Acoustic Black Hole Research Articles

Broadband shock vibration absorber based on vibro-impacts and acoustic black hole effect

A vibration absorber combining the acoustical black hole (ABH) effect with vibro-impacts is presented as a separate device that can be mounted on a primary structure for passive vibration mitigation. The vibration isolator is shown to be effective for low-frequency shock vibration attenuation. Numerical and experimental results are reported, considering a rectangular honeycomb plate as the primary structure, on which the device, consisting of a circular ABH plate with one or several impact masses, is attached. A numerical model is developed using a finite element space discretization combined with a conservative scheme in a penalty approach for the contact dynamics. The analysis shows that the device has broadband efficiency thanks to the combined effects of energy transfer due to impacts and high attenuation in mid and high-frequency range due to the ABH effect. A detailed parametric study underlines that the tuning of the linear fundamental frequency of the impact mass plays an important role. Besides, considering several impact masses is shown to improve the efficacy. An experimental setup is then used in order to demonstrate the effectiveness of the device. The results confirm the vibration mitigation trends obtained from the numerical model that could then be used as a design tool for the attenuator. Finally, an analysis based on the Shock Response Spectrum (SRS), widely used in the field of aerospace engineering, is performed, underlining the attenuator is able to reduce rapidly and efficiently the vibrations due to a mechanical shock.

Acoustic characteristics of composite structures with 2D acoustic black holes and stiffened plates

Acoustic black hole (ABH) structure, a novel technology for modulating flexural waves, offers promising solutions for controlling noise caused by structural vibrations. In this paper, we introduce acoustic black holes into traditional stiffened plate structures and design a new type of ABH stiffened plate structure, which combines the wave manipulation ability of ABH with the load-bearing capacity of the stiffened structures. We establishes finite element models for calculating the free space sound field and closed cavity sound field of an ABH stiffened plate, builds corresponding experimental platforms, and tests them under point load excitation. The results show that ABH stiffened plate has great vibration and noise reduction effects, with a 5–10 dB reduction in sound radiation power above the cut-on frequency and a maximum noise reduction of about 30 dB. Through quantitative analysis from multiple perspectives, it is shown that ABH have an inherent mechanism of increasing damping level and reducing coupling between structure and sound field. Studying the influence of different media on the acoustic radiation of ABH stiffened plates, results further reveal that ABH have noise reduction effects in water media, and increasing the application scenarios of the ABH stiffened plate structures.

Study on the Influence Factors on Harvesting Capacity of a Piezoelectric Vibration Energy Harvesting System Covered on Curved Beam with Acoustic Black Hole

The acoustic black hole (ABH) structures have been shown to have great potential for energy harvesting. Within an ABH, the bending wave velocity decreases rapidly and the phase accumulates, resulting in localised energy accumulation. It is very significant that the energy can be harvested and power can be supplied for microelectronic devices. How to improve energy harvesting capacity is a problem that needs to be solved. Previous research on energy harvesting capacity of straight beams and flat plates with ABH has yielded a wealth of results. However, in practical engineering, curved beams are also commonly found. Given the differences in structure, it is of practical significance to study the influence factors on harvesting capacity of the piezoelectric vibration energy harvesting system covered on curved beam with acoustic black hole. First, the vibration characteristics of curved beam with ABH are analysed by the finite element method and localised energy accumulation is observed. Then, energy harvesting capacity is studied by means of the electromechanical coupling model in FEA; it has been found that energy harvesting capacity is lower in high frequency. The reason of this problem is analysed and solved by dividing the size of the piezoelectric sheet in an array layout. Based on this, the influence of array layout of piezoelectric cells on the energy harvesting capacity of the system is focused on. In addition, the influence of resistance value, material property, and curvature of curved beam on the energy harvesting capacity is analysed. Some meaningful results are obtained. These results provide the guidance to the design and optimisation for an energy harvesting system covered on curved beam with ABH.

Numerical computation of damped vibration and sound radiation for structures having a porous layer sandwiched by double walls with an acoustic black hole

Vibration and noise reduction are important technology to realize comfort products with safe structures. In this paper, we carry out numerical simulation of damped vibration and sound radiation for structures having a porous material sandwiched by double walls. The double walls are consisted of a steel base plate and a steel cover plate. The cover plate in double walls has a Krylov type acoustic black hole. Damping material is on the surface of the black hole. One point in the base plate is excited by white noise. Numerical analysis is performed to clarify changes of vibration transmission from the base plate to the cover plate due to the acoustic black hole using FEM and MSKE method(Modal Strain and Kinetic Energy method) proposed by Yamaguchi et al. Sound radiation from the cover plate is also investigated. The vibration transmission were reduced for the model having acoustic black hole in the cover plate. Vibration energy flows into the black hole in the cover plate. This causes that vibration acceleration of the cover plate is damped effectively. Sound radiation from the cover plate also decreased. This is because high damping and shorter wave length of vibration in the acoustic black hole.

