Numerical realization of a semi-active virtual acoustic black hole effect

The Acoustic Black Hole (ABH) structure has been developed as a promising approach for passive vibration attenuation and noise control. The basic theory of ABH effect hinges on the geometry thickness gradual decreasing according to the power law. This feature of structure reduces flexural wave speed, resulting in "trapping" flexural wave into ABH indentation to achieve energy focalizing. In this work, the FE model of a plate embedded with ABH indentation and damp structure is established and excited by a series of harmonic forces respectively. The characteristics of energy distribution in this plate in full frequency band are investigated by the power flow method and wavenumber domain analysis. By transforming the spatial vibratory energy into wavenumber domain, the ABH effect is analyzed and compared with a uniform panel. Meanwhile, the dissipation effect of vibration and sound radiation energy has been studied with addition of damping material. Furthermore, the energy harvesting and dissipation performances of a plate embedded with heterogeneous ABH array are investigated in order to demonstrate the influence of ABH structure parameters and configuration. The research will be beneficial for the vibration energy control in full frequency band.

Interpretation of the Acoustic Black Hole effect based on the concept of critical coupling

This presentation explores the acoustic black hole (ABH) effect achieved through variation of duct wall impedance. Unlike the method of Mironov and Pislyakov, which utilizes tapered annular rings within the duct to achieve the ABH effect, this approach leverages mechanical impedance variation to create the desired acoustic behavior. By engineering the impedance profile along the duct wall, an axially decreasing local phase speed results that mimics the event horizon of a black hole, causing sound waves to be significantly attenuated and trapped. Governing equations are derived and solved using both the WKB approximation and numerical methods. The solutions reveal that specific impedance profiles can effectively decelerate and absorb acoustic waves, resulting in substantial reduction in sound transmission. Verification is performed using the fully coupled structural acoustic finite element algorithm Sierra/SD. This research further paves the way for the development of innovative acoustic materials and devices which utilize ABH principles to achieve enhanced noise control and acoustic wave manipulation. [Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under Contract DE-NA-0003525.]

An Acoustic Black Hole (ABH) is a scatterer, embedded in a panel, allowing passive vibration control without adding mass. In practice, it is achieved by means of a local reduction in thickness (axisymmetric pit with a parabolic profile) and the addition of a thin viscoelastic coating in a central region of uniform thickness (plateau). The vibration absorption induced by the ABH, allows the design of stiff, light and non-resonant panels. In this paper, we propose to study this effect for a three-layer sandwich panel (glass fiber skin/honeycomb core/glass fiber skin), which gives rise to both bending and shear effects. The equations of motion of the thick, symmetrical sandwich panel with variable characteristics is obtained within the framework of the zig-zag theory by applying Hamilton’s principle. These equations lead to a sixth-order analytical model, allowing the dispersion curves to be obtained, and then an analytical model of the scatterer inserted in an infinite panel. The analysis of the local modes of the scatterer allows the evaluation of its absorption; the interpretation of the ABH effect is carried out using its scattering matrix. The model allows in particular the analysis of the effect of shear on the ABH effect. Experimental tests, based on vibrational maps obtained by vibrometry, are used to discuss and validate the predictions.

In the recent decade, there are increasing numbers of reported studies on the use of acoustic black hole (ABH) effects for the damping of flexural vibrations in engineering structures. This effect can be obtained by the use of a high-order power-law profile change in the thickness of the structure to induce a gradual decrease in incident wave velocity, to almost zero, along the propagation distance. ABHs have great potential for mitigating vibrations and noise. This paper covers the review of one- and two-dimensional ABHs and the experimental and numerical study of 3D printed beams with embedded ABH features. Specimens fabricated with the 3D additive manufacturing process were studied due to its expanding applications towards aerospace and automotive industries. However, limitations to the smoothness of the end product may affect the ABH effect, since the printing is typically via bottom-up layering where geometric accuracy is only up to the layer thickness. The vibration response of the 3D printed ABH beams, which has not been reported previously, is analyzed using the mean quadratic velocity and structural intensity parameters.

