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We consider a coupled hybrid system whose main application is the problem of the active control of noise. The model describes the interaction of acoustic vibrations in the interior of a given two-dimensional cavity with the mechanical vibrations of two damped strings located in a part of the boundary of the cavity, in which suitable feedbacks are acting. Our main result is that the total energy associated to this model decays exponentially as time goes to infinity. [ABSTRACT FROM AUTHOR]

We consider stability in an inverse problem of determining the material coefficient matrix for a coupled system that describes acoustic interactions, by the Riemannian geometrical approach. The stability is proved by the Carleman estimates and observability inequalities. [ABSTRACT FROM AUTHOR]

Structural finite element coupled with the conjugated infinite element method is an efficient numerical technique for solving the acoustic radiation problem due to the vibration of underwater objects. However, for large complex structures, the total acoustic mesh would become very large if the artificial boundary is too far away from the structural wetted surface. Thus, the calculation time can become too long to confine the application of the conjugated infinite element method. On the other hand, if the artificial boundary is close to the structural wetted surface, it will lead to computation accuracy losing due to the near-field effects. Consequently, it is essential to present some guidelines based on the physical mechanism of structural acoustics to choose a suitable artificial boundary that optimizes calculation accuracy and efficiency. In present work, the evanescent wave theory of an infinite length cylindrical shell is adopted to theoretically analyze the decay characteristic of evanescent waves in near field. Then, the effect of the position of artificial boundary on computation accuracy of conjugated infinite element for a finite length ring-stiffened cylindrical shell is numerically investigated. Results suggested that for the cylindrical shell mentioned in this study, the artificial boundary can be placed at least 0.4 times the acoustic wavelength away from the structural wetted surface. What's more, for high frequencies or large-scale structures, the required non-dimensional distance between the artificial boundary and the structural wetted surface increases. [ABSTRACT FROM AUTHOR]

Structural-acoustic optimization procedures can be used to find the optimal design for reduced noise or vibration in many real-world scenarios. However, the time required to compute the structural-acoustic quantity of interest often limits the size of the model. Additionally, structural-acoustic optimization using state-of-the-art evolutionary algorithms may require tens of thousands of system solutions, which add to the limitations for large full-scale systems. To reduce the time required for each function evaluation, parallel processing techniques are used to solve the system in a highly scalable fashion. The approach reduces the analysis time by solving the system using a frequency-domain formulation and distributing solution frequencies amongst processors to solve in parallel. To demonstrate, the sound radiated from a curved panel under the influence of a turbulent boundary layer is minimized in the presence of added point masses, which are varied during the optimization procedure. The total mass is also minimized and the Pareto front relating the trade-off between added mass and reduced noise is determined. Solver scaling information is provided that demonstrates the utility of the parallel processing approach. [ABSTRACT FROM AUTHOR]

Aimed at the complex information in videos and low detection efficiency, an actions detection model based on neighboring Gaussian structure and 3D LARK features is put forward. We exploit a double hierarchical multi-structure space-time statistical matching model (DMSM) in temporal action localization. First, a neighboring Gaussian structure is presented to describe the multi-scale structural relationship. Then, a space-time statistical matching method is proposed to achieve two similarity matrices on both large and small scales, which combines double hierarchical structural constraints in model by both the neighboring Gaussian structure and the 3D LARK local structure. Finally, the double hierarchical similarity is fused and analyzed to detect actions. Besides, the multi-scale composite template extends the model application into multi-view. Experimental results of DMSM on the complex visual tracker benchmark data sets and THUMOS 2014 data sets show the promising performance. Compared with other state-of-the-art algorithm, DMSM achieves superior performances. [ABSTRACT FROM AUTHOR]