Preliminary study of Acoustic Black Holes implemented in CT/MRI housings including validation experiments

An increasing number of interventional use of computer tomograph (CT) scanners and magnetic resonance tomographs (MRI) leads to the need for particularly quiet medical imaging in order to minimize the stress level for patients as well as medical staff. Due to the many different imaging sequences, noise reduction measures have to consider a wide range of different frequencies. Especially the use of Acoustic Black Holes (ABHs) seems to be a promising strategy to reduce vibration-induced noise. The objective of this study is to extend conventional measures for minimizing structural vibrations, usually realized by applying composite stiffeners, by adding ABHs directly into these stiffening components. This allows vibration maxima to be minimised efficiently. In the first step, the vibroacoustic properties of composite materials are investigated numerically and the created simulation model is validated by experiments. Based on these results, the influence of stiffeners on the structural behaviour is investigated and an optimised design for minimum surface velocities is found. To further reduce surface velocities in the frequency range where the human ear is particularly sensitive, ABHs are tested. In this context it is investigated whether ABHs can be advantageously combined with conventional stiffeners. Keywords: Medical devices, Acoustic black holes, Noise and vibration

A sonic black hole structure with perforated boundary for slow wave generation

Sonic black hole (SBH) allows for effective and broadband sound absorption in a retarding duct structure, in which two indispensable physical processes take place: wave retarding and energy dissipation. In this study, an SBH structure with perforated boundary (SBH-PB) is investigated, in which perforated acoustic boundaries are incorporated into the SBH to enhance the black hole effects through a simplified structural design. To predict acoustic performance and understand the physical mechanisms, this paper employs the transfer matrix method (TMM) to model and analyze the physics characteristics of an SBH-PB. Compared to the original form of SBH, the proposed SBH-PB is shown to entail better sound absorption owing to the increased energy dissipation by the small holes, while requiring a smaller number of inner rings. The validity of the TMM prediction is verified by experiments. Finally, the slow wave phenomenon in the retarding structure is also further studied and visualized by numerical simulation and experiment. The proposed structure holds promise for sound wave manipulation and the development of acoustic noise control devices.

Sound insulation design of vehicle side window glass based on acoustic black holes

Acoustic black holes (ABHs) can reduce the mechanical vibration and radiated sound of a structure while simultaneously reducing its mass. By considering the practical application, a new vehicle side window glass is designed using ABH array to optimize its sound insulation performance in this paper. The finite element method (FEM) is used to analyze the response and modes of the glass to validate its ABH effect. The accuracy of the Finite Element-Stational Energy Analysis (FE-SEA) model is verified using Gaussian expansion method (GEM) and Statistical modal Energy distribution Analysis (SmEdA). Using the verified FE-SEA, the transmission loss (TL) of the new side window glass and the influence of the number of ABH layers are analyzed. It is observed that the overall TL of the new glass is better than that of the traditional one, which can highly improve the TL at the coincidence frequency and meanwhile reduce the weight. It provides a new technique to design vehicle glass to improve vehicle sound insulation performance.

Design of full vehicle PBNR based on side windows with embedded acoustic black holes

Acoustic black holes (ABH) have been proven to be a lightweight structure with wide application prospects in wave tuning, energy recovery, and vibration and noise reduction. The side window glass is one of the important noise propagating paths into vehicle interior. In this paper, a new side window glass based on ABHs is designed and the influence of the new side window glass on the full vehicle PBNR is analyzed. A full vehicle SEA model is established and verified using the PBNR test. The transmission paths contributing the most noise energy to the driver's head cavity under excitations at different positions are analyzed. The results show that the side window glass is the main weak component needed to be optimized in the analysis frequency band. The influence of the new type of side window glass on the whole vehicle PBNR is studied based on the full vehicle SEA model. It is observed that by comparing with the traditional glass, the new glass can significantly improve the PBNR at the coincidence frequency of glass under different paths and meanwhile reduce the weight of the glass. It can provide a new technology for improving vehicle interior noise control.

