State Key Laboratory of Acoustics

We propose a scheme for generating high-efficient acoustic focusing capable of circumventing obstacles in the propagating medium. This distinct feature that is highly desirable for practical applications is realized by employing two symmetrical Airy beams, and a different type of acoustic lens is designed by using a zero-index medium to provide the required phase profile with extremely high resolution. Furthermore, the scheme has the flexibility of generating tunable focal length. We anticipate our design to open possibilities for the design of acoustic lens and have potential applications in various important scenarios such as biomedical imaging/therapy and non-destructive evaluation.

Based on anisotropic density-near-zero metamaterials, we demonstrate a planar hyperlens with resolution beyond the diffraction limit in both one and two lateral dimensions. In contrast to the cylindrical hyperlens with elliptical dispersions of finite anisotropy, the proposed planar hyperlens is designed with flat near-zero dispersion that supports wave tunneling with extremely high phase velocity for infinite large transverse wave vectors. Therefore, the acoustic evanescent waves immediately concentrate into the designed oblique path till the output surface, leading to a subwavelength resolution. Prototype hyperlens is constructed with a membrane-network by means of equivalent lumped-circuit model, and the subwavelength magnifying performance for a pair of one-dimensional line objects as well as the complex two-dimensional structure is demonstrated. This method provides diverse routes to construct hyperlens operating without the limitation on imaging region in practical applications.

In this work, we propose two adaptive receivers achieving enhanced range estimation capability through a joint exploitation of the oversampling and the spillover of target energy in adjacent range samples. To this end, a proper discrete-time model for the received signal is introduced. Then, the generalized likelihood ratio test (GLRT) and the so-called two-step GLRT are derived and assessed. The performance analysis, conducted using both simulated data and real recorded datasets, is aimed at assessing the effectiveness of proposed solutions, also in comparison with existing detectors sharing range estimation capabilities. The illustrative examples highlight that better detection performance and increased range estimation accuracy can be achieved by exploiting the oversampling at the price of an additional processing cost.

The extraordinary transmission in density-near-zero (DNZ) acoustic metamaterials (AMs) provides possibilities to manipulate acoustic signals with extremely large effective phase velocity and wavelength. Here, we report compact transformable acoustic logic gates with a subwavelength size as small as 0.82λ based on DNZ AMs. The basic acoustic logic gates, composed of a tri-port structure filled with space-coiling DNZ AMs, enable precise direct linear interference of input signals with considerably small phase lag and wavefront distortion. We demonstrate both theoretically and experimentally the basic Boolean logic operations such as OR, AND, XOR, and NOT with wide operational frequency ranges and controllability, by adjusting the phase difference between two input signals. More complex logic calculus, such as “I1 + I2 × I3,” are also realized by cascading of the basic logic gates. Our proposal provides diverse routes to construct devices for acoustic signal computing and manipulations.

In this study, the authors deal with the problem of adaptive detection of point‐like targets in Gaussian disturbance with unknown but persymmetric structured covariance matrix induced by the space and/or time symmetry of the sensing system. In this framework, they devise and assess two selective receivers exploiting the Rao test and the generalised likelihood ratio test design criteria. The performance assessment, conducted by Monte Carlo simulation, has shown that the proposed receivers can significantly outperform their unstructured counterparts and guarantee enhanced rejection performance of unwanted signals with respect to their natural competitors.

We design a two-dimensional broadband acoustic omnidirectional absorber (AOA) simply comprising homogeneous anisotropic metamaterials to enlarge the absorption cross-section of a smaller core with matched acoustic impedance as the previous AOAs do, which generally involve complicated design of gradient-index or negative-index materials. Furthermore, AOAs with asymmetric/large geometrics can be realized conveniently, which will otherwise require a complex redesign of parameters. The proposed scheme is also extendable to three-dimensional cases. An implementation using angularly distributed fins was demonstrated experimentally, showing the broadband functionality of the designed absorber. Such metamaterial-based acoustic absorbers may have potential applications in various fields such as acoustic energy concentration, noise control, etc.

We have designed a one-way acoustic mirror comprising anisotropic zero-index media. For acoustic beam incident at a particular angle, the designed structure behaves like a high-efficient mirror that redirects almost all the incident energy into another direction predicted by the Snell's law, while becoming virtually transparent to beams propagating reversely along this output path. Furthermore, the mirror can be tailored to work at arbitrary incident angle by simply adjusting its geometry. Our design, with undirectional reflection functionality and flexible working angle, may offer possibilities in space isolations and have deep implication in various scenarios like ultrasound imaging or noise control.

