Asymmetric Acoustic Transmission Research Articles

Acoustic vortex filter based on tunable metasurfaces

In this paper, we present an acoustic vortex filter (AVF) based on tunable metasurfaces, which can selectively filter the incident multiplexed vortices that carry different orbital angular momentum (OAM). Our metasurface-based AVF is composed of an upper acoustic metasurface (UAM) and a lower acoustic metasurface, of which the intrinsic topological charge (ITC) can be tuned by mechanically rotating the UAM along its central axis. Due to the critical order of the propagating vortex modes in waveguide, controlling the ITC of the AVF allows for the selective filtering of incoming multiplexed acoustic vortex beams based on the sound vortex diffraction in phase-gradient metasurface, which endows the vortex filter the capability that let the incident vortex of specific OAM pass through it. In the following demonstration, both in theory and experiment, we design the AVF and effectively filter the acoustic vortices with two opposite topological charges by simply altering the orientation angle of the UAM. Based on this, we further demonstrate its application in asymmetric acoustic wave transmission. Our work offers an approach to selectively filter the incident acoustic vortex, which improves the capability to control the acoustic OAM via metasurfaces.

Angle-dependent broadband asymmetric acoustic transmission in a planar device

Asymmetric manipulation of acoustic transmission is of fundamental interest for wave physics, and has attracted rapidly-growing attentions owing to the potential applications in diverse scenarios. Here we propose to realize angle-dependent asymmetric acoustic transmission by designing a planar structure comprising a gradient-index layer and a layer of homogeneous medium with relatively-lower index. We analytically derive the working frequency and angle range of the device with unidirectional mechanism. And the simulated results show that the proposed device gives rise to high-efficiency broadband asymmetric transmission by allowing acoustic waves normally incident on one side to pass, while behaving as an acoustic barrier blocking waves obliquely incoming from both directions as angle of incidence exceeds a critical angle. Bearing the advantages of simple design, broad bandwidth and switchable functionality, our scheme opens a route to the design of novel acoustic devices capable of adapting various circumstances, and may find applications in noise control, medical detection, etc.

Efficient asymmetric acoustic transmission based on the combination of a binary metasurface and a gradient index metasurface

We design and demonstrate the asymmetric acoustic transmission based on the combination of a binary metasurface (BM) and a gradient index metasurface (GIM). It yields efficient asymmetric acoustic transmission for two opposite incident directions: nearly total transmission for positive incidence but nearly total reflection for negative incidence. The underlying mechanism is ascribed to the diffraction characteristics of the BM and the GIM. We exemplify the BM and the GIM by coating unit cells consisting of three-layers of acoustic labyrinthine metamaterials, which provide full phase control and total transmission. Numerical simulations agree well with the theoretical analysis.

Tunable asymmetric acoustic transmission device based on acoustic grating and sonic crystal with point defect

A numerical demonstration is given of the feasibility of a newly designed tunable asymmetric acoustic transmission device, for which the acoustic transmission coefficients for perpendicularly incident left- and right-running waves are different at a particular single frequency within a forbidden zone. The proposed device consists of a hybrid, sonic-crystal-based resonant structure with horizontal periodicity and a one-dimensional periodic rectangular acoustic grating with vertical periodicity. The former structure exhibits a wide bandgap through which only waves with particular combinations of their frequency and propagation direction can tunnel. The direction dependence of the acoustic transmission is based on the geometrical asymmetry of the placement of the grating, and on the narrowband correspondence between the diffraction angle of the grating and the allowed transmission direction through the sonic-crystal-based structure. Angular and spectral tuning are possible by changing the grating and/or the sonic crystal periodicity. The device has potential applications in underwater acoustics for asymmetric transmission systems and secure communication systems.

ASSYMETRIC ACOUSTIC WAVE TRANSMISSIONS USING MODE CONVERSION

We propose a design for asymmetric acoustic transmission using mode conversion in a waveguide with gratings. First, we studied dispersion diagrams to determine the waveguide widths and clarified the mechanism of asymmetric acoustic transmission using the cutoff of a higher-order mode. Then we analyzed the sound pressure level and showed that asymmetric acoustic transmission is feasible by utilizing the mode conversion and cutoff. In addition, we investigated the structural dependence of the transmission rates for bidirectional propagation.

