For several years, acoustic topological insulators have received attention due to their unique ability to manipulate acoustic waves. However, acoustic wave manipulation due to acoustic topological insulators is based on Bragg scattering. This results in the wavelength being bounded by the lattice constant. In this paper, a new ultra-low frequency acoustic topological insulator structure is proposed using the labyrinth model in acoustic metamaterials. With a lattice constant of a0 = 60 mm, the unidirectional transmission frequency possessing a topologically protected edge state can be reduced to 684 Hz. This frequency is much lower than that of other structures with equal lattice constants. The length of the resonant cavity can be greatly increased by utilizing a triangular labyrinth structure at the symmetric position. Subwavelength Dirac cones can be constructed in the energy band structure by utilizing the local resonance effect. Topological phase transitions characterized by opposite valley Chern numbers can be achieved by changing the rotation angle of the scatterer. The results of simulations verify the existence of a topology-protected unidirectional transmission edge state on topological boundaries. The research in this paper provides a efficient structure for controlling low-frequency acoustic waves.

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 400–3200[Formula: see text]Hz and above 0.7 between 250[Formula: see text]Hz and 400[Formula: see text]Hz. The total length of the proposed ABH is 155[Formula: see text]mm, in which the thickness of the MPPs is 0.5[Formula: see text]mm, the pore size is 0.3[Formula: see text]mm 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.

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 127.8[Formula: see text]Hz, the upper limit is increased by 694.4[Formula: see text]Hz, and the bandgap width is increased by 822.2[Formula: see text]Hz 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.

In order to achieve low-frequency and broadband sound absorption simultaneously, we propose a structure that combines a sonic black hole with multilayer micro-perforated panels. Firstly, we present finite element models for composite structures based on sonic black holes and micro-perforated panels and describe the sound absorption mechanism of the composite structure by comparing the sound absorption phenomena of micro-perforated panels with sonic black holes and micro-perforated panels with ordinary circular tubes. Secondly, the effects of the end coordinates of the sonic black hole, the number of panels and the parameters of the micro-perforated panels are discussed. Thirdly, the theoretical model of the proposed structure is developed using the transfer matrix method. Finally, the sound absorption test of the proposed structure is carried out using impedance tubes. The test results show that the sound absorption coefficient of the sample with a geometric length of 203 mm reaches 0.8 at 223 Hz and stabilizes above 0.9 at 398-1600 Hz. The sound absorber based on a sonic black hole and multi-layer micro-perforated panels has excellent sound absorption performance and has great research potential and application value.

Two-dimensional materials with unique properties have received much attention in recent years. Current research is exploring their potential applications in various fields. Acoustic topological insulators have been an emerging branch in the field of acoustic metamaterials in recent years. Its strong unidirectional transmission capability has potential applications in acoustic sensors, filters, and beamwaves. However, to realize a wide range of applications of acoustic topological materials, it is necessary to solve the problems of multi-band and wide bandgap. To this end, this paper proposes a jointly resonant acoustic topological insulator structure. The structure achieves multi-band filtering, and three filterable bandgaps can be obtained below 6200 Hz. Meanwhile, the total width of the three filterable bandgaps accounts for 65.5

Previous studies have introduced topological concepts to the field of acoustics, greatly contributing to the development of acoustic wave-controlled transmission. It has evolved from acoustic wave transmission in a single frequency band to low-frequency and multiband transmission. This paper presents research on acoustic wave transmission control and the observation of transmission process properties. The paper proposes an acoustic topological insulator based on a composite resonant cavity that can change its topological boundary state and achieve controlled transmission of acoustic waves by adjusting the scatterer's rotation angle. At the same scale, this structure has a high internal space utilization and can make full use of the rotational scattering mechanism. In an air environment, the structure can create an acoustic topological transmission channel that can transmit acoustic waves in four bands. It has the advantage of having a wide frequency band. Furthermore, this study investigates the impact of channel defects on the transmission of acoustic waves, highlighting the fault-tolerant nature of these topological channels. These properties make it possible to control sound wave transmission along a predetermined path. These findings support the advancement of controllable acoustic wave transmission, which holds promise for applications in acoustic signal processing, acoustic stealth, and other related fields.