Previously, we proposed an improved multiband-structured subband adaptive filter (IMSAF) algorithm to accelerate the convergence rate of the MSAF algorithm. When the projection order and/or the number of subbands is increased, the convergence rate of the IMSAF algorithm improves at the cost of increased complexity. Thus, this paper proposes several approaches to reduce the complexity of the IMSAF algorithm, both in error vector calculation and matrix inversion operation. Specifically, three approaches are developed to efficiently calculate error vector. The first approach gives an approximate filtering, whereas the other two approaches can provide a fast exact filtering with or without update of the weight vector explicitly based on a recursive scheme. The decorrelation property of IMSAF is determined, and two simplified variants are developed to reduce the complexity as by-products, i.e., the simplified IMSAF (SIMSAF) and pseudo IMSAF algorithms. Then, we discuss the problem of solving a linear system of equations. The performance advantages, limitations, and preferable applications of various methods are analyzed and discussed. Computer simulations are conducted in the context of system identification to determine the principle and efficiency of the proposed fast algorithms.

Nonreciprocity in acoustics is of paramount importance in many practical applications and has been experimentally realized using nonlinear media, moving fluids, or time modulation, which regrettably suffer from large volumes and high-power consumption, difficulty in integration, and inevitable vibrations or phase noise. In modern Hamiltonian theory, the violation of system's reciprocity can be achieved via asymmetric Peierls phases, which typically involves with non-Hermiticity or time-reversal symmetry breaking. Here, we propose a framework for designing nonreciprocal acoustic devices based on the asymmetric Peierls phases that can be fully controlled via active acoustic components. The fully controlled Peierls phases enable various high-performance acoustic devices, including non-Hermitian extensions of isolators, gyrators, and circulators, which are otherwise impossible in previous approaches that are bound by Hermiticity or passivity. We reveal that the transmission phases in isolators are equivalent to the Peierls phase plus a constant. The nonreciprocal phase delay in gyrators and the unirotational transmission behavior in circulators result from the gauge-invariant Aharonov-Bohm phases determined by Peierls phases. Our work not only uncovers multiple intriguing physics related to Peierls phases but also provides a general approach to compact, integratable, nonreciprocal acoustic devices.

Silica-based aerogels are widely regarded as promising sound-absorbing materials due to their low density and high specific surface area. However, their hard surface and small pores hinder sound wave penetration, resulting in a relatively poor sound absorption performance. To overcome this limitation, our study employs melamine foam (MF) as a scaffold to construct a gradient aerogel composite acoustic absorber. This innovative design significantly leads to a low average density (31.3–117.4 mg/cm3) and large density gradient up to 13.7 mg/cm4 (approximately 2.7 times difference compared with the lowest density), and its sound absorption properties are greatly improved, achieving an average absorption coefficient of 87% over the entire frequency band and 95% above 2000 Hz for 30 mm samples. In addition, the best noise reduction coefficient can reach 0.59. For demonstration, simulations further reveal the role of the pore size in enhancing sound absorption. The large pores in the foam skeleton facilitate the coupling of sound waves into the structure, while the small pores in the aerogel effectively block sound wave transmission, providing additional pathways for acoustic energy dissipation. Moreover, the incorporation of aerogel significantly enhances the foam’s mechanical properties. In terms of thermal insulation, the presence of aerogel markedly improves the foam’s insulating performance. This gradient design not only expands the potential applications of aerogels in sound absorption and thermal insulation but also provides a novel approach for the development of advanced acoustic materials.

An acoustic focusing lens incorporated with periodically aligned subwavelength grooves corrugated on its spherical surface has been developed. It is demonstrated theoretically and experimentally that acoustic focusing achieved by using the lens can suppress the relative side-lobe amplitudes, enhance the focal gain, and minimize the shifting of the focus. Use of the lens coupled with a planar ultrasound transducer can generate an ultrasound beam with enhanced acoustic transmission and collimation effect, which offers the capability of improving the safety, efficiency, and accuracy of targeted surgery implemented by high intensity focused ultrasound.

