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In this paper, we design a truly exact perfect absorbing layer (PAL) for domain truncation of the two-dimensional Helmholtz equation in an unbounded domain with bounded scatterers. This technique is based on a complex compression coordinate transformation in polar coordinates and a judicious substitution of the unknown field in the artificial layer. Compared with the widely used perfectly matched layer (PML) methods, the distinctive features of PAL lie in that (i) it is truly exact in the sense that the PAL-solution is identical to the original solution in the bounded domain reduced by the truncation layer; (ii) with the substitution, the PAL-equation is free of singular coefficients and the substituted unknown field is essentially nonoscillatory in the layer; and (iii) the construction is valid for general star-shaped domain truncation. By formulating the variational formulation in Cartesian coordinates, the implementation of this technique using standard spectral-element or finite-element methods can be made easy as a usual coding practice. We provide ample numerical examples to demonstrate that this method is highly accurate and robust for very high wavenumbers and thin layers. It outperforms the classical PML and the recently advocated PML using unbounded absorbing functions. Moreover, it can fix some flaws of the PML approach. Ministry of Education (MOE) Published version The work of the first author was partially supported by the Strategic Priority Research Program of Chinese Academy of Sciences grant XDA25010402. The work of the second author was partially supported by Singapore MOE AcRF Tier 2 Grants MOE2018-T2-1-059 and MOE2017-T2- 2-144.

Based on the magnetoacoustic coupled dynamics theory, the wave function expansion method is used to solve the problem of acoustic wave scattering and dynamic stress concentration around the two openings in e-type piezomagnetic composites. To deal with the multiple scattering between openings, the local coordinate method is introduced. The general analytical solution to the problem and the expression of the dynamic stress concentration are derived. As an example, the numerical results of the dynamic stress distribution around two openings with equal diameters are given. The effects of the parameters, such as the incident wave number and the spacing between the openings, on the dynamic stress concentration factor are analyzed.

Time-domain acoustic scattering problems in two dimensions are studied. The numerical scheme relies on the use of the Convolution Quadrature (CQ) method to reduce the time-domain problem to the solution of frequency-domain Helmholtz equations with complex wavenumbers. These equations are solved with the method of fundamental solutions (MFS), which approximates the solution by a linear combination of fundamental solutions defined at source points inside (outside) the scatterer for exterior (interior) problems. Numerical results show that the coupling of both methods works efficiently and accurately for multistep and multistage based CQ. ISSN:1991-7120 ISSN:1815-2406

International audience; This chapter discusses the role of energy absorption in a reverberation chamber and some related applications. The composite Q-factor is the key indicator of the total amount of losses in a reverberation chamber since it quantifies the energy stored (per unit of time) per unit of transmitted/dissipated power. In a first part of this chapter, we settled the definition of this composite quality factor and described a method to estimate it in different ways with a set of two antennas. The scattering parameters at antenna ports enable to estimate the composite Q-factor in three different ways and enables to check for the enhanced backscattered coefficient. Obviously, an accurate enough estimation requires a high number of RC states. We suggest that averaging over small frequency bands is a solution to minimize intrinsic statistical variation of Q-factors estimated from a limited set of stirrer positions. The first application consists of evaluating the average absorption cross section of any piece of material within the RC. This absorption coefficient is retrieved from the contrast of quality factors measurement before and after putting the material in the chamber. For simple geometries of the inserted absorber, it is possible to estimate its efficiency. Moreover, from the knowledge of the intrinsic EM properties of homogeneous and thick (with regard to the skin depth) materials, we showed that the measured AACS computed from the modification of the Q-factor is consistent with the theoretical one. This last fact was at the origin of a specific calibration procedure for mm-wave RC dedicated to animal exposure. This is the second application of this chapter. In order to calibrate the EM dose during exposure, the power density in the RC must be controlled and proved to be consistent with the expected temperature rise. Using a rectangular homogeneous phantom, mimicking the properties of animal’s skin, we show that the temperature rise at its surface and measured with an infrared camera was indeed predictable. On a theoretical point of view, the measurement of the Q-factor, the knowledge of intrinsic parameters of the phantom and the properties of the diffuse field in the RC fully determine the heat deposition on the phantom surface. Solving the heat-equation gives then the correct temperature rise prediction with a reasonable accuracy. Chapter Contents: • 6.1 Losses and Q-factor • 6.1.1 Role of losses in a RC • 6.1.2 Definition of the Q-factor • 6.1.3 Origin of losses and their contribution to the Q-factor • 6.1.4 Q-Factor measurements • 6.1.4.1 Test setup for Q-factor measurements • 6.1.4.2 Q-Factor measurements using the full scattering matrix • 6.1.4.3 Q-Factor measurements using the reflection coefficient from each antenna • 6.1.4.4 QTx and QRx measurements • 6.1.4.5 Experimental results of Q-factor estimation • 6.2 The average absorption effective area of an object in a RC • 6.2.1 The Q-factor of an object in a RC and its average absorption cross section • 6.2.2 Theoretical absorbing cross section of a rectangular piece of absorbing material • 6.2.3 Measurement examples • 6.3 Dosimetry for animals • 6.3.1 Using RC for animal exposure • 6.3.2 Theory about heat transfer in a RC • 6.3.2.1 The phantom • 6.3.2.2 The heat equation • 6.3.2.3 The electrothermal coupling • 6.3.2.4 The 1-D approximation • 6.3.2.5 Solution of the 1-D heat equation • 6.3.3 Experimental validation • 6.4 Discussion • References. © The Institution of Engineering and Technology 2021.