Acoustic black hole (ABH) is a technique capable of manipulating the propagation of flexural wave, and the sonic black hole (SBH) is a kind of ABH which is used to manipulate sound wave in a fluid medium. In this paper, we propose an SBH structure with labyrinthine units and combine with micro-perforated panel (MPP) to form a composite sound absorption structure. The sound absorption mechanism of the absorption structure is deeply investigated using numerical and simulation methods. The simulation reveals the sound absorption mechanism by acoustic streaming effects of composite sound absorption structure. We analyze the flow characteristics of the acoustic medium under acoustic excitation, and the effect of the flow field on the distribution of the acoustic field, and the energy dissipation distribution. Our theoretical results show that the sound absorption is attributed to the effects of sound energy focusing of ABH, the local resonance of MPP, and the acoustic energy localization and dissipation effect of labyrinthine units caused by large flow velocity gradients. Finally, the proposed composite sound absorption structure has good sound absorption performance, which is also confirmed by impedance tube experiments. It can provide a new way of thinking for the design and optimization of the SBH structure.

This paper investigates the acoustic streaming effects (ASEs) and mechanisms behind the transmittance of sound in a metallic micro-cavity acoustic black hole (ABH) structure with an abrupt cross-section and examines the sound field flow characteristics inside the micro-cavity ABH under the sound excitation, such as the velocity, acceleration and pressure fields. And the sound transmission mechanisms are characterized by the ASEs which can be obtained by solving Navier–Stokes equations. The numerical results show that the sharp increase in the velocity and acceleration at the ABH tip position is the main reason for the focusing of the sound energy. And the dramatic increase in the tip cross-section reduces the acoustic streaming velocity, which is the main reason for the attenuation of the sound energy. Additionally, the thermoviscous effect of the acoustic boundary layer can also dissipate the low-frequency sound energy. The sound insulation experiment shows that the proposed micro-cavity ABH structure has a sound transmission loss (STL) of over 15[Formula: see text]dB in the low-frequency regime. This research reveals the mechanisms of the ASE’s work on the sound transmission properties of the micro-cavity ABH and provides new insight into low-frequency sound wave suppression. The ABH structure proposed in this paper has excellent strength, bearing capacity and long lifecycle, so it can be applied in the construction industry through its integrated design of structure and performance.

Acoustic topological insulators, as a type of acoustic metamaterial, possess special acoustic wave manipulation capabilities. However, an acoustic topological insulator based on Bragg scattering requires its lattice constant to be equivalent to the wavelength. This means that constructing acoustic topological insulators efficiently in the low-frequency range is a challenge. To this end, this paper proposes a method to construct sub-wavelength acoustic topological insulators by adding spatially folded resonant cavities at symmetric locations. It can easily reduce unidirectional transmission frequency bands with topological protection to subwavelength scales. On this basis, it is even possible to realize effective control of the unidirectional transmission band over a great range by adjusting the length parameter of the folded resonant cavity. The control range can reach more than 4000Hz. Experimental results from finite element simulations further verify that this approach does not affect the topologically protected edge states and the topologically unidirectional acoustic transmission properties. This work provides a method for the construction of low-frequency acoustic topological insulators, as well as an efficient method for the accurate regulation of unidirectional transmission frequency bands over a wide range of frequencies.

Acoustic black hole (ABH) structures are widely used for vibration and acoustic waves control due to their ability to guide the zero reflection of elastic waves and the concentration of wave energy. However, ABH can hardly suppress the ultra-low-frequency waves. We propose the acoustic cloister to break the low-frequency limit of the cutoff frequency and realize the perfect ABH effect while suppressing the ultra-low frequency waves. Thus, the waves can be localized within this structure and realize the ultra-low frequency ultra-broadband bandgap. We theoretically elaborate the bandgap mechanism of the acoustic cloister and demonstrate the good robustness of the acoustic cloister, which is beneficial for generating stable ultra-low frequency nonlinear bandgaps. Nonlinear buckling theory has been applied to explain the ultra-low frequency nonlinear bandgaps of 3–22 and 24–28 Hz that appear in the experiments, which reduces the wave transmission by 20–40 dB, and it has been demonstrated that the bending stresses appeared in the experiments can generate and greatly extend ultra-low frequency bandgaps. In torsional excitation experiments, the acoustic cloister structure attenuates wave transmission in the 3–100 Hz range by 10–80 dB. Our work makes a significant contribution to advances in vibration and acoustic wave control.

