforations is compared with that of the traditional MPP with the same porosity. It is demonstrated that the change in hole shape significantly modifies the fluid velocity field and the flow resistivity in/of the hole, and hence the sound absorption of the proposed MPP with petal-shaped perforations can outperform that of the traditional MPP in the considered case. This work proposes a general MPP theory that not only contains the classical Maa's theory for circular MPP, but also accounts for the MPP with petal-shaped perforations.

Controllably asymmetric beam splitting via gap-induced diffraction channel transition in dual-layer binary metagratings

In this work, we designed and studied a feasible dual-layer binary metagrating, which can realize controllable asymmetric transmission and beam splitting with nearly perfect performance. Owing to ingenious geometry configuration, only one meta-atom is required to design for the metagrating system. By simply controlling air gap between dual-layer metagratings, high-efficiency beam splitting can be well switched from asymmetric transmission to symmetric transmission. The working principle lies on gap-induced diffraction channel transition for incident waves from opposite directions. The asymmetric/symmetric transmission can work in a certain frequency band and a wide incident range.Compared with previous methods using acoustic metasurfaces, our approach has the advantages of simple design and tunable property and shows promise for applications in wavefront manipulation,noise control and acoustic diode.

Acoustic silencing in a flow duct with micro-perforated panel liners

Despite the increasing interest in Micro-perforated panels (MPPs) for various noise control applications, the acoustic behavior of MPP liners in flow ducts has not been fully apprehended. On the top of this is the lack of understanding on the influence of various design arrangements and system parameters on the performance of MPP-based acoustic silencing devices. Incorporating previously developed acoustic impedance formulae within the general framework of the Patch Transfer Function (PTF) framework, these issues are investigated in this paper in the context of a MPP liner, flush-mounted inside a flow duct wall. Numerical analyses reveal the effects of the grazing flow and different partition arrangements in the backing cavity of MPP liners on their silencing performance as well as the underlying physical phenomena. Capitalizing on the efficiency of the modelling approach, a system optimization is carried out. The numerically predicted noise attenuation results are validated through comparisons with experimental measurements under various grazing flow velocities. While shedding light on the underlying sound attenuation mechanism, studies provide guidelines for the design of MPP silencers in flow ducts.

An experimental and theoretical study on environment-friendly sound absorber sourced from nettle fibers

This is a comprehensive study on a sustainable sound absorber sourced from nettle fibers. The fibers were thoroughly characterized for properties linked to its performance as an effective and efficient sound absorber, like density, surface roughness, surface morphology, physical strength, and thermal stability. Nettle fibers were converted into non-woven form and characterized for its acoustical characteristics. The obtained results of sound absorption coefficients were analyzed to determine the optimum thickness for optimum performance during practical noise control applications. Further, a comprehensive comparison of experimental results and theoretical results obtained from empirical models were compared. The genetic algorithm and Allard model was used to estimate the characteristics impedance and complex wavenumber. These characteristics were further used to visualize the effect of the air gap behind the sound absorber. The results obtained showed nettle fiber to be an effective sound absorber. The results of theoretical and experimental studies matched well. The average noise reduction coefficient up to 0.72 was achieved, which was further boosted to 0.80 on incorporating a 30 mm air gap. The performance of nettle fiber with optimum thickness was compared with commercial glasswool, and waste jute fibers. The comparison established nettle to have better sound absorption properties.

Sound absorption of acoustic resonators with oblique perforations

Low-frequency airborne noise reduction is an issue of major concern in most practical cases due to the limiting space constraints. The applicability of acoustic resonators that not only work in this frequency range but can also be tuned is of great interest in many noise control applications such as muffler devices, noise barriers, or building isolation walls. This Letter studies the acoustic behavior of perforated panel absorbers with oblique perforations. Unlike more complex devices, the proposed absorber uses a simple concept that relies on the increase in the effective length of the panel by using perforations aligned obliquely with respect to the panel surface. In doing so, a shift of the resonance frequency toward low frequencies along with an increase in the sound absorption can be achieved provided that the geometrical characteristics of the absorber are properly chosen. A simple predictive model that relies on the fluid-equivalent theory was developed to investigate the acoustic properties of these absorbers, measurements in an impedance tube over additive manufactured samples serving to confirm the previous assertions. Preliminary results show the potential of these absorbers and encourage their further development for practical purposes.