Time domain acoustic scattering problems in two and three dimensions are studied. The numerical scheme consists in the use of Convolution Quadrature method to reduce the time domain problem to solve frequency domain Helmholtz equations with complex wavenumbers. These equations are solved with the method of fundamental solutions (MFS), which approximates the solution by a linear combination of fundamental solutions defined at source points inside (outside) the scatterer for exterior (interior) problems. Numerical results show that the coupling of both methods works efficiently and accurately for multistep and multistage based CQ.

It is known that the steady acoustic radiation force on a scatterer due to incident time-harmonic waves can be calculated by evaluating certain integrals of velocity potentials over a sphere surrounding the scatterer. The goal is to evaluate these integrals using far-field approximations and appropriate limits. Previous derivations are corrected, clarified, and generalized. Similar corrections are made to textbook derivations of optical theorems.

A dual frequency identification sonar was used to estimate density and size of potential deep diving odontocetes' prey in deep sea scattering layers in Hawaii. Size distributions data showed a bimodal pattern, with a population of data above the 50 cm of length, possibly big squids, and a population of data below 50 cm, mainly micronekton. The number of animals bigger than 50 cm was correlated with the length of the peak of the frequency distribution. Data suggest that big predators, like squids that constitute prey for beaked, pilot, and sperm whales, are more likely to be found where they have better chance to find micronectonic prey.

Early reflections are known to be important to musicians performing on stage, but acoustic measurements are usually made on empty stages. This work investigates how a chamber orchestra setup on stage affects early reflections from the stage enclosure. A boundary element method (BEM) model of a chamber orchestra is validated against full scale measurements with seated and standing subjects in an anechoic chamber and against auditorium measurements, demonstrating that the BEM simulation gives realistic results. Using the validated BEM model, an investigation of how a chamber orchestra attenuates and scatters both the direct sound and the first-order reflections is presented for two different sized "shoe-box" stage enclosures. The first-order reflections from the stage are investigated individually: at and above the 250 Hz band, horizontal reflections from stage walls are attenuated to varying degrees, while the ceiling reflection is relatively unaffected. Considering the overall effect of the chamber orchestra on the direct sound and first-order reflections, differences of 2-5 dB occur in the 1000 Hz octave band when the ceiling reflection is excluded (slightly reduced when including the unobstructed ceiling reflection). A tilted side wall case showed the orchestra has a reduced effect with a small elevation of the lateral reflections.