Vibrational acoustic black holes (ABHs) have shown great promise for reducing structural vibrations and sound radiation in light fluids. However, it is still unknown whether the acoustic black hole (ABH) effect can be materialized in heavy fluids. This paper discusses this issue by developing a semi-analytical model on a simply supported ABH plate that vibrates and radiates sound into water. The proposed model is validated by finite element models and used to investigate the vibration and sound radiation properties of the ABH plate in different frequency ranges. The results show that the ABH effect can be systematically manifested in heavy fluids, as reflected by a significant increase in structural damping and a decrease in vibration and sound radiation. Numerical analysis of the radiation damping and mass loading effects shows that the radiation damping has little effect on the vibration reduction of the water-loaded plate. However, the mass loading effect mitigates the low-frequency drawback of conventional ABH structures in air, resulting in a broadband reduction in structural vibration and sound radiation from the water-loaded ABH plate.

A 2D Daubechies wavelet model on the vibration of rectangular plates containing strip indentations with a parabolic thickness profile

Acoustic Black Hole effect (ABH) is a passive vibration damping technique without added mass based on flexural waves properties in thin structures with variable thickness. A common implementation is a plate edge where the thickness is locally reduced with a power law profile and covered with a viscoelastic layer. The plate displacement in the small thickness region is large and easily exceeds the plate thickness. This is the origin of geometric nonlinearity which can generate couplings between linear eigenmodes of the structure and induce energy transfer between low and high frequency regimes. This phenomenon may be used to increase the efficiency of the ABH treatment in the low frequency regime where it is usually inefficient. An experimental investigation evidenced that usual ABH implementation gives rise to measurable geometric nonlinearity and typical nonlinear phenomena. In particular, strongly nonlinear regime and wave turbulence are reported. The nonlinear ABH beam is then modeled as a von Kármán plate with variable thickness. The model is solved numerically by using a modal method combined with an energy-conserving time integration scheme. The effects of both the thickness profile and the damping layer are then investigated in order to improve the damping properties of an ABH beam. It is found that a compromise between the two effects can lead to an important gain of efficiency in the low frequency range.

The acoustic black hole (ABH) has been developed in recent years as an effective, passive, and lightweight method for attenuating bending wave vibrations in beams and plates and reducing the sound radiation and structural-acoustic response of structures. The ABH effect utilizes a local change in the plate or beam thickness to reduce the bending wave speed and increase the transverse vibration amplitude. Attaching a viscoelastic damping layer to the ABH results in effective energy dissipation and vibration reduction. Surface-averaged mobility and radiated sound power measurements were performed on an aluminum plate containing an array of 20 two-dimensional ABHs with damping layers and compared to a similar uniform plate. Detailed laser vibrometer scans of an ABH cell (including the ABH and surrounding homogeneous plate) were also performed to analyze the vibratory characteristics of individual ABH cells and compared with mode shapes calculated using finite elements. The results showed that the surface-averaged mobility was reduced by up to 14 dB for the fully damped ABH plate compared to a uniform reference plate while also reducing the mass of the plate. The results demonstrated that the dynamics of plates with embedded ABHs can be characterized by low, mid, and high frequency ranges, with low-order local ABH modes contributing significantly to low frequency ABH performance. The effects of damping layer thickness and diameter were also investigated to assess ABH performance optimization. It was shown that the damping layer can have the added benefit of mass loading the ABH and enhancing low frequency performance. The results will be useful for designing the low frequency performance of future ABH systems and describing ABH performance in terms of design parameters.

The acoustic black hole (ABH) effect in waveguides is studied using frequency-domain finite element simulations of a cylindrical waveguide with an embedded ABH termination composed of retarding rings. This design is adopted from an experimental study in the literature, which surprisingly showed, contrary to the structural counterpart, that the addition of damping material to the end of the waveguide does not significantly reduce the reflection coefficient any further. To investigate this unexpected behavior, we model different damping mechanisms involved in the attenuation of sound waves in this setup. A sequence of computed pressure distributions indicates occurrences of frequency-dependent resonances in the device. The axial position of the cavity where the resonance occurs can be predicted by a more elaborate wall admittance model than the one that was initially used to study and design ABHs. The results of our simulations show that at higher frequencies, the visco-thermal losses and the damping material added to the end of the setup do not contribute significantly to the performance of the device. Our results suggest that the primary source of damping, responsible for the low reflection coefficients at higher frequencies, is local absorption effects at the outer surface of the cylinder.