Switchable asymmetric acoustic transmission based on topological insulator and metasurfaces

Dynamically switching the symmetric and asymmetric acoustic transmission (AAT) is significant in sound manipulation. In this paper, we present the design of switchable AAT in a certain frequency band, which is composed of an acoustic valley hall (AVH) topological insulator and acoustic metasurface (AMS). The underlying mechanism of AAT is attributed to the valley-selective excitation of AVH edge modes and the arbitrary refraction angle controls of the AMS. The acoustic transmission can only occur when the AVH edge modes are excited by controlling the direction of the incident sound waves with the help of AMS. By moving the AMS, the device can shift from symmetric transmission to asymmetric transmission within the designed frequency band. Importantly, the acoustic transmission through the device is topologically protected, which is robust to bends and defects. The AMS based on the spiral-coiling structure is chosen to reach a high acoustic transmission. The performance of switchable AAT is validated with full-wave simulations. Our work may open a new method for introducing the advantages of topological insulators to AAT.

Diffraction control in a non-Hermitian acoustic grating

Losses in media have become fresh elements in the designing of asymmetric acoustic transmission and reflection without the nonlinear effect. In this paper, we demonstrate the diffraction control by equipping fine-tuned losses in a non-Hermitian acoustic grating. Compared to the Hermitian grating without losses, the positive first order diffracted wave with negative incidence will be suppressed perfectly, but the negative first order diffracted wave with positive incidence will be enhanced counterintuitively in the lossy diffraction grating. The physical mechanism is that losses can modulate the magnitudes of Bloch wave vectors through the interplay with the real modulation of the refractive index. Absorption is demonstrated as a powerful way to control the acoustic diffraction.

Ultra-broadband asymmetric acoustic transmission in a straight open duct

We propose a mechanism of broadband asymmetric acoustic transmission in a straight open duct, by breaking spatial symmetry using two geometrically-asymmetric components filled with natural material placed on two sides of duct. Thanks to low loss and dispersion of natural material, the resulting device enables high-efficiency and ultra-broadband transmission for plane waves coming from one port while maintaining an open passage for light, heat and background fluid. Our design with simplicity, ultra-lightweight, optical and thermal transparency and ventilating functionality opens up possibility for novel acoustic one-way devices and may have potential in diverse applications such as duct noise control.

Broadband and broad-angle asymmetric acoustic transmission by unbalanced excitation of surface evanescent waves based on single-layer metasurface

Currently, complicated structure, incident-angle selectivity, and narrow frequency band are the key drawbacks of the asymmetric acoustic transmission (AAT) devices. Here we tackle these problems by proposing a class of single-layer lossy acoustic metasurfaces. The broadband AAT performance is realized in a broad range of incident angles. When the incident angle is in the range between two critical values, which are derived in this paper, an external sound wave can be converted into an evanescent mode, and the total internal reflection occurs for backward sound. The incident sound wave can be negatively refracted for forward sound if the evanescent mode conversion condition is broken, representing the realization of the AAT. However, the AAT phenomenon cannot be observed outside of the range defined above. The proposed design of highly efficient broad-angle AAT can find applications in sound sensing and noise control.

Frequency-selective asymmetric transmission via the lossy acoustic metasurface

Asymmetric acoustic transmission (AAT) has been demonstrated using diverse acoustic metasurfaces (AMs). Here, we show that an emerging class of lossy AMs enables frequency-selective AAT. The asymmetric and symmetric transmissions can be realized at interested operating frequencies in specific incident angle ranges. The underlying mechanism is attributed to the inherent loss inside the labyrinthine structure of different arrangements. Our design introduces the freedom degree of frequency control for the achievement of the AAT devices.