Knowing the optical properties of a sample is important in many scientific fields, such as space science, climate studies, and medicine. In many of these applications, the samples are fragile, unique or available in limited quantities, and have to be subsequently studied by additional techniques, implying that sample preservation is important. Established light scattering single particle measurements involve attaching the sample to a holder or measuring a laminar flow of particles, neither of which allows a controlled, unperturbed non-destructive measurement. Acoustic levitation is a state-of-the-art and non-contacting approach capable of assisting light scattering measurements. However, a full 4π measurement (i.e., a measurement from any direction on the full 4π solid angle) has hither been impossible due to levitation instabilities. Here we present and describe the instrument capable of performing a full 4π light scattering measurement. We measure light scattering properties of millimeter-sized samples at any direction. This is enabled by introducing a novel non-contacting sample holder based on acoustic levitation, which allows a disturbance-free measurement of an orientation-controlled sample. The instrument is scalable and currently employs polarized visible light (400–700 nm). It also measures beam and sample stability as well as temperature and humidity, to ensure consistency of measurements. We demonstrate the 4π capabilities of the instrument by measuring an angular map of light scattering from a polystyrene foam sample, as well as a multi-angular measurement (two semicircular measurements 90° apart) of a sample consisting of agglomerated 500 nm silica spheres. The upper left 2 × 2 submatrix of the Mueller matrix is measured from the sample along the (polar) scattering angle in semi-circular sweeps, changing the azimuthal scattering angle for each sweep. Our results allow for verification of theoretical models by mimicking the conditions of the simulations and by making the measurements directly comparable to model predictions. © 2020 Academy of Finland: 325806, 325805 European Research Council, ERC: 320773 Government Council on Grants, Russian Federation Svenska Kulturfonden The research leading to these results has received funding from the ERC Advanced Grant 320773 entitled “Scattering and Absorption of Electromagnetic Waves in Particulate Media” (SAEMPL). GM acknowledges Svenska Kulturfonden research grant for doctoral studies. Research by KM, AP, and MG supported, in part, by Academy of Finland projects No. 325805 and 325806. Research at the Ural Federal University is supported by the Act 211 of the Government of the Russian Federation, agreement No. 02.A03.21.0006.

We investigate two density questions for Sobolev, Besov and Triebel--Lizorkin spaces on rough sets. Our main results, stated in the simplest Sobolev space setting, are that: (i) for an open set $\Omega\subset\mathbb R^n$, $\mathcal{D}(\Omega)$ is dense in $\{u\in H^s(\mathbb R^n):{\rm supp}\, u\subset \overline{\Omega}\}$ whenever $\partial\Omega$ has zero Lebesgue measure and $\Omega$ is "thick" (in the sense of Triebel); and (ii) for a $d$-set $\Gamma\subset\mathbb R^n$ ($0, Comment: 38 pages, 6 figures

The Royal Netherlands Navy operates a low-frequency active sonar consisting of a combination of a towed sonar source and receiver array in order to detect and classify objects underwater. The operation involves transmitting high power underwater sound towards an object and detecting the echo reflected by the object. Operating a towed source at high source levels implicates that sound is inadvertently radiated towards the vessel which is towing the source. A structural-acoustic interaction between the underwater noise and the ship's hull causes the ship structure to be excited. The structure borne sound is radiated into the crew's accommodation as airborne noise by the floors, walls and ceilings and may be perceived as disturbing. Using analytical and empirical transfer functions, a model was made for the irradiation of the ship's hull, the hull's acoustic response, the propagation of structure borne noise through the ship structure and the radiation of airborne noise into accommodation spaces. The model results were verified with measurements acquired during a sea trial. The validated noise transfer model may be used to assess the effectiveness of noise control measures aiming to reduce the noise exposure levels in accommodation spaces © Proceedings of the 26th International Congress on Sound and Vibration, ICSV 2019. All rights reserved.