Theoretical and numerical investigation into the suppressing mechanism of radiated noise in an ABH-cavity system

The problem of radiated noise of a coupling system between acoustic space and flexible structure is very common in practical applications. In this paper, the acoustic black hole (ABHs) structure is used for reducing the radiated noise of the plate-cavity system as an efficient passive control technology. The suppression mechanism of the ABH-cavity system is investigated. Theoretical analysis shows that enhanced radiated noise suppressing effect can be obtained by rearranging the numbers and locations of ABHs compared to its uniform counterpart at cavity modal frequencies. Due to this suppression mechanism, ABH structure exhibits a reduced radiation efficiency of the panel, which can be explained by the wavenumber analysis that lots of high wavenumber components are generated and effective wavenumbers in the radiation region are reduced.

Eigenvalue analysis and impact response analysis for T-shaped structures with new acoustic black hole including residual thickness supported by nonlinear concentrated springs

This paper describes numerical analysis using FEM with Model Strain Energy Method for structures having a T-shaped cross section supported by nonlinear concentrated springs under impact load. An edge of the rib plate in the T-shaped structure has an Acoustic Black Hole with residual thickness. A viscoelastic damping layer is covered on the black hole. Finite element for the nonlinear springs with hysteresis are expressed and are connected to the T-shaped structures modeled by linear solid finite elements in consideration of complex modulus of elasticity. We calculated modal loss factors and transient responses using eigenmodes including coupled motions between the nonlinear springs and the T-shaped structures. From the dominant modes and the impact responses, we clarified effects of the black hole with residual thickness on the nonlinear damped responses. By adding the black hole to the structures, modal loss factors increase. Higher modal loss factors are obtained due to set the residual thickness. As the amplitude of the impact force, the responses become complicated due to the nonlinearity, and include more super harmonic and subharmonic components. These nonlinear components are reduced by adding the black hole. The less nonlinear components are given due to set the residual thickness.

PROPELLER INFLUENCE ON SURFACE PRESSURE FLUCTUATION UNDER BOUNDARY LAYER INGESTION CONDITION

Research on propeller noise under boundary layer ingestion condition is essential for designing new aircraft concepts. Measurements of pressure spectra on a flat plate below a propeller give important information about the noise coming from the blades. These also are important for designing surface noise mitigation structures, such as acoustic black holes, meta-materials, and liners. This paper investigates the effects of a propeller on the flat plate surface pressure under boundary layer ingestion, using bottom of the wind tunnel channel as flat plate. Tripping devices on the wind tunnel bottom wall control the boundary layer thicknesses. The surface pressure is measured using an instrumented plate positioned underneath the propeller. The plate has 87 pressure taps to measure the unsteady and steady pressure using the remote microphone probe (RMP) technique. The results show the influence of the propeller inflow, the advance ratio, and the number of blades on the different regions underneath the propeller, and on coherent turbulent structures of the incoming flow. The boundary layer thicknesses affects the static pressure upstream of the propeller - creating high pressure regions. The propeller affects the turbulent structures downstream of the blades, increasing its correlation length.

Micro-perforated mufflers based on the Acoustic Black Hole effect

Attenuation of acoustic disturbance with compact and low-weight acoustical devices is a demanding requisite in industries such as vehicle exhaust mufflers, turbofan liners or ventilation systems. Acoustic Black Holes (ABH) is a concept that has been studied originally in the field of vibration control for the design of a structure based on a power-law thickness profiles along its edges. The idea of wave trapping and dissipation has been extended to the field of duct acoustics using compact devices that avoid blocking the air flow. Based on this idea, in this work we propose the study of a micro-perforated silencer that should prevent reflection and transmission of the sound waves over a wide range of audible frequencies. The use of micro-perforations over the facing cavities should help to extend performance towards the low frequency range. The acoustic behavior of the ABH is assessed from theoretical and numerical modeling. The performance can be greatly improved when the physical constituting parameters are optimized simultaneously. Particle swarm optimization is a computational method inspired by the behavior of natural systems that is able to provide a satisfactory solution with enhanced total dissipation.

Free-layer damping influence on the evanescent wave in a strip containing periodic acoustic black holes

This work demonstrates the transformation of wave motions in a periodic two-dimensional (2D) ABH structure with one-dimensional (1D) lattice subject to the damping dissipation. The free-layer damping (FLD) treatment is employed to achieve the damping dissipation. Numerical results, alongside analyses based on the local dispersion relation model, expose the effect of dissipation on bandgap (BG), which is always accompanied by the ABH-featured local resonance. Analyses show that the dissipative periodic ABH strip enables comparable attenuation intensity as that in its corresponding non-dissipation periodic ABH strip due to fact that the periodic ABH strip has two types of BG: (i) one shows infinite group velocity and constant lattice phase; (ii) another is accompanied by the discontinuous dispersion curves for the same flexural wave. The minimal attenuation inside these two BGs is less affected by the dissipation level. The discontinuity phenomenon inside the BG is attributed to the coupling between different flexural wave evanescent modes. As a result, the reduction of minimal attenuation inside the BG is less sensitive to the dissipation level. This attenuation variation subject to the damping dissipation inside the BG is distinguished from the weakened attenuation in traditional local resonance BG, in which there always exists a phase jump by π/a. Along with the ABH-enhanced dissipation, the dissipative periodic ABH strip enables broadband attenuation by inducing wide complex waves, while obtaining considerable attenuation intensity.