Subwavelength asymmetric transmission device for underwater acoustic waves with multiple incident directions

Asymmetric acoustic transmission devices with less restrictions on incident waves, i.e. incident direction and wavelength, are an inevitable trend of its future development. In this study, we reveal the mechanism for the subwavelength asymmetric transmission for underwater sound with multiple incident directions in an assembly system, constituting of a cascade solid-fluid superlattice coupled with an acoustic grating (AG) immersed in water. Theoretical analysis and numerical simulations verified that acoustic waves are almost totally reflected for negative incidence with multiple incident directions, while having a high transmission for positive cases. Moreover, our design can work within a remarkable broad bandwidth and possesses a preserved waveform in the subwavelength region. The unique feature is attributed to the extraordinary ultra-wide forbidden transmission of the cascade structure, as well as the diffraction effect of AG at low frequencies. The perfect subwavelength performance of our design suggests its potential practical application in underwater communication and signal transmission.

Broadband high-index prism for asymmetric acoustic transmission

Narrow bandwidth and wavefront distortion are two shortcomings of some existing asymmetric acoustic transmission devices. In this letter, we propose a high-index prism for realizing broadband asymmetric acoustic transmission with uniform plane wave outputs. Numerical simulations and experiments are carried out to verify the theoretical prediction. The experimentally measured acoustic pressure fields, the energy transmission contrast, and the refracted angle in the high transmission direction are in good agreement with the numerically simulated results. Both the simulated and measured energy transmission contrast exceeds 10 dB within the broad frequency range of 2000 to 3500 Hz. The broadband performance is attributed to the spiral unit cell employed to construct the prism, which features a high refractive index within a broad bandwidth. Besides, the transmitted waves in the high transmission direction are uniform plane waves. Our approach provides a practical method to design a broadband asymmetric acoustic transmission device with uniform plane wave outputs and has potential in various applications, such as noise control and medical ultrasound.

Asymmetric transmission of acoustic waves in a waveguide via gradient index metamaterials

We demonstrate that asymmetric acoustic wave transmission in a waveguide can be achieved via gradient index metamaterials (GIMs). We theoretically prove that the acoustic wave can be efficiently converted to surface waves (SWs) via GIMs. The GIMs in a waveguide can allow the transmission of acoustic waves in one direction but block them in the other direction. This theory is validated by experiments. Our findings may provide new applications in various scenarios such as high-efficiency acoustic couplers and noise control.

Asymmetric acoustic transmission with a lossy gradient-index metasurface

We theoretically and experimentally present the asymmetric acoustic transmission phenomenon in a lossy gradient-index metasurface (GIM). The lossy GIM design is a reflectionless planar layer for acoustic waves freely transmitted for positive incidence (PI) for incident angles from 10° to 40° and strongly attenuated for negative incidence (NI) for incident angles from −10° to −40°. The underlying mechanism is ascribed to the loss-induced suppression of the high diffraction order of the period grating. The diffraction order associated with the period grating takes a value of m=0 (without multiple reflections) for PI and m=2 (with multiple reflections) for NI, and hence, the asymmetric acoustic transmission occurs. We exemplify the lossy GIM by the coating unit cells consisting of three-layers of acoustic labyrinthine metamaterials, which provide a full phase control and around 26% transmission loss. The experimental results agree well with the theoretical analysis and numerical simulations. Our design may offer applications in noise control and acoustic sensors.

Multi-objective optimization of asymmetric acoustic transmission with periodical structure

Asymmetric acoustic wave propagation is important for control and manipulation of the acoustic wave signals in various devices. However, previous approach to find optimal asymmetric acoustic transmission (AAT) is through repeatedly adjusting the geometrical parameters, thus causing time-consuming. Here we propose a study on the multi-objective optimization of the AAT, aiming to achieve the widest working frequency range (fr) and the highest transmittance peak (η) with regard to the design variables. For this purpose, the Radial Basis Function (RBF) neural work and finite element (FE) method are applied to obtain the valuable data in the procedure. Furthermore, local sensitivity analysis of design parameters on the fr and η are analyzed. Ultimately, the Non-Dominated Sorting Genetic Algorithm II (NSGA-II) is adapted for getting the Pareto-optimal solutions. The optimization results show great improvement for the overall performance of the AAT, which could be potentially significant in designing various sound devices.