The Royal Netherlands Navy operates a low-frequency active sonar consisting of a combination of a towed sonar source and receiver array in order to detect and classify objects underwater. The operation involves transmitting high power underwater sound towards an object and detecting the echo reflected by the object. Operating a towed source at high source levels implicates that sound is inadvertently radiated towards the vessel which is towing the source. A structural-acoustic interaction between the underwater noise and the ship's hull causes the ship structure to be excited. The structure borne sound is radiated into the crew's accommodation as airborne noise by the floors, walls and ceilings and may be perceived as disturbing. Using analytical and empirical transfer functions, a model was made for the irradiation of the ship's hull, the hull's acoustic response, the propagation of structure borne noise through the ship structure and the radiation of airborne noise into accommodation spaces. The model results were verified with measurements acquired during a sea trial. The validated noise transfer model may be used to assess the effectiveness of noise control measures aiming to reduce the noise exposure levels in accommodation spaces © Proceedings of the 26th International Congress on Sound and Vibration, ICSV 2019. All rights reserved.

International audience; This paper details the extension of a metric-based anisotropic mesh adaptation strategy to the boundary element method for problems of 3D acoustic wave propagation. Traditional mesh adaptation strategies for boundary element methods rely on Galerkin discretizations of the boundary integral equations, and the development of appropriate error indicators. They often require the solution of further integral equations. These methods utilise the error indicators to mark elements where the error is above a specified tolerance and then refine these elements. Such an approach cannot lead to anisotropic adaptation regardless of how these elements are refined, since the orientation and shape of current elements cannot be modified. In contrast, the method proposed here is independent of the discretization technique (e.g., collocation, Galerkin). Furthermore, it completely remeshes at each refinement step, altering the shape, size, and orientation of each element according to an optimal metric based on a numerically recovered Hessian of the boundary solution. The resulting adaptation procedure is truly anisotropic and independent of the complexity of the geometry. We show via a variety of numerical examples that it recovers optimal convergence rates for domains with geometric singularities. In particular, a faster convergence rate is recovered for scattering problems with complex geometries.

In this paper, we use continuum mechanics to develop an analytic treatment of elastic wave scattering from an embedded cylinder and show that a classic treatise on the subject contains important errors for oblique angles of incidence, which we correct. We also develop missing equations for the scattering cross section at oblique angles and study the sensitivity of the scattering cross section as a function of elastodynamic contrast mechanisms. We find that in the Mie scattering regime for oblique angles of incidence, both elastic and density contrast are important mechanisms by which scattering can be controlled, but that their effects can offset one another, similar to the theory of reflection at flat interfaces. In comparison, we find that in the Rayleigh scattering regime, elastic and density contrast are always complimentary toward increasing scattering cross section, but for sufficiently high density contrast, the scattering cross section for incident compressional and y-transverse modes is nearly independent of elastic contrast. The solution developed captures the scattering physics for all possible incident elastic wave orientations, polarizations, and wavelengths including the transition from Rayleigh to geometric scattering regimes, so long as the continuum approximation holds. The method could, for example, enable calculation of the thermal conductivity tensor from microscopic principles which requires knowledge of the scattering cross section spanning all possible incident elastic wave orientations and polarizations at thermally excited wavelengths.

The scattering cross section for a cluster of scatterers can be calculated using various methods, either exactly or by invoking various approximations. Of special interest are methods in which the scattering properties of individual members of the cluster are used. The underlying question is: Can the contribution to the cluster's cross section from any one member of the cluster be identified? Except for situations in which all effects of multiple scattering are ignored, no such method of identification has been found.

International audience; Ultrasonic telemetry imaging systems are used to monitor such immersed structures as main vessels of nuclear reactors. The interaction between acoustic beams and targets involves scattering phenomena, mainly specular reflection and tip diffraction. In order to assist in the design of imaging systems, a simulation tool is required for the accurate modeling of such phenomena. Relevant high-frequency scattering models have been developed in electromagnetic applications, in particular, the geometrical optics (GO), Geometrical Theory of Diffraction (GTD) and its uniform corrections (UAT and UTD), Kirchhoff approximation (KA) and Physical Theory of Diffraction (PTD). Before adopting any of them for simulation of scattering of acoustic waves by edged immersed rigid bodies, it is important to realize that in acoustics the characteristic dimension to the wave length ratio is usually considerably smaller than in electromagnetics and a further study is required to identify models' advantages, disadvantages and regions of applicability. In this paper their numerical comparison is carried out. As the result, the most suitable algorithm is identified for simulating ultrasonic telemetry of immersed rigid structures.