This paper proposes an extended resonant structure to solve the problem that topological acoustic waveguides have a narrow bandwidth at low frequencies. This acoustic structure consists of a two-dimensional structure and a resonant cavity in the three-dimensional direction, and its essence is to extend the resonant cavity in the two-dimensional structure to the three-dimensional direction. The problem that the size of the resonant cavity is limited by the size of the two-dimensional structure can be solved by this special extension. At the same time, the resonant cavity can be maximized in the three-dimensional direction. The topological properties of the original structure are not affected as long as the radius of the resonant cavity is widened without changing the symmetry of the overall composite structure. The rotating scatterer remains a reliable method for realizing topological phase transitions. The effect of the resonant cavity length on the band position is obtained using the finite element method, and it is demonstrated that the topological acoustic waveguide has a wide operating band at low frequencies. Simulation results show that this structure still has a bandgap width of 100 Hz at a low frequency of 350 Hz. The topological acoustic waveguide structure proposed in this paper can provide a new idea for the study of low-frequency broadband acoustic topology, which promotes the control of low-frequency acoustic waves by the topological acoustic waveguide.

Acoustic topological insulators have received much attention in recent years, due to the topologically protected edge propagation modes they possess. However, in the current research, the boundary between 3D topological insulators and 2D topological insulators is too clear. This leads to the fact that the advantages of three-dimensional space cannot be well applied to two-dimensional structures. Therefore, this paper proposes a method to add resonant cavities in two-dimensional acoustic topological insulators to a three-dimensional direction. Taking advantage of the three-dimensional space, the arbitrary construction of resonant cavities can be realized. Together with the two-dimensional multilevel resonance, the construction of subwavelength Dirac cones in an arbitrary range can be realized theoretically. This variation is continuous, which means that arbitrary regulating of the subwavelength unidirectional transmission frequency band can be realized. Meanwhile, different from the ordinary two-dimensional acoustic topology frequency band regulating, this regulating method using a three-dimensional resonant cavity does not need to change the shape and size of the original scatterer, but only needs to change the overall length of the three-dimensional resonant cavity. In terms of the feasibility of the scheme, band regulating is truly realized. The results of finite element simulations suggest that this may also provide a new method for the construction of multi-band acoustic topological insulators. This paper explores the two-dimensional acoustic topology from a new perspective, realizing the arbitrary control of acoustic topological insulators in the subwavelength domain over an unlimited range of unidirectional transmission bands. It also provides a new potential method for the construction of multi-band acoustic topological insulators. This will provide great opportunities for the construction and application of novel acoustic topological components.

Achieving ultra-low and ultra-broad-band sound absorption and full-band sound insulation is a major challenge. Here, we propose a composite structure of a multilayer micro-perforated plate and acoustic black holes to achieve this purpose. Combining the stable sound absorption effect of the multilayer micro-perforated plate in the full frequency band and the sound insulation effect of the acoustic black hole in the low frequency and the excellent sound absorption effect in the high frequency, the excellent sound control effect of 600–3150 Hz absorption coefficient greater than 0.8 and 100–3150 Hz sound transmission loss greater than 50 dB is achieved. The acoustic properties of different components and different acoustic black hole outlet were evaluated by finite element method, and the principles of sound absorption and insulation of the composite structure were elaborated. Finally, the results of finite element method are verified by impedance tube experiments. This work can make further progress in elucidating the acoustic properties of the ABH and open up new avenues in the control of ultra-low and ultra-wide frequency acoustic waves.

In recent years, numerous studies have investigated the impact of imperfections on topological insulators, and certain defects have been observed. Former topological insulators have a limited range of topological transport bands and an irregular lattice structure, which hampers their usefulness in technological applications. Here, it’s implemented the topological transport of sonic waves in a phononic crystal array structure. This paper has shown that changing the rotating scatterer's angle can alter the topological phase, resulting in lattice band inversion. This establishes a topological channel that enables acoustic conduction. It is feasible to create multifarious channel structures and devise acoustic transmission systems with varying positions. The introduction of rotating scatterers enables the regulation of the influx of acoustic waves into targeted channels. And the robustness of its edge states was verified by using the finite element method. These outcomes furnish the sound wave control mechanism with a novel formation and urge the development of programmable sound wave regulation. Based on these properties, the technique of regulating topological wave propagation through the use of a topological insulator can also be extended to other systems, including filters and acoustic devices.

In recent years, there has been significant development in acoustic waveguides through the introduction of topological phase correlation concepts in acoustics. Many studies have shown the properties of acoustic unidirectional transmission. This paper presents a new ring-shaped acoustic topological insulator that is multi-band, unlike previous structures. The structure improves internal space utilization with the same dimensions, reducing acoustic wave conduction frequency in air environments. Additionally, the introduction of a multistage ring-shaped resonant cavity enables the conduction of acoustic waves in multiple frequency bands. This paper employs the rotational scattering mechanism to invert the topological phase of the lattice. This allows for the construction of two crystals with opposite phases, which can be used to create a topological channel for the transmission of acoustic waves. At the topological interface, the acoustic transmission losses within the four bands are small. Furthermore, this paper verifies by simulation that the defects have little effect on acoustic transmission. The paper's research offers potential for multiband acoustic control. (c) 2024 Author(s).

