Reflected Wavefront Modulation with Phase Array by Using Acoustic Metasurface

Acoustic metasurface has attracted increasing attention due to the ability of manipulating the transmitted and reflected phase of waves to generate various acoustic functionalities with planar layer of sub-wavelength structure.We design an acoustic metasurface with a semi-closed and nested slotted tube array,and it possesses the capacity of modulating the reflected phase with sub-wavelength thickness (about λ/23).The reflected phase shifts can be obtained from 0 to 2π by different rotation angles of internal slotted tubes.The theoretical results agree well with the numerical results by a finite element method.The results show that some excellent wavefront manipulations are demonstrated with the phase array by using acoustic metasurface such as anomalous reflection and sub-wavelength flat focusing.The design may offer a path for acoustic manipulation and promote the potential applications of acoustic metasurface in low frequency noise control,acoustic imaging and cloaking.

Bidirectional asymmetric acoustic focusing with two flat acoustic metasurfaces

We design an asymmetric transmission system(ATS)with two flat acoustic metasurfaces(AMs)to yield bidirectional asymmetric acoustic focusing(BAAF).The acoustic waves could be focused on both sides of the ATS with different focal lengths and intensities.To achieve high intensity energy concentration,the accelerating acoustic beams are selected to realize the BAAF.The working bandwidth of the BAAF based on our ATS could reach ～0.4 k Hz.It is found that by adjusting the distance between two flat AMs,the focal length and intensity of the bidirectional focusing could easily be modulated.Because the distance between two flat AMs is large enough,the BAAF even could be converted into a unidirectional acoustic focusing.The proposed BAAF may find applications in non-destructive evaluation,biomedical imaging and medical diagnosis.

Experimental realization of fractal fretwork metasurface for sound anomalous modulation

Natural creatures and ancient cultures are full of potential sources to provide inspiration for applied sciences.Inspired by the fractal geometry in nature and the fretwork frame in ancient culture,here we design the acoustic metasurface to realize sound anomalous modulation,which manifests itself as an incident-dependent propagation behavior:sound wave propagating in the forward direction is allowed to transmit with high efficiency while in the backward direction is obviously suppressed.We quantitatively investigate the dependences of asymmetric transmission on the propagation direction,incident angle and operating frequency by calculating sound transmittance and energy contrast.This compact fractal fretwork metasurface for acoustic anomalous modulation would promote the development of integrated acoustic devices and expand versatile applications in acoustic communication and information encryption.

Acoustics Performance Research and Analysis of Light Timber Construction Wall Elements Based on Helmholtz Metasurface

Based on the efficient sound absorption characteristics of Helmholtz resonance structures in the range of medium and low frequency acoustic waves,this paper investigates an effective solution for light timber construction walls with acoustic problems.This study takes the light timber construction wall structure as the research object.Based on the Helmholtz resonance principle,the structure design of the wall unit,impedance tube experiment and COMSOL MULTIPHYSICS simulation calculation were carried out to obtain the change rule of acoustic performance of the Helmholtz resonance wall unit structure.The research results show that the overall stability of sound insulation of the structure is improved,and the frequency range with sound transmission loss more than 50 dB in the experimental group is 640–1600 Hz,while in the control group is 500–906 Hz and 1238–1600 Hz;the sound absorption performance of the structure is obviously better than that of the ordinary structure,especially in the low frequency acoustic wave range of 100–320 Hz,the sound absorption coefficient of the experimental group is more than 0.49,while the sound absorption coefficient of the control group is less than 0.1.It is expected that these results will contribute to the optimization of the acoustic performance of light timber construction walls and have high application and popularization value.

Broadband design of acoustic metasurfaces for the stabilization of a Mach 4 boundary layer flow

A piecewise acoustic metasurface is designed to suppress the first mode while marginally amplifying the Mack second mode in a Mach 4 flat-plate boundary layer(BL)flow.The results of linear stability theory(LST)and the eN method demonstrate the stabilization effect and transition delay performance,respectively.However,the direct numerical simulation(DNS)results indicate that the designed broadband acoustic metasurface actually weakly excites the first mode with a slightly larger fluctuating pressure amplitude at the surface,which is in contrast to the analysis of LST.The discrepancies are found to lie in the‘roughness’effect caused by the recirculation zones inside the microslits and the alternating expansion and compression waves induced at the slit edges,which significantly amplifies the first mode.For further clarification of the competitive mechanism between the acoustic stabilization and‘roughness’destabilization effects of metasurfaces on the first mode,a carefully designed metasurface is installed at the maximum growth rate region,which excites the first mode on the metasurface but inhibits its development downstream.