The finite element method is used to model acoustic scattering from rough poroelastic surfaces. Both monostatic and bistatic scattering strengths are calculated and compared with three analytic models: Perturbation theory, the Kirchhoff approximation, and the small-slope approximation. It is found that the small-slope approximation is in very close agreement with the finite element results for all cases studied and that perturbation theory and the Kirchhoff approximation can be considered valid in those instances where their predictions match those given by the small-slope approximation.

Date of Conference: 28 Aug.-2 Sept. 2017 Conference name: 25th European Signal Processing Conference (EUSIPCO), 2017 We introduce a time-frequency scattering method using hyperbolic tangent function for vessel sound classification. The sound data is wavelet transformed using a two channel filter-bank and filter-bank outputs are scattered using tanh function. A feature vector similar to mel-scale cepstrum is obtained after a wavelet packed transform-like structure approximating the mel-frequency scale. Feature vectors of vessel sounds are classified using a support vector machine (SVM). Experimental results are presented and the new feature extraction method produces better classification results than the ordinary Mel-Frequency Cepstral Coefficients (MFCC) vectors. © EURASIP 2017.

The influence of spatial diversity in acoustic scattering properties on estimates of the effective scatterer diameter (ESD) applied to soft biological tissues is investigated. This study is based on two-dimensional simulations of scattering media, beginning with random distributions of simple disk structures where all scattering features are known exactly. It concludes with an analysis of histology maps from healthy and fatty rabbit liver. Further, the liver histology is decomposed using an orthonormal basis to separate acoustic scattering at various spatial scales and observe how it influences ESD estimates. Overall, the goal is to quantitatively interpret ESD results for diagnostic assessments despite wide variations in tissue scatterer properties.

An extended calibration target has been developed for calibrating the intensity output of a multibeam echo sounder (MBES). The target was constructed of chain links arranged similar to a curtain, providing an extended surface target with a mean scattering strength of −17.8 dB at 200 kHz. The target was used to calibrate a 200 kHz MBES, and the MBES was subsequently used to collect seafloor backscatter over sand and gravel seafloors. Field results were compared with calibrated split-beam echo sounder measurements at an incidence angle of 45°. The results suggest that the chain target is a viable MBES calibration tool.

In present paper principles of acoustic scattering in case hardened components were discussed. Improvements of ultrasonic testing method and instrument development opportunities were proposed. Through these investigations, it would be possible to provide an advanced method of ultrasonic testing of case hardening quality, which might be applied in Russian metallurgical industry market. Comparative analysis of characteristics of existing instrument and the proposed instrument was conducted. This article demonstrates all important advantages of the proposed method of nondestructive testing for quality control of heavy-loaded components.

This paper presents theoretical and experimental work on concurrent active noise and vibration control for a ventilation duct. The active noise control system is used to reduce the air-borne noise radiated via the duct outlet whereas the active vibration control system is used to both reduce the structure-borne noise radiated by the duct wall and to minimise the structural feed-through effect that reduces the effectiveness of the active noise control system. An elemental model based on structural mobility functions and acoustic impedance functions has been developed to investigate the principal effects and limitations of feed-forward active noise control and decentralised velocity feedback vibration control. The principal simulation results have been contrasted and validated with measurements taken on a laboratory duct set-up, equipped with an active noise control system and a decentralised vibration control system. Both simulations and experimental results show that the air-borne noise radiated from the duct outlet can be significantly attenuated using the feed-forward active noise control. In the presence of structure-borne noise the performance of the active noise control system is impaired by a structure-borne feed-through effect. Also the sound radiation from the duct wall is increased. In this case, if the active noise control is combined with a concurrent active vibration control system, the sound radiation by the duct outlet is further reduced and the sound radiation from the duct wall at low frequencies reduces noticeably.