Bloch wavefunctions in crystals experience localization within the bulk when disorder is introduced, a phenomenon commonly known as Anderson localization. This effect is considered universal, being applicable to all types of waves, quantum or classical. However, the interaction between disorder and topology-a concept that has profoundly transformed many branches of physics-necessitates revisiting the original Anderson localization picture. For instance, in the recently discovered topological Anderson insulator, the introduction of disorder induces topological boundary states that can resist localization due to protection from line-gap topology. While line-gap topology applies to both Hermitian and non-Hermitian systems, non-Hermitian systems uniquely exhibit point-gap topology, which has no Hermitian counterparts and leads to the non-Hermitian skin effect. Here, we experimentally demonstrate disorder-induced point-gap topology in a non-Hermitian acoustic crystal. This crystal, with non-Hermitian disorder in nearest-neighbor couplings, exhibits the non-Hermitian skin effect, where all eigenstates localize at a boundary. Interestingly, the boundary where localization occurs-either the left or right-depends on the strength of the disorder. As the disorder strength increases, the direction of boundary localization can be reversed. Additionally, we observe a "bipolar" skin effect, where boundary localization occurs at both the left and right boundaries when disorder is introduced in next-nearest-neighbor couplings. These findings experimentally reveal a non-Hermitian mechanism of disorder-induced localization that goes beyond the conventional framework of Anderson localization.

In this study, a new nonlinear acoustic method that makes full use of the sensitivity of higher harmonics caused by internal microcracks is proposed to locate multiple cracks in a one-dimensional bar. The tested bar was coupled with an acoustic emitter, the frequency used by the transducer is the eigenfrequency of the bar, and the boundary of the emitter can be treated as free. A mass load was added to the other end of the bar to provide an asymmetrical boundary condition. The receiver was scanned along the propagation direction of acoustic wave. The relationships between the amplitude, the phase of the harmonics, and the positions of the cracks can be used to locate multiple cracks in the bar. The higher harmonic frequencies induced due to crack nonlinearity are different from the eigenfrequencies of the bar. This creates an amplitude difference in the vibration on each side of the cracks and can be used to locate multiple cracks in a one-dimensional bar. Numerical calculations were carried out to verify the method and possible approaches to improve the accuracy of this method are discussed.

In the fast moving scene, the underwater acoustic communication (UWAC) channel has extremely complex time-varying characteristics. The lack of standardized channel model severely limits the development of mobile UWAC technology. In this paper, the channel time-varying impulse response (TVIR) is estimated based on the measured mobile UWAC data, and the correlation coefficient, spread function and envelope of the channel are analyzed. The analysis results show that the fast movement between the transmitter and the receiver will lead to serious deviation and amplitude variation of the multipath arrival time, which will cause large Doppler frequency offset and spread, and lead to the decline of channel correlation. In this paper, the statistical characteristics of the measured channel are analyzed, which is helpful to the development of mobile UWA channel model.

Path planning plays an important role when an Autonomous Underwater Vehicle (AUV) is performing a cruising task such as underwater Simultaneous Localization and Mapping (SLAM). A path planning method accomplish the goals of avoiding obstacles and finding shortest paths, but may meet the problems that the AUV trapped into local optima and low efficiency. A novel path planning method based on APF-RRT* is proposed, which introduces gravity and repulsion functions of the APF algorithm to the RRT* algorithm. The proposed method avoids the problem of local optima trapping and low efficiency. Considering the impact of water flow and the movement of dynamic obstacles on the AUV, these two factors are introduced to the gravity and repulsion function, which improves the effectiveness of the algorithm in the underwater environment. Furthermore, Bézier curve is used to smooth the planned path to make the path more consistent with the kinematics of the AUV. The simulation results show that this algorithm can quickly complete path searching and generate reachable paths in both static and dynamic underwater environments. After path optimization, it is more suitable for AUV driving and operation in underwater environments.

