Topology optimization of chiral metamaterials with application to underwater sound insulation

Chiral metamaterials have been proven to possess many appealing mechanical phenomena,such as negative Poisson's ratio,high-impact resistance,and energy absorption.This work extends the applications of chiral metamaterials to underwater sound insulation.Various chiral metamaterials with low acoustic impedance and proper stiffness are inversely designed using the topology optimization scheme.Low acoustic impedance enables the metamaterials to have a high and broadband sound transmission loss(STL),while proper stiffness guarantees its robust acoustic performance under a hydrostatic pressure.As proof-of-concept demonstrations,two specimens are fabricated and tested in a water-filled impedance tube.Experimental results show that,on average,over 95%incident sound energy can be isolated by the specimens in a broad frequency range from 1 k Hz to 5 k Hz,while the sound insulation performance keeps stable under a certain hydrostatic pressure.This work may provide new insights for chiral metamaterials into the underwater applications with sound insulation.

Research Progress of Underwater Soundabsorbing Material

This article provides an overview of underwater sound-absorbing materials mainly applied with polyurethane matrix.It mainly elaborates on the underwater sound mecha-nism,commonly used underwater sound-absorbing materials and structures,as well as new underwater sound-absorbing material structures derived from local resonance pho-nonic crystals,such as phononic crystals,local resonance phonon wood piles,and meta-material sound-absorbing structures.This provides a broader development space and direction for the future development of underwater sound-absorbing materials.

Error analysis and calibration for underwater sound intensity measuring system

Based on measuring the cross-spectrum density of sound pressure between two hydrophones, the facility for underwater sound intensity measurement is investigated and designed. According to the principle of two-hydrophone method for intensity measurement, the error analysis is carried out. Given the method of sound intensity measurement calibration for this underwater sound intensity measurement facility, the uncertainty of intensity measurement by this facility is evaluated. It is shown that the analysis and evaluation are agreeable to the experimental results.

Grating‐like anechoic layer for broadband underwater sound absorption

To address the challenging task of effective sound absorption in the low and broad frequency band for underwater structures,we propose a novel grating‐like anechoic layer by filling rubber blocks and an air backing layer into metallic grating.The metallic gratings are incorporated into the anechoic layer as a skeleton for enhanced viscoelastic dissipation by promoting shear deformation between rubber and metal plates.The introduction of an air backing layer releases the bottom constraint of the rubber,thus intensifying its deformation under acoustic excitation.Based on the homogenization method and the transfer matrix method,a theoretical model is developed to evaluate the sound absorption performance of the proposed anechoic layer,which is validated against finite element simulation results.It is demonstrated that a sound absorption coefficient of the grating‐like anechoic layer of 0.8 can be achieved in the frequency range of 1294-10000 Hz.Given the importance of sound absorption at varying frequencies,the weighted average method is subsequently used to comprehensively evaluate the performance of the anechoic layer.Then,with structural density taken into consideration,an integrated index is proposed to further evaluate the acoustic properties of the proposed anechoic layer.Finally,the backing conditions and the boundary conditions of finite‐size structures are discussed.The results provide helpful theoretical guidance for designing novel acoustic metamaterials with broadband low‐frequency underwater sound absorption.

Target localization and parameters estimation by sonar system with explosions as underwater sound sources

Underwater target localization and parameters(azimuth and range) estimation by the method of utilizing explosions as underwater sound sources are described in this paper.The narrow beam reverberation model of the target echo signal is researched to estimate the target azimuth in reverberation background.Estimation errors of target azimuth and range are studied and proved to approximately meet Gauss distribution.Then the variance formula of target range error is deduced.Simulation experiments are applied to research the target range error and its standard deviation,and a series of measures to improve the estimation accuracy of target range are proposed.It is confirmed by the data processing results of simulations and lake experiments that the proposed method can accurately locate underwater target at a long distance on the condition of a certain underwater explosion range error.