In this Letter, a proof is provided for the reciprocity between modal scattering coefficients of the acoustic waveguides connected by a junction enclosure. The result holds for all waveguide modes and for junction enclosures with locally reactive boundary conditions away from the interfaces between the junction and waveguides. Also provided is a physical interpretation of the reciprocity of the modal scattering coefficients. The scattering of two-dimensional waveguide modes by a right-angled bend in a rectangular duct is used as an illustrating example.

The article of record as published may be found at https://doi.org/10.1121/1.4789358 Internal waves and bathymetric variation create time- and space-dependent alterations in the ocean acoustic waveguide, and cause subsequent coupling of acoustic energy between propagating normal modes. In this paper, the criterion for adiabatic invariance is extended to the case of an internal solitary wave (ISW) encountering a sloping bathymetry (i.e., continental shelfbreak). Predictions based on the extended criterion for adiabatic invariance are compared to experimental observations from the Asian Seas International Acoustics Experiment. Using a mode 1 starter field, results demonstrate time-dependent coupling of mode 1 energy to higher adjacent modes, followed by abrupt coupling of mode 5–7 energy to nonadjacent modes 8–20, produces enhanced mode coupling and higher received levels downrange of the oceanographic and bathymetric features. Numerical simulations demonstrate that increasing ISW amplitude and seafloor slope enhance the coupling of energy to adjacent and nonadjacent modes. This enhanced coupling is the direct result of the simultaneous influence of the ISW and its proximity to the shelfbreak, and, compared to the individual effect of the ISW or shelfbreak, has the capacity to scatter 2–4 times the amount of acoustic energy from below the thermocline into the upper water column beyond the shelfbreak in realistic environments. The ASIAEX and NLIWI experiments were supported jointly by the National Science Council of Taiwan and the U.S. Office of Naval Research.

The backscattered travel-time structure is obtained by cross-correlating air-acoustic ocean surf noise recorded on microphone pairs (separation ∼2 m) on the beach. The scatterer is a 20 cm radius Polyvinyl chloride pipe 2.5 m landside of the microphone array. Arrivals corresponding to the time-difference (travel-time difference between two scatterer-receiver paths) and scattered (travel time for receiver-scatterer-receiver path) waves emerge in the cross-correlation functions in a backscattering configuration. Theoretically, only a microphone pair is needed to locate the scatterer using the time-difference and scattered travel times. Localization of the scatterer is demonstrated with the microphone array on the beach.

Orientador: Ricardo Caetano Azevedo Biloti Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Matemática Estatística e Computação Científica Resumo: Difrações sísmicas são induzidas por descontinuidades e pequenas estruturas na subsuperfície, portanto o imageamento de difrações pode extrair informações valiosas para a identificação de falhas, canais, fraturas e objetos enterrados. Investigamos a possibilidade de imagear e caracterizar difrações utilizando métodos de reconhecimento de padrões. Para este fim, propomos a construção de um descritor baseado em atributos estatísticos dos aspectos dinâmicos e cinemáticos de operadores difração que melhor distinguem reflexões de difrações. Estes atributos são utilizados para classificação automática dos pontos imagem de uma sessão sísmica através de algoritmos supervisionados e não-supervisionados. Avaliamos o método para dados sintéticos e um dado GPR. Os resultados indicam que o método é robusto, mesmo para dados com baixa razão sinal-ruído Abstract: Seismic diffractions are mainly induced by discontinuities and small structures, so diffraction imaging can extract valuable information to identify subsurface scattering features, such as faults, channels, fractures and buried objects. We investigate the possibility to image and characterize diffractions using pattern recognition methods. To this end, we look at kinematical and dynamical aspects of diffraction operators and we propose a descriptor based on statistical attributes of kinematical and dynamical aspects of diffraction operators that better distinguish diffractions from reflections. These descriptors are used to perform automatic classification of imaging points on a seismic section using supervised and unsupervised algorithms. We evaluate the method using synthetic and GPR data. Results indicate that the method is robust even for low signal-to-noise ratio datasets Doutorado Matemática Aplicada Doutora em Matemática Aplicada CAPES