The local resonant phononic crystal structure is significantly smaller than the control wavelength so it is of major importance for controlling the sound field in the low-frequency band. However, lightweight and small local resonant phononic crystal structures still present a great challenge. In this paper, we propose a metamaterial for low-frequency damping. The band structures and eigenmodes are calculated using the finite element method, and the generation mechanism of the resonant band gap is analyzed. The results show high transmission losses in the band gaps of 26.8-30.8 Hz and 33.7-180.1 Hz. In addition, the effect of geometric parameters on the bandgap frequency is discussed, and the control of the low-frequency bandgap can be achieved by adjusting the geometric parameters. The results of the study provide a new design idea for obtaining phononic crystal structures with low-frequency band gaps.

In this paper, we propose a multi-layer micro-perforated panel structure based on a curled space for broadband sound absorption at low frequencies, which increases the number of perforated panel layers in a limited space using a curled space. The absorption coefficients of the structure under plane wave conditions were calculated using the transfer matrix method and the finite element method. It is demonstrated that the multi-layer micro-perforated panel structure can ensure high absorption (consistently over 90%) in the frequency range of 400 similar to 5000 Hz. The sound absorption mechanism of the multi-layer micro-perforated panel structure is investigated by using the acoustic impedance along with the reflection coefficient of the complex frequency surface. In addition, we also discuss the effects of the micro-perforated panel parameters on the structural sound absorption coefficient. The results show that the proposed multi-layer micro-perforated panel structure provides an excellent solution for sound absorption in a limited space.

In order to improve and broaden the absorption performance of Sonic Black Hole (SBH), we propose a modified SBH structure with micro-perforated panels which is embedded the multiple resonator cavities, and investigate the sound absorption effect of the structure. Characteristics of acoustic streaming effects are embodied in sound pressure and flow velocity distribution. Further, proposed SBH structure can achieve high sound absorption under 6.0 kHz. Based on the symmetry of the structure, the Two-dimensional axisymmetric Finite Element Method (FEM) model is proposed to study the sound absorp-tion mechanism, which shows that the change of flow velocity gradient of acoustic medium leads to the loss of acoustic energy inside the structure. By discussion on acoustic streaming effects of different fre-quencies, the resonator cavity and micro-perforated panels contributes more to the sound absorption performance at low frequencies, and SBH produces a sound absorption advantage at high frequencies. The theoretical calculation and simulation result match and prove the correctness of the sound stream-ing effect. Sound absorption tests by acoustic impedance tube confirm that the proposed structure can maintain a sound absorption coefficient of over 0.5 in the range of 0.05-6.0 kHz. The combination of micro-perforated panels and SBH structure is widely applicable to muffler design and has good applica-tion prospects. & COPY; 2023 Elsevier Ltd. All rights reserved.

This paper proposes a ventilation barrier for low-frequency sound insulation with a double-opening Helmholtz’s periodic structure. The energy band characteristics of the unit structure and the sound transmission loss of the ventilation barrier are calculated using the finite element method. The research results show that the ventilation barrier exhibits excellent sound insulation in the range of 323–803[Formula: see text]Hz, and the sound insulation band is consistent with the bandgap of the unit structure. In order to reveal the sound insulation mechanism, we have performed a modal analysis of the unit structure. Further, the effects of different dimensional parameters on the structural bandgap are analyzed by changing the structure. The results show that extending the length of the structure inlet or outlet can effectively reduce the start frequency of the low-frequency bandgap, thus achieving the regulation of the sound insulation band. The proposed low-frequency sound insulation ventilation barrier has a simple structure with great application prospects. In addition, the proposed ventilation barrier provides a new idea for noise control in low-frequency ventilation environments.

The potential value of acoustic topological insulators is of wide interest because of their special topology-protected edge-state unidirectional transmission properties. In this article, a frequency band-controlled acoustic topological insulator based on local dimensional coordination is designed. It is capable of controlling the unidirectional transmission frequency range by controlling the position of the band gap within a certain range. The energy band structure and eigenmodes are analyzed by using the finite element method. The range of control of the band gap position by the local dimension length is derived. The control of the band gap over the unidirectional propagation frequency is experimentally verified. This work provides a way to precisely control the unidirectional transmission frequency of topology-protected edge states in a specific band, opening up new possibilities for their potential value.