An overview of research efforts to understand the effects of underwater sound on cetaceans

Cetaceans are aquatic mammals living in an environment that is more suited for hearing than vision.As such,their sensory systems largely utilize acoustic cues for navigation,communication,foraging,and predator avoidance.However,the elevation of underwater sound levels from increased human activities has adversely affected cetaceans’use of sound to perform vital life functions.To address those impacts,scientific studies have been conducted to understand the behavioral,psychoacoustical,physiological,and physical responses by cetaceans that have been exposed to anthropogenic sound.These studies range from captive experiments involving auditory thresholds and noise-induced threshold shifts,to field observations of behavioral disturbance from sound exposure,to post-mortem examinations of physical manifestations in stranded animals.Over the years,results from these studies have assisted regulatory agencies in developing a series of criteria and thresholds for cetacean conservation and management around the world.This paper provides a high-level overview of worldwide research efforts that have been dedicated to understanding the effects of underwater sound on cetaceans.The review is not intended to be exhaustive but rather to capture major efforts and significant findings in this field.In addition,the review excludes synthesis studies and modeling exercises that do not involve direct research on target species.

A method to accurately measure and respond to an underwater sound pulse in a wide dynamic range

This paper presents a method to control the gain within pulse and accurately measure the amplitude of an underwater sound pulse in a wide dynamic range. In the method a loop composed of a gain controlled unit and a microcomputer is employed.This method also gives satisfactory results when there exist distortion of signal cnvelop and fluctuation of signal amplitude. The basic mathematical model of instantaneous amplitude-gain control and amplitude measurement and its hardware structure are presented. The calibration method of the system for retransmitting (responding) with required sound level is given as well.

Simulating Underwater Plasma Sound Sources to Evaluate Focusing Performance and Analyze Errors

Focused underwater plasma sound sources are being applied in more and more fields. Focusing performance is one of the most important factors determining transmission distance and peak values of the pulsed sound waves. The sound source’s components and focusing mechanism were all analyzed. A model was built in 3D Max and wave strength was measured on the simulation platform. Error analysis was fully integrated into the model so that effects on sound focusing performance of processing-errors and installation-errors could be studied. Based on what was practical, ways to limit the errors were proposed. The results of the error analysis should guide the design, machining, placement, debugging and application of underwater plasma sound sources.

Analysis of underwater decoupling properties of a locally resonant acoustic metamaterial coating

This paper presents a semi-analytical solution for the vibration and sound radiation of a semi-infinite plate covered by a decoupling layer consisting of locally resonant acoustic metamaterial. Formulations are derived based on a combination use of effective medium theory and the theory of elasticity for the decoupling material. Theoretical results show good agree- ments between the method developed in this paper and the conventional finite element method （FEM）, but the method of this paper is more efficient than FEM. Numerical results also show that system with acoustic metamaterial decoupling layer exhibits significant noise reduction performance at the local resonance frequency of the acoustic metamaterial, and such performance can be ascribed to the vibration suppression of the base plate. It is demonstrated that the effective density of acoustic metamaterial decoupling layer has a great influence on the mechanical impedance of the system. Furthermore, the resonance frequency of locally resonant structure can be effectively predicted by a simple model, and it can be significantly affected by the material properties of the locally resonant structure.

Parallel optimization of underwater acoustic models:A survey

Underwater acoustic models are effective tools for simulating underwater sound propagation.More than 50 years of research have been conducted on the theory and computational models of sound propagation in the ocean.Unfortunately,underwater sound propagation models were unable to solve practical large-scale three-dimensional problems for many years due to limited computing power and hardware conditions.Since the mid-1980s,research on high performance computing for acoustic propagation models in the field of underwater acoustics has flourished with the emergence of high-performance computing platforms,enabling underwater acoustic propagation models to solve many practical application problems that could not be solved before.In this paper,the contributions of research on high-performance computing for underwater acoustic propagation models since the 1980s are thoroughly reviewed and the possible development directions for the future are outlined.