Reconfigurable source illusion device for airborne sound using an enclosed adjustable piezoelectric metasurface

Source illusion is an important issue in acoustic fields that has significant applications in various practical scenarios.Recent progress in acoustic metasurfaces has broken the limitation of manipulating large-scale waves at subwavelength scales and enables a better illusion capability,while there is still a problem that most previous studies are hampered by a lack of tuning capability.Here we propose a reconfigurable source illusion device capable of providing azimuthallydependent phase delay in real-time via changing the static voltage distribution.The resulting device is implemented by employing an adjustable piezoelectric metasurface with a subwavelength thickness that can achieve a full 2π-phase shift while maintaining efficient transmittance.The effectiveness of our mechanism is demonstrated via two distinctive source illusion phenomena of shifting and transforming a simple point source without changing the device geometry.We anticipate that our methodology,which does not require a large device size or a complicated phased array,will open up new avenues for the miniaturization and integration of source illusion devices and may promote their on-chip applications in a variety of fields,such as acoustic camouflage and manipulation precision.

Refined acoustic holography via nonlocal metasurfaces

Holography can provide the desired wavefront phase and/or amplitude for imaging,particle manipulation,bacteria trapping,and cell patterning in optics and acoustics.However,previous work on acoustic holography is mostly based on local design optimization,either using active control of the sound source or relying on the structural design to provide the desired wavefront.Achieving precise control over the acoustic field remains a significant challenge.Here,we realize refined single-plane symmetric binary amplitude,asymmetric intensity gradient amplitude,and bi-objective hologram through the non-local holographic imaging theory that considers the acoustic coupling of structural units in detail.By taking into account the self-radiation and mutual radiation between many small units on a plate of well-designed thickness,as well as the transmission through the plate's apertures,we can effectively regulate the sound field behind the plate.We demonstrate the effectiveness of our approach through numerical simulations and experiments,showcasing a circle,a black hole,and a bi-objective with a circle and a square hologram.Notably,the acoustic black hole hologram precisely reconstructs the intensity gradient distribution at two bright spots.This non-local holographic imaging theory is valuable for the fine-intensity regulation of the sound field and is expected to be applied in ultrasound diagnosis and treatment,medical imaging,and other fields.

Controlling acoustic orbital angular momentum with artificial structures:From physics to application

Acoustic orbital angular momentum(OAM)associated with helicoidal wavefront recently attracts rapidly-growing attentions,offering a new degree of freedom for acoustic manipulation.Due to the unique dynamical behavior and inherent mode orthogonality of acoustic OAM,its harnessing is of fundamental interests for wave physics,with great potential in a plethora of applications.The recent advance in materials physics further boosts efforts into controlling OAM-carrying acoustic vortices,especially acoustic metasurfaces with planar profile and subwavelength thickness.Thanks to their un-conventional acoustic properties beyond attainable in the nature,acoustic artificial structures provide a powerful platform for new research paradigm for efficient generation and diverse manipulation of OAM in ways not possible before,enabling novel applications in diverse scenarios ranging from underwater communication to object manipulation.In this article,we present a comprehensive view of this emerging field by delineating the fundamental physics of OAM-metasurface interac-tion and recent advances in the generation,manipulation,and application of acoustic OAM based on artificial structures,followed by an outlook for promising future directions and potential practical applications.

Efficient conversion of acoustic vortex using extremely anisotropic metasurface

Vortex wave and plane wave,as two most fundamental forms of wave propagation,are widely applied in various research fields.However,there is currently a lack of basic mechanism to enable arbitrary conversion between them.In this paper,we propose a new paradigm of extremely anisotropic acoustic metasurface(AM)to achieve the efficient conversion from 2D vortex waves with arbitrary orbital angular momentum(OAM)to plane waves.The underlying physics of this conversion process is ensured by the symmetry shift of AM medium parameters and the directional compensation of phase.Moreover,this novel phenomenon is further verified by analytical calculations,numerical demonstrations,and acoustic experiments,and the deflection angle and direction of the converted plane waves are qualitatively and quantitatively confirmed by a simple formula.Our work provides new possibilities for arbitrary manipulation of acoustic vortex,and holds potential applications in acoustic communication and OAM-based devices.