The increasing prevalence of railway transportation has exacerbated rail vibration, which predominantly occurs in the wide frequency range of 500–2500 hz. While conventional rail absorbers effectively suppress rail vibration at specific frequencies, broadband suppression remains challenging. An innovative rail absorber utilizing acoustic black hole (ABH) effects is presented in this paper. The absorbers, distributed periodically along the rail length, entail exceptional flexural wave focusing effects through thickness tailoring to the power-law profile. Numerical analyses reveal three distinctive flexural wave suppression mechanisms enabled by this absorber: bandgaps near pinned–pinned resonance frequencies arising from periodically mounting, ABH-enabled broadband energy focusing and damping at medium and high frequencies, and dynamic absorber effects at specific resonance frequencies. The integration of these suppression effects leads to effective broadband rail vibration control. Parametric studies are conducted to provide guidance for absorber optimal design, and experiments validate the effectiveness of the ABH absorbers in broadband rail vibration control, advancing the development of next-generation rail absorbers.

Ring-shaped acoustic black holes for broadband vibration isolation in plates

The waveguide acoustic black hole (WAB) effect is a promising approach for controlling wave propagation in various applications, especially for attenuating sound waves. While the wave-focusing effect of structural acoustic black holes has found widespread applications, the classical ribbed design of waveguide acoustic black holes (WABs) acts more as a resonance absorber than a true wave-focusing device. In this study, we employ a computational design optimization approach to achieve a conceptual design of a WAB with enhanced wave-focusing properties. We investigate the influence of viscothermal boundary losses on the optimization process by formulating two distinct cases: one neglecting viscothermal losses and the other incorporating these losses using a recently developed material distribution topology optimization technique. We compare the performance of optimized designs in these two cases with that of the classical ribbed design. Simulations using linearized compressible Navier-Stokes equations are conducted to evaluate the wave-focusing performance of these different designs. The results reveal that considering viscothermal losses in the design optimization process leads to superior wave-focusing capabilities, highlighting the significance of incorporating these losses in the design approach. This study contributes to the advancement of WAB design and opens up new possibilities for its applications in various fields.

Many rooms struggle with the absorption of low-frequency sound due to its long wavelengths. The integration of existing solutions into these spaces is often challenging due to their intricate installations and large depths. To address this problem, a new type of resonance absorber has been developed: the Distributed Mode Absorber (DMA). It consists of a thin vibrating front panel and a volume of enclosed air behind it. This straightforward structure can be utilized to create acoustically functional furniture that can be seamlessly incorporated into rooms. This article is devoted to the structural optimization of the DMA front panel, using the Acoustic Black Hole (ABH) effect known within structural dynamics. A numerical model is constructed using a Finite Element Analysis (FEA) and examined numerically. Several geometric parameters of the ABH are studied with regard to their influence on the vibrations of the front panel. Prototypes are developed and manufactured based on these insights. The quality of the numerical model is verified during the subsequent validation. Finally, the sound absorption of the improved DMA is compared with that of the reference DMA.

制振は,安全な構造を創り,快適性を備えた工業製品を実現するために,工業的,環境的にも重要な技術である.自動車や建築物においては,吸音二重壁構造を用いて,振動や騒音の対策をしていることがある.Krylov により鋼板の先端に向かい伝搬する振動が理論的に無反射となる波動ブラックホール効果が提案されている.本報告では.吸音二重壁構造のベースプレートに波動ブラックホールと制振材を付与した構造の振動伝達特性を FEM および MSKE 法により,数値解析をした.また,ベースプレートの一組の対辺を固定した.