Narrow Results

We study the sound perturbation of Unruh's acoustic geometry and present an exact expression for the quasinormal modes of this geometry. We found that the quasinormal frequencies are pure-imaginary which give a purely damped modes.

This paper is motivationally based on the study revealing the characteristics of Acoustic Schwarzschild BHs in respect of particle dynamics, and weak gravitational plasma lensing. We discuss the particle dynamics by studying the effective potential, ISCO, for massive particle and photon motion. We consider the weak gravitation field to study the gravitational lensed photons. This purpose of lensing is served by taking under consideration three fields of plasma uniform plasma, singular isothermal sphere and a nonsingular isothermal sphere. Each field is separately incorporated to calculate the plasma deflection angle, which is further utilized in the image magnification associated with the source brightness for uniform plasma and singular isothermal sphere as a selective case. All the obtained results are compared with the Schwarzschild black hole case as a standard.

In order to solve the problem of strong penetration and difficult attenuation of low-frequency sound wave in traditional materials, several three-dimensional acoustic black hole superstructures are designed. First of all, multi-stage acoustic black holes are designed. It is found that their sound insulation coefficient is about 0.9 in the frequency range of 50–1600 Hz when the ration of the outlet tip diameter to the inlet diameter is $d/D=12/99=0.121$. Then, the acoustic black hole thin and light superstructure was designed by embedding many acoustic black hole units in an array on the 10 mm thick plate. The sound insulation coefficient of two samples embedded 81 or 144 acoustic black holes is above 0.96 in the frequency range of 50–1600 Hz. To facilitate processing and engineering applications, we designed acoustic black hole wedge-shaped plate superstructures, and found that the average sound insulation of these acoustic black hole superstructures is 30 dB in the frequency range of 50–1600 Hz. These superstructures will be widely used in anechoic rooms, factories and aviation.

In this paper, a phononic crystal with acoustic black hole (ABH) characteristics is designed based on the compression effect of ABHs on acoustic wavelengths. The simulation results show that the lower limit of the first bandgap of the phononic crystal with ABH is reduced by 127.8Hz, the upper limit is increased by 694.4Hz, and the bandgap width is increased by 822.2Hz compared with that of the phononic crystal without ABH. The mechanism of bandgap expansion is discussed based on the mechanism of bandgap formation and the acoustic modulation effect of the ABH. The influence of the geometric and material parameters of the ABH on the bandgap is analyzed. The ABHs offer a new way of optimizing phononic crystals, and this work can be used as a reference for their design.

The acoustic black hole has good sound insulation performance in low frequency range. The transmission and insulation characteristics of acoustic black hole is investigated by experiments. First, we study the transmission and insulation characteristics of the acoustic black hole by numerical simulation. Second, we studied the sound transmission characteristic of multi-level acoustic black hole. Finally, the sound transmission and insulation characteristics of the acoustic black hole are studied by experiments. The influence of the acoustic black hole tip’s diameter on sound insulation coefficient is studied. The sound transmission characteristics of the first, two and three level acoustic black holes are also studied by experiments. Our numerical results show that low frequency acoustic energy can be effectively focused at the tip of the acoustic black hole, and can be effectively insulated by the acoustic black hole. Our numerical results are verified by the experimental results. Our study can provide a feasible method for controlling the low frequency noise.

We use numerical and experimental methods to investigate the low frequency sound insulation characteristic of designed thin acoustic black hole (ABH). The numerical results show that the sound energy focusing effect plays a leading role in low frequency sound insulation of designed ABH, and the reflection at the edge of ABH is the main reason of sound insulation in medium and high frequencies. Experimental results display that the Sound Transmission Loss (STL) of the designed ABH is higher than 30 dB below 700 Hz, which shows that the isolated acoustic waves are more than 95%. The low frequency sound insulation performance of proposed ABHs is much better than the traditional acoustic materials, which has great potential applications for low frequency sound insulation.

In order to achieve broadband sound absorption, we propose a composite structure combining acoustic black holes (ABHs) and micro-perforated panels (MPPs). Here, we adopt both simulant and experimental methods to describe the sound transmission mechanism of the proposed composite ABH. This mechanism allows the ABH to have a sound absorption coefficient of over 0.8 in the range of 400–3200Hz and above 0.7 between 250Hz and 400Hz. The total length of the proposed ABH is 155mm, in which the thickness of the MPPs is 0.5mm, the pore size is 0.3mm and the porosity is 0.1 and 0.16, overcoming the length size and bandwidth limit for sound wave suppression in current ABHs. This work can further progress in elucidating the acoustic characteristics of ABH and open new avenues in ultra-broad-band sound wave control.

Acoustic black hole (ABH), as a new wave manipulation technique, shows excellent applications in vibration and noise reduction of structures. Nowadays, most ABHs use materials with a fixed elastic modulus, limiting their low-frequency performance. Herein ABH plates with variable elastic modulus (VM-ABH) is designed, and its vibration and acoustic radiation characteristics are investigated by using numerical analysis. The results show that the vibration response of VM-ABH has a decrease of 5–13.2dB relative to that of the uniform texture ABH (UT-ABH) in the frequency range of 10–5000Hz, and the degree of energy aggregation is significantly improved. Moreover, the sound pressure level was reduced by 3.6 dB. Meanwhile, by linearly varying the elastic modulus in the center region of the VM-ABH, the effects of gradient index and terminal elastic modulus on the damping characteristics and dynamic response are revealed. The research results provide new objects for the study of vibration and noise reduction of ABH.