Flexible phased-array ultrasound transducers (PAUTs), promising for nondestructive testing in biomedical and industrial applications, are classified as stretchable or bendable. Stretchable PAUTs offer a superior solution for complex curved surfaces but face substantial variations in piezoelectric element pitches, particularly on surfaces with small radii, complicating position correction algorithms. Meanwhile, real-time measurement of the pitches remains a technical difficulty. In contrast, bendable PAUTs with limited variable pitches are currently more practical for engineering applications. This paper introduces an innovative bendable PAUT featuring a constant-pitch-preservation (CPP) design. It includes a flexible 12 × 12 piezo-composite element array bonded with silicone, allowing conformity to surfaces with varying curvatures while maintaining constant element pitches. This design enables accurate progressive time delays for precise ultrasound beam steering and focusing. Individual backing and matching blocks for each piezoelectric element enhance detection performance. Experimental results from pulse-echo inspections and sector scans validate its effectiveness in high-quality imaging.

Channel equalization plays a crucial role in underwater acoustic (UWA) communications. However, there is a performance gap between conventional equalization methods and the optimal minimum mean square error (MMSE) solution. Recently, a turbo equalization scheme enabled by a soft frequency-domain equalizer (SFDE) based on the Vector Approximate Message Passing (VAMP) algorithm, was introduced for single-carrier UWA communications. The VAMP-SFDE performs selfiterations between an inner soft equalizer (ISE) and an inner soft slicer (ISS), and approaches the performance of an MMSE equalization. Experimental results for single-input single-output deep-sea vertical UWA communications verified the superiority of the VAMP equalizer. In this paper, we extend the investigation to shallow-water multiple-input multiple-output (MIMO) horizontal UWA communications, where the multipath is generally more severe than that in vertical transmissions. As the signal-to-noise ratio (SNR) value is critical to the equalization performance, we incorporate an SNR estimation via the expectation-maximization (EM) algorithm into the VAMP-SFDE, leading to the EM-VAMP-SFDE. Experimental results showed the proposed EM-VAMP-SFDE not only outperformed traditional equalizers but also achieved extra gain over existing VAMP-SFDE with preset SNR value.

In this work, a complex-valued deep neural network (ℂ-DNN) aided channel tracking approach is proposed for underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) communications. The experiments with the at-sea-measured WATERMARK dataset indicate that, the complex-valued model can perform a near-optimal channel tracking, as well as saving 50% spatial resources compared to the real-valued counterpart. Besides, the model transfer with UWA environment mismatch is also validated in this paper.

ABSTRACT Denoising is a critical step in signal processing. We develop a method for random noise reduction in active source seismic data using spectrum reconstruction. Two methods are developed for modifying the observed data’s amplitude spectrum: one substitutes it with the source wavelet’s amplitude spectrum, whereas the other involves multiplying the source wavelet’s amplitude spectrum with the observed data’s amplitude spectrum. By reconstructing the modified amplitude spectrum while preserving the observed data’s phase spectrum, noise suppression is achieved. Extensive testing with theoretical models, synthetic shot gathers, and field data indicate a notable improvement in the signal-to-noise ratio (S/N) compared with the traditional band-pass filtering method. This method proves particularly effective for enhancing the S/N in the context of active source wide-angle seismic data used in offshore structural studies, eliminating the need for data segmentation based on offset, and thereby improving processing efficiency. Our method relies solely on a single complete cycle of the source wavelet, making it a purely data-driven solution. It has broad applications in processing active source or controlled source data with consistent source wavelets, including but not limited to seismic exploration, acoustic detection, and signal denoising in various ground-penetrating radars used on Mars, the moon, and earth.

With the development of technology, SLAM technology has been widely used in AUV trajectory tracking, where precise trajectory is the basis for AUV to perform underwater tasks such as 3D underwater environment reconstruction and navigation. Visual SLAM extracts image feature points to achieve robot trajectory tracking. However, due to the poor quality of underwater images, the effect of visual SLAM in underwater environments is not ideal. In this context, an improved image enhancement method for underwater robot SLAM is proposed. The method introduces a grayscale image enhancement algorithm to improve the quality of underwater grayscale images. The high-quality images enable the robot to extract more effective feature points for pose estimation. Considering the difference in object features between underwater and ground environments, an ORB vocabulary suitable for underwater environments is trained for ORB-SLAM3 loop closure, which associates the current data with all underwater data to perform relocalization and provides more effective data for backend optimization to eliminate cumulative errors. The effectiveness of the image enhancement is verified by comparing the number and matching of ORB feature points before and after enhancement. AQYACOL dataset is used to run ORB-SLAM3 in both structured and unstructured underwater environments, and the results show that the proposed method improves the robustness of ORB-SLAM3 in underwater operation.