Incremental Learning Based Adaptive Filter for Nonlinear Distributed Active Noise Control System

Active control of noise for multi-channel applications is affected by the existence of nonlinear primary and secondary paths. There is a degradation in the performance of linear multi-channel active noise control (LMANC) systems based on minimization of sum of squared errors obtained from multiple sensors in presence of nonlinear primary path (NPP) and nonlinear secondary path (NSP) conditions. The NPP and NSP problems are more prominent and challenging for multi-point noise control applications owing to different locations of silent zones and acoustic coupling between secondary sources and error sensors. In order to surmount this problem, an incremental strategy based nonlinear distributed ANC (NDANC) system is developed in this article. The adaptive exponential functional link network (AE-FLN) is employed as an adaptive control unit at the acoustic sensor nodes (ASNs) for the design of NDANC system. The incremental co-operation scheme is utilized to provide uniform noise cancellation in presence of NPP and NSP conditions. Simulation study is conducted extensively to demonstrate the efficiency of the proposed system for different practical NPP and NSP scenarios. The detailed computational load analysis and subjective evaluation of reduction in perceptual noise levels are performed for different real noise conditions.

Feedforward multichannel virtual-sensing active control of noise through an aperture: Analysis on causality and sensor-actuator constraints.

The multichannel implementation of the auxiliary-filter-based virtual-sensing (AF-VS) technique for active noise control applications is revisited and realized in the paper. Frequency-domain analysis based on random primary noise proves that the multichannel virtual-sensing active noise control (MVANC) technique can achieve optimal control at the desired virtual locations even if the signals at the physical and virtual microphones are not causally related. Further analysis on a number of sensor-actuator configurations shows that the MVANC technique achieves optimal control at the desired locations as long as the number of secondary sources does not exceed that of the physical error microphones. Furthermore, the simulations with measured transfer functions and real-time experiments conducted on a four-channel system validate the frequency domain analyses.

Improved interference-type sound barriers: Use of hyperbolic phase modulation

When sound barriers are constructed in parallel on both sides of a noise source, the reduction in noise through one barrier can be impaired because of constructive interference of sound reflected from the other. To eliminate this drawback, sound barriers with phase gradient surfaces have been designed in previous works. Without using any absorbing material, these barriers feature a linear phase profile with height and work by steering the reflected beam downward when sound impinges on them. Here, another interference-type barrier that possesses a nonlinear hyperbolic phase profile is further proposed, which aims to make the reflected wavefronts converge between the barriers, reducing the amount of noise energy that can escape. Results indicate that such a design has a significantly higher insertion loss than the linear case, and could have application in environmental noise control.

Model-based Bayesian direction of arrival analysis for sound sources using a spherical microphone array.

In many room acoustics and noise control applications, it is often challenging to determine the directions of arrival (DoAs) of incoming sound sources. This work seeks to solve this problem reliably by beamforming, or spatially filtering, incoming sound data with a spherical microphone array via a probabilistic method. When estimating the DoA, the signal under consideration may contain one or multiple concurrent sound sources originating from different directions. This leads to a two-tiered challenge of first identifying the correct number of sources, followed by determining the directional information of each source. To this end, a probabilistic method of model-based Bayesian analysis is leveraged. This entails generating analytic models of the experimental data, individually defined by a specific number of sound sources and their locations in physical space, and evaluating each model to fit the measured data. Through this process, the number of sources is first estimated, and then the DoA information of those sources is extracted from the model that is the most concise to fit the experimental data. This paper will present the analytic models, the Bayesian formulation, and preliminary results to demonstrate the potential usefulness of this model-based Bayesian analysis for complex noise environments with potentially multiple concurrent sources.

A Review of Active Noise Control Applications on Noise Barrier in Three-Dimensional/Open Space: Myths and Challenges

Active noise control (ANC), with counteracting sound in exact equal magnitude and opposite phase to the noise to be controlled, is often considered as a potential solution for solving complex noise problems. However, there are both myths and challenges in its implementations. In a crowded city like Singapore, many noise sources from construction site and subway track are located very close to the residential and commercial buildings. It was suggested by few researchers that by placing suitable control speakers at the construction site (working principle of ANC), the noise from the construction site could be prevented from propagating to the surrounding buildings. Similarly, for viaduct or subway track, by placing control speakers along the viaduct or track, the noise generated by the passing trains or vehicles could be reduced based on the principle of ANC technique. However, implementation of ANC technique on these noise issues is not easy as all of these noise control problems involve multiple noise sources with complex or transient frequency spectrum in large three-dimensional/open space. Therefore, the main intention of the present paper is to discuss the current state of the art of this topic as well as to examine the potential application and limitation of the ANC technique in mitigating unwanted noise, particularly in large three-dimensional/open space and with cooperation of passive noise barrier.