Thin metamaterial using acoustic black hole profiles for broadband sound absorption

The multi-pancake absorber is a thin acoustic metamaterial composed of periodically arranged thin annular cavities separated by the plates and connected by a main pore, enabling low frequency sound absorption. However, with a constant main pore radius, this structure exhibits tonal absorption behavior characterized by narrow peaks with varying amplitudes due to an imbalance in losses between the modes. This work investigates a geometric variation of the main pore, focusing on the designs with a gradually decreasing radius known as acoustic black hole profiles, to achieve broadband absorption. An analytical model based on the transfer matrix approach and lumped elements is proposed to predict the acoustic properties. Validation is conducted through thermo-visco-acoustic finite element simulations and impedance tube measurements. Multiple main pore profiles are investigated and the complex frequency plane representation is used to improve the balance of losses. An understanding of the influence of the main pore radius at the entrance and backing of the sample, allows design of a decreasing main pore radius profile that results in a high absorption coefficient value across a broad frequency range. Non-linear decreasing main pore profiles can be used, if necessary, to accentuate losses and achieve absorption peaks of similar amplitude.

Dynamic modeling of vibration behavior and power flow of a plate structure embedded with an ABH indentation

A two-dimensional (2D) semi-analytical model is established for the analyses of vibration behavior and energy transmission of the plate structure with a power-law-profiled thickness variation, referred to as an Acoustic Black Hole (ABH) plate. Under the general Rayleigh-Ritz framework, a modified version of 2D Fourier series is employed as the admissible function of the transverse displacement of the ABH plate featuring a thickness variation. Thanks to such modified Fourier series, a high-order spatial derivative continuity over the entire solution domain is available to express the internal forces and bending moments, further calculating the corresponding structural intensity and power flow within the ABH plate. The proposed model allows an accurate prediction of the modal characteristics and dynamic response, which is verified by the FEM and experiment, respectively. Several numerical analyses are proceeded and the typical dynamic phenomena, including the wave compression and efficient vibrational suppression of the 2D ABH plate are observed. The structural intensity (SI) vector arrows of the ABH plate are given, which represent the magnitude of the vibrational energy and the flowing direction in a visual pattern. The energy transmission behaviors and the manipulation of power flow by the ABH are revealed in an energy perspective. The typical energy focalization and dissipation phenomena of the ABH plate are analyzed in the frequency domain. Finally, the enhancement of the energy transmission by the ABH feature on the plate is explored by the normalized power flow. It is noted that the proposed model offers a visual energy transfer analysis tool for the 2D ABH structures, which may provide a novel perspective in various studies of ABH structures.

A symplectic analytical wave propagation model for forced vibration of thin plate with acoustic black hole indentation

In recent years, acoustic black holes (ABHs) have been revealed as an effective bending wave control technology and widely used to regulate the vibration and noise reduction performance of plate structure. However, the isolation effect of most traditional ABH plate can only be aroused at frequencies above the cut-off frequency. In order to widen the frequency range of the ABH effect, this paper embedded single and multiple ABH indentations at the end of plate. For the forced vibration response of the ABH indentation plate, an analytical wave propagation model was proposed based on the symplectic method. The governing equations of bending vibration of thin plate was first introduced into the symplectic dual system, then the symplectic analytical solution of wave propagation parameters and waveforms were obtained. A Gaussian-discretization scheme was used to discretize the ABH indentation, whereas the proposed method was used to evaluate the wave amplitude. The vibration response was finally obtained using the superposition principle and wave propagation, reflection, and transmission relationships. The effectiveness of the proposed method was verified using the finite element results. The results showed that the ABH indentation plate, especially the plate with multiple ABH indentations, had significant vibration reduction performance. Considering that embedding ABH indentation can weaken the structural rigidity of the plate, the ABH indentation plate with reinforcing stiffeners was then proposed to alleviate this limitation, and the influence of the width and quantity of reinforcing stiffeners on the vibration response was studied. The results indicated that the increase of width will to some extent weaken the ABH effect, and the quantity of stiffeners had a significant impact on the ABH effect.