Broadband asymmetric acoustic transmission through an acoustic prism

Narrow bandwidth and complex structure are the main shortcomings of the existing asymmetric acoustic transmission devices. In this letter, a simple broadband asymmetric acoustic transmission device is proposed by using an acoustic prism filled with xenon gas. The sound pressure field distributions, the transmission spectra, and the prism angle effect are numerically investigated by using finite element method. The proposed device can always realize asymmetric acoustic transmission for the wave frequency larger than 480 Hz because the wave paths are not influenced by the wave frequencies. The asymmetric acoustic transmission is attributed to normal refraction and total reflection occur at different interfaces. Besides, relatively high transmission efficiency is realized due to the similar impedance between the acoustic prism and background. And the transmitted wave direction can be controlled freely by changing the prism angle. Our design provides a simple method to obtain broadband asymmetric acoustic transmission device and has potentials in many applications, such as noise control and medical ultrasound.

Asymmetric acoustic transmission in an open channel based on multiple scattering mechanism

We report both experimentally and theoretically that asymmetric acoustic transmission is realized by an open channel with two triangle cavities of different sizes on each side immersed in air. This exotic phenomenon stems from asymmetric multiple scatterings induced by the triangle cavities, which is different from the previous devices based on acoustic metasurfaces. In addition, the influences of the temperature and machining inaccuracies on the asymmetric transmission effect are investigated in detail, and the asymmetric acoustic transmission still exists with the excitation of the cylindrical acoustic source. Our device is very simple and easy to be realized, and opens a new way to design the devices of the asymmetric acoustic transmission, which has great potential applications in acoustic rectifiers and acoustic diodes.

Broadband asymmetric acoustic transmission in a single medium by an array of heat sources

We report the realization of a broadband asymmetric acoustic transmission with six different-temperature heat sources in air. This exotic effect arises from the desired refractive index in propagation paths induced from heat sources of different temperatures and asymmetrical distribution, which avoids acoustic impedance differences between the heat sources and air and has no reflection energy loss. In addition, the influence of the viscosity of air, the thermal convection, and the temperature and length of the heat sources on the asymmetric transmission effect is investigated in detail. The results show that the proposed device has the advantages of broad bandwidth, high transmission contrast, and simple structure, which enable it to provide more schemes for sound manipulation. It has excellent potential applications in acoustic devices.

Ultra-broadband asymmetric acoustic transmission with single transmitted beam**Project supported by the Major Program of the National Natural Science Foundation of China (Grant No. 51239005), the National Natural Science Foundation of China (Grant No. 11404147), the Natural Science Foundation of Jiangsu Province of China (Grant No. BK20140519), China Postdoctoral Science Foundation (Grant No. 2015M571672), the Scientific Research Project for Graduate Students

We report both experimentally and numerically that ultra-broadband asymmetric acoustic transmission is realized by a brass plate and a right triangle reflector immersed in water. This exotic phenomenon arises from the asymmetric excitation of the leaky asymmetric zero-order Lamb mode in the brass plate induced by the incident angle of external bulk waves. The results show that the bandwidth of the asymmetric acoustic transmission could reach 2000 kHz, and the positive transmitted wave is only a single acoustic beam. The device has the advantages of ultra-broadband, single transmitted beam, and simpler structure, which has great potential applications in ultrasonic devices.

Controls of pass-bands in asymmetric acoustic transmission**Project supported by the National Basic Research Program of China (Grant No. 2012CB921504), the Major Program of the National Natural Science Foundation of China (Grant No. 51239005), the National Natural Science Foundation of China (Grant Nos. 11174142 and 11404147), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20140519), the China Postdoctoral Science

The controls of the pass-bands in an asymmetric acoustic transmission system are investigated numerically and experimentally, and the system consists of a periodical rectangular grating and two uniform brass plates in water. We reveal that the pass-band of the asymmetric acoustic transmission is closely related to the grating period, but is affected slightly by the brass plate thickness. Moreover, the transmittance can be improved by adjusting the grating period and other structural parameters simultaneously. The control method of the system has the advantages of wider frequency range and simple operation, which has great potential applications in ultrasonic devices.