Pentamode-Based Coding Metasurface for Underwater Acoustic Stealth

Coding metasurface draws amounts of research interests due to its potential for achieving sophisticated functions in wave manipulation by using simple logical unit cells with out-of-phase responses. In this paper, we present a novel acoustic coding metasurface structure for underwater sound scattering reduction based on pentamode metamaterials. The metasurface is composed of two types of hexagonal pentamode unit cells with phase responses of 0 and π respectively. The units are arranged in random 1-bit coding sequence to achieve low-scattering underwater acoustic stealth effect. Full-wave simulation results are in good accordance with the theoretical expectation. The optimized arrangement resulted in the distribution of scattered underwater acoustic waves and suppression of the far field scattering coefficient over a wide range of incident angles. We show that pentamode-based coding metasurface provides an efficient scheme to achieve underwater acoustic stealth by ultrathin structures.

Energy-distributable waterborne acoustic launcher for directional sensing

Highly directional launch and intensity adjustment of underwater acoustic signals are crucial in many areas such as abyssal navigation,underwater signal communication,and detection for marine biology.Inspired by the phenomenon that aquatic animals like dolphins detect and track prey with high resolution,we propose an energy-distributable directional sensing strategy which can achieve parallel needle-like transmitting sound beams with adjustable energy based on out-coupling valley-polarized edge states.The acoustic spin angular momentum and energy flow distribution at different interfaces inside the phononic crystal are provided and they show tight coupling.Furthermore,a sound beam with a width of 20°and an acoustic intensity enhancement factor≈6.6 are observed in the far field.As an application,we show that this device can be used as an acoustic energy distributor.This communication pattern with excellent functionalities and performance provides a desirable idea for high-energy-level directional collimated underwater sensing and underwater acoustic energy distribution.

Developments of parabolic equation method in the period of 2000–2016

Parabolic equation （PE） method is an efficient tool for modelling underwater sound propagation, particularly for problems involving range dependence. Since the PE method was first introduced into the field of underwater acoustics, it has been about 40 years, during which contributions to extending its capability has been continuously made. The most recent review paper surveyed the contributions made before 1999. In the period of 2000-2016, the development of PE method basically focuses on seismo-acoustic problems, three-dimensional problems, and realistic applications. In this paper, a review covering the contribution from 2000 to 2016 is given, and what should be done in future work is also discussed.

Sound scattering from underwater elastic spherical shell covered with multilayered medium approximated acoustic cloak

The anechoic performance and mechanism of underwater elastic spherical shell covered with coating are studied at low frequencies.The acoustic cloak is anisotropic material,which can be designed with homogeneous isotropic materials on the basis of effective medium approximation theory.The analytic expression of scattering acoustic field from the shell covered with multilayered medium is formulated and the scattering form function,resonance mode,acoustic field distribution are computed,the scattering characteristics and mechanism of transmission are analyzed.The results show that the direction of sound transmission inside the multilayered medium is changed,the acoustic field is deflected gradually,and the acoustic energy flux is guided around the target,which reduces the scattering intensity at low frequencies,the acoustic intensity of target＇s surface is very weak.Excepting the first resonance peak in spectrum produced by the zero order partial wave,the other resonance modes of elastic spherical shell are not excitated and the multilayered medium can suppress the resonance of the spherical shell effectively.

The WKBZ mode approach to sound propagation in the two-axis underwater channel

The two-axis underwater channel often exists in deep ocean. Sound propagation in the two-axis underwater channel is a benchmark problem for computational methods of underwater acoustics. In this paper, the generalized phase-integral (WKBZ) normal mo de approach is extended to deal with this kind of problem. Numerical results show that the extended WKBZ approach is effective.

Application and measurement of underwater acoustic reciprocity transfer functions in reverberant sound field

The reciprocity measurement theory in anomalous reverberant sound fields was investigated.An improved method Was proposed due to the interrelated errors.The source volume velocity Was corrected by spatial average of measurement results and evaluation of the reverberant sound field influence on acoustic energy density.The result was validated in underwater experiment,corrected reciprocity measurement results were almost the same as direct measurement results.It indicates that reverberant sound field does not affect the validitv of the principle,but influences the obtainment of source volume velocity,then influences the measurement of transfer functions with the principle.The proposed method is simple and effective in anomalous reverberant sound fields.The study mav be valuable for the applications which are based on the principle.