In this paper, we investigate the evolution of classical wave propagation in the canonical acoustic black hole by a numerical method and discuss the details of the tail phenomenon. The oscillating frequency and damping time scale both increase with the angular momentum l. For lower l, numerical results show the lowest WKB approximation gives the most reliable result. We also find that the time scale of the interim region from ringing to tail is not affected obviously by changing l.

The “acoustic black hole” (ABH) phenomenon can be exploited for flexural vibration suppressions in beam and plate structures. Conventional ABH structures, however, are tied with the inherent structural weakness due to the low local stiffness required and possibly high stress concentration caused by the small residual cross-section thickness of the ABH taper, thus hampering their practical applications. In this study, the dynamic and static properties of a compound ABH beam are investigated through numerical simulations. It is shown that, whilst ensuring an effective ABH effect, the compound ABH structure allows a significant improvement in the static properties of the structure. For the former, the compound design is shown to outperform its counterpart in the conventional ABH configuration in terms of the damping enhancement and the vibration suppression. For the latter, the compound ABH structure is also shown to provide much better static properties in terms of structural stiffness and strength. Meanwhile, the structural damping can be further improved by using an extended platform at the tip of tailored profile, which improves the structural strength but reduces the structural stiffness at the same time. Therefore, when choosing the platform length, a balance needs to be struck among the desired ABH effect and the mechanical properties of the structure.

The phenomenon of acoustic black hole (ABH) exhibits unique and appealing features when bending waves propagate along a structure with a tailored power-law thickness profile. The ABH-induced wave retarding and energy focussing are conducive to effective wave manipulation and energy harvesting. Using a PZT-coated ABH beam as a benchmark, this paper investigates the electromechanical coupling between the PZT patches and the host beam and explores the resultant energy conversion efficiency for potential energy-harvesting (EH) applications. An improved semi-analytical model, considering the full coupling among various electromechanical components in the system, is proposed based on Timoshenko deformation assumption and validated through comparisons with FEM and experimental results. Numerical analyses are then conducted to show typical ABH-specific features as well as the influence of the PZT layout on the electromechanical coupling of the system and the corresponding EH efficiency. Results show that ABH effects entail effective and broadband EH upon proper design of the system with due consideration of the PZT layout in relation to the wavelength and frequency range. Some design guidelines on the installation of PZTs are provided in view of maximization of the ABH benefits and the energy-harvesting performance.

An acoustic black hole (ABH) resonator is regarded as an efficient approach for controlling vibration caused by flexural wave energy. In this paper, the beam models with periodic ABH beam resonators are designed. Both the vibration absorption and isolation performances are investigated. Theoretical models based on the Transfer Matrix Method are presented to evaluate the reflection coefficient, which is validated both by the semi-analytic method combined with the Finite Element Method (FEM) and the Impedance Matrix Method. Meanwhile, FEM models of periodic ABH beam resonators acting as the beam terminator and isolator are established and analyzed. The results show that the periodic ABH beam resonators are of a better vibration reduction performance in lower frequency and have wider bandgaps for lower reflection coefficient and higher transmission loss than the single wedge. Moreover, with the increasing number of periods, the advantages of the periodic ABH beam resonators in reducing vibration become more obvious. Through the complex plane and dynamic analyses, it shows that multimode coupling and meta-damping effect lead to superior performance since the enriched modal content is introduced by the periodic ABH beam structure. This effect is also verified by the experimental result. Besides, the study also reveals the paradoxical relationship between vibration absorption and isolation performances. Additionally, parametric studies are conducted to disclose the effects of structural parameters. Based on the analyses, two approaches are proposed to enhance the vibration reduction performances, including the composite beam resonators and compound beam resonators. This paper illustrates a promising vision for applying the periodic ABH beam resonators to various vibration control fields.

This paper studies the bandgap properties and wave attenuation mechanisms of periodic beams embedded with a combination of acoustic black holes (ABHs) and local resonators (LRs). ABH refers to a retarding structure with a decreasing, power-lawed thickness profile, which gradually reduces the local phase velocity of incoming bending waves and thus traps the structural vibration energy within a confined area. Combining LR with ABH provides a practical approach to enhance structure vibration attenuation. To characterize the combined effects of ABH and LR, an energy-based formulation that uses B-splines as admissible functions is proposed. The B-spline basis functions can be allocated in a unique way such that the power-lawed variation of the beam profile can be accurately described despite the sharp thickness reductions and strong wave fluctuations in the ABH part. The vibration characteristics of the periodic beam are investigated under two scenarios: the resonance frequency of the LRs is tuned to coincide with the passband of the beam or the stopband of the beam. Improved vibration attenuations are observed in both scenarios, but the coupling behaviors and the underlying mechanisms are drastically different. To seek a clear explanation, an equivalent model of three degrees of freedom is established. By correlating the dynamics of the equivalent model with those of the beam model, it is found that the ratio between the stiffness of the resonator and that of the host beam plays an important role in forming new bandgaps. When the resonance of the LRs occurs in the passband of the ABH beam, the new bandgaps are a super-positioned effect of the original ABH bandgap and the LR bandgap. When the resonance of the LRs occurs outside the ABH bandgap, interactions between the LRs and the host beam are greatly enhanced, leading to an interesting frequency-splitting effect that dominates the formation of new bandgaps. Finally, the vibration responses of the proposed beam are investigated through experiments.