Evolutionary strategy to optimize sonic black hole profiles in duct terminations

Sonic black holes (SBH) at duct ends constitute a particular type of anechoic termination. Acoustic waves get slowed down by means of a set of rings separated by cavities, whose inner radii diminish following a power law. Ideally, such design produces no wave reflection. In practice, however, this is not possible though the reflection coefficient remains low for a broadband frequency range. In this work, an evolutionary strategy (ES) is used to optimize the shape of SBH profiles and to try to improve the results of the standard power-law design. We test several cost functions for the reflection coefficient and impose constraints on the radii to fasten convergence. Different optimization problems are addressed. The first one is to determine the optimum exponent for the classical power-law profile. In the second one, the inner ring radii are only imposed to be monotonically decreasing from the SBH entrance. This results in new designs that overcome the performance of the optimum power-law profile at almost all frequencies. To perform the computations, we use a transfer matrix method for the SBH and resort to a derandomized ES with covariance matrix adaptation, which can easily deal with multiobjective optimization at a very reasonable computational cost.

Elastic wave reflection in beams with acoustic black hole termination under axial excitations

The wave speed in structures can be controlled by altering their stiffness through variations of the geometry and dimensions of the cross-section along its length. Theoretically, slowing down the elastic waves can lead to a zero-reflection effect, which is referred to as acoustic black hole (ABH). ABH has been demonstrated in metallic structures with isotropic-like material properties; while the effects of material anisotropy such as wood mechanical properties on the ABH effect are yet to be explored. We demonstrate numerically and experimentally the ABH effect in wood beams (Pinus radiata) of different lengths by applying an exponential reduction of the circular cross-section. The beams are knot-free and made of cuts of 40mm, 80mm, 100mm, 120mm and 160mm length along the grain, which could lead to a small Poisson ratio by about one order of magnitude. An increase in moisture content of the ABH wood beams by about 8-10% of the dry weights(N=5 samples) and a higher volume fraction of fibrous grains have been shown to reduce the wave reflection, as the drier and lower volume fraction the fibre, the higher the reflection becomes. This work can lead to design of novel elastic wave-guides for timber structures as used in buildings and wooden infrastructure or construction processes

The Modeling of Disc-shaped Acoustic Black Hole Plate and Vibration Characteristics

The Acoustic Black Hole (ABH) structure has great potential in vibration and noise control due to the characteristics of high damping and energy accumulation. A dish-shaped two-dimensional acoustic black hole plate is proposed in order to reduce the possibility of the structural damage happened in conventional embedded ABH in the centre of the plate. The dish-shaped acoustic black hole plate only cuts ABH on the edge. The finite element method is often used to establish the model of acoustic black hole plates. However, it is high cost and low efficiency in computation. A semi-analytical theoretical model of the disc-shaped acoustic black hole plate based on Gaussian expansion method is established in order to improve computation efficiency. Meanwhile, the shape function filtering method is proposed to solve the problem of non-convergence when Gaussian expansion method is used to model a non-rectangular plate. A finite element model of the disc-shaped acoustic black hole plate is established and used to verify the accuracy of the proposed semi-analytical theoretical model. The results show that the dish-shaped acoustic black hole plate has excellent energy accumulation effect and can be used further as Dynamic Vibration Absorber (DVA) to reduce the vibration of host structure.

Principles of progressive slow-sound and critical coupling condition in broadband sonic black hole absorber

Recent advances in sonic black hole (SBH) provide new opportunities for controlling sound waves and designing wave manipulation devices. SBH is a device that consists of partitions with gradually decreasing inner radii inserted into an acoustic duct. Several studies have reported that SBH can achieve a broadband sound absorption coefficient close to 1, avoiding the issue of alternating high and low absorption coefficients observed in traditional sound absorbers. However, the fundamental mechanisms and principles behind this behavior are not yet fully understood. This study aims to investigate the detailed sound absorption mechanisms of SBH, including the progressive slow-sound effect and the critical coupling condition that leads to broadband sound absorption. To achieve this goal, an analytical model based on the effective medium approach is developed to investigate the layer-by-layer retardation in sound propagation. The sound absorption coefficient is then determined based on the surface impedance calculation. The effective medium analysis reveals that SBH enables a unique condition to progressively decelerate wave propagation across its layers. As a result, the critical coupling condition becomes more easily established with smoothly increasing SBH partitions and more discretised layers, as elucidated by the complex frequency analysis results. The physical insights gained from this study reveal the distinctive features of SBH compared to classical sound absorbers, paving the way for its engineering applications.