Novel Proportionate Adaptive Filters with Coefficient Vector Reusing

In this paper, a new family of adaptive filtering algorithms is presented, which aims to combine the small misalignment resulting from the reuse of past weight vectors with the fast convergence arising from the proportionate adaptation and logarithmic cost functions. This family of algorithms is obtained as a solution to a deterministic constrained optimization problem, by using the Lagrange multipliers technique, which differs from the traditionally employed stochastic gradient technique. Two special cases are proposed, namely the improved mu-law proportionate least mean logarithmic square with reuse of coefficients (IMPLMLS-RC) algorithm and the improved mu-law proportionate least logarithmic absolute difference with reuse of coefficients (IMPLLAD-RC) algorithm. An energy conservation relationship is established, which can be employed to perform stochastic transient analyses of the proposed algorithms. Simulations in system identification and active noise control applications show the advantages of the IMPLMLS-RC and IMPLLAD-RC algorithms over the traditional LMS and LAD, and the recently proposed LMLS and LLAD, with respect to both steady-state performance and robustness against impulsive noise.

Low-frequency perfect sound absorption achieved by a modulus-near-zero metamaterial

We have analytically proposed a mechanism for achieving a perfect absorber by a modulus-near-zero (MNZ) metamaterial with a properly decorated imaginary part, in which the perfect absorption (PA) is derived from the proved destructive interference. Based on the analysis, an ultrathin acoustic metamaterial supporting monopolar resonance at 157 Hz (with a wavelength about 28 times of the metamaterial thickness) has been devised to construct an absorber for low-frequency sound. The imaginary part of its effective modulus can be easily tuned by attentively controlling the dissipative loss to achieve PA. Moreover, we have also conducted the experimental measurement in impedance tube, and the result is of great consistency with that of analytical and simulated ones. Our work provides a feasible approach to realize PA (>99%) at low frequency with a deep-wavelength dimension which may promote acoustic metamaterials to practical engineering applications in noise control.

A thin multi-order Helmholtz metamaterial with perfect broadband acoustic absorption

We propose a multi-order Helmholtz metamaterial with deep-subwavelength thickness in which perfect continuous acoustic absorption is achieved within 400 Hz ∼ 2800 Hz. The metamaterial is composed of multiple detuned cells, each of which is constructed by several perforated plates inserting into the cavity of a Helmholtz resonator (HR) and hence gains multiple individually-tuned high-order peaks besides the original HR peak. By precisely designing each peak of the cells, the extra-broadband perfect absorption can be obtained. This kind of metamaterials could possess broad applications in noise control engineering owing to the extraordinary absorption performance and high flexural stiffness.

A wideband triple-layer microperforated panel sound absorber

Microperforated panels are light, clean and tunable sound absorbers, which fundamentals were already established in the previous century. They consist of a distribution of minute holes of diameter d on a panel of thickness t with perforation ratio ϕ in front of an air layer of thickness D. Therefore, the frequency band of absorption of a single-layer microperforated panel depends on (d, t, ϕ, D). In noise control applications, such a simple structure provides absorption in the frequency range of interest if holes are of sub-millimetric diameter. The former single-layer microperforated panels had two main weaknesses: narrow frequency band of absorption and a high manufacturing cost. However, nowadays these deficiencies can be easily surpassed. The absorption frequency band can be drastically increased by means of multiple-layer structures. The manufacturing cost may be reduced with the application of techniques currently used in the advanced circuitry industry. This is illustrated by the design of a triple-layer microperforated panel yielding absorption in almost four and a half octaves. The parameters of the triple-layer microperforated panel are optimised by Simulated Annealing to provide maximum absorption within a prescribed frequency band.

Design of periodic foam structures for acoustic applications: Concept, parametric study and experimental validation

Reticulated foams are widely used in noise control applications. However, difficulties in controlling the foaming process makes it difficult to control the microstructure and consequently the absorption characteristics of stochastic foams. Present work is a step toward designing an appropriate microstructure of foam to obtain desired acoustics performance using the concept of periodic foams. Periodic foam structures are created from designed unit cells by uniform spatial repetition of the unit cell in the three-dimensional domain. Three different unit cell configurations are created, characterized and compared using the Johnson-Champoux and Allard (JCA) model. These three unit cell configurations are further used to create periodic foam structures and prototypes of those structures are fabricated using additive manufacturing techniques. The measured characteristic impedance, complex wave propagation constant, absorption coefficient, and transmission loss for the fabricated samples are in close agreement with the JCA model predictions. Designed periodic foam structures proposed in this study offer a novel way to tailor the absorption spectrum to specifications in noise control and other acoustics applications.