Steel and lead piles immersed in seawater and their toes made non-reflective (for clarity of the results) are driven by an axisymmetric harmonic force. Axial and radial vibrations travel from head to toe. Radial vibration, which radiates sound into the surrounding medium, is computed using Membrane thin-shell vibration theory. At 1 kHz, a typical construction steel pile (radius 38 cm, wall thickness 2.5 cm) has a phase velocity of 4943 m/s and in cold seawater (sound speed 1485 m/s), the Mach wave radiates at $$a\hbox {cos}(1485 /4943) = 73^{\circ }$$ from the pile axis. According to ray theory, a Mach wave is received at any position providing a $$73^{\circ }$$ line from that position intersects the pile (at what was the emitting point). The vibration energy at this emitting point has travelled there from the pile head. For a given slant range ( R) to a receiver, ray theory predicts that a Mach wave is not received below a minimum colatitude (COMIN). For a typical steel pile, it is found that as R increases beyond the pile length ( L), COMIN increases rapidly from $$0^{\circ }$$ , passes through $$68^{\circ }$$ at $$10 \times L$$ , and asymptotes to $$73^{\circ }$$ as R increases further. For the same pile made of lead, the phase velocity at 1 kHz is 1332 m/s, and no Mach wave is radiated. For both steel and lead piles, radiated SPLs were calculated using both exact and far-field approximate radiation theories as functions of colatitude, at slant ranges from 10 to 1000 m. For the steel piles, the far-field approximate model (which omits the Mach wave) underestimates SPL by up to 20 dB if the receiver's colatitude exceeds COMIN. For the lead piles however, the far-field approximate model is accurate to within 1 dB. [ABSTRACT FROM AUTHOR]

The paper presents a combined experimental and numerical investigation of the vibrational response of a flanged cylinder structure due to a time-harmonic point excitation. The principle focus of this study was to (1) determine the relative sensitivity of the vibrational response of the cylinder due to variations in the experimental configuration (such as using different vibrational sources, excitation points or complicating structural configurations), and (2) ascertain the level of detail required in the numerical models to accurately replicate the experimental results. Overall, good agreement was achieved between the measured and modelled mode shapes/frequencies up to approximately 1500 Hz, while the experimental results were shown to be largely insensitive to the excitation point or type of mechanical shaker employed. Comparative identification of the measured and modelled cylinder mode shapes allowed for other measured frequency response peaks which did not exhibit discernible modal patterns to be identified from the FEM analysis as either bending modes or internal plate modes for the structure. Finally, it was observed that the contact condition used to model the bolted plates in the structure had a significant effect on the predicted plate modal frequencies, while small amounts of mass loading in the experimental configuration were predicted to cause significant frequency shifts for certain modes. [ABSTRACT FROM AUTHOR]

Fahy's and Pretlove's modelling techniques for a vibro-acoustic system have been employed in many research works. This is the first article addressing the differences between these modelling techniques for a nonlinear vibro-acoustic system. Pretlove's technique is used for vibro-acoustic systems with one flexible panel only (or 2D enclosures); Fahy's is applied to vibro-acoustic systems with multiple flexible panels (or 3D enclosures). The main difference between them is that all acoustic boundary conditions can be satisfied in the 2D modelling technique, while only rigid acoustic boundary conditions can be satisfied in the 3D modelling technique. The multi-level residue harmonic balance method, which was recently developed and modified from the classical harmonic balance method, is employed here. The advantage of this method is that it requires less computation effort when solving the nonlinear equations generated in the entire solution process. The comparisons between various transmission loss and nonlinear vibration results generated from the two models are investigated. [ABSTRACT FROM AUTHOR]

Objectives: For turbo spin echo (TSE) sequences to be useful at ultra-high field, they should ideally employ an RF pulse train compensated for the B inhomogeneity. Previously, it was shown that a single k-point pulse designed in the small tip-angle regime can replace all the pulses of the sequence (static k-points). This work demonstrates that the B dependence of T-weighted imaging can be further mitigated by designing a specific k-point pulse for each pulse of a 3D TSE sequence (dynamic k-points) even on single-channel transmit systems Materials and methods: By combining the spatially resolved extended phase graph formalism (which calculates the echo signals throughout the sequence) with a gradient descent algorithm, dynamic k-points were optimized such that the difference between the simulated signal and a target was minimized at each echo. Dynamic k-points were inserted into the TSE sequence to acquire in vivo images at 7T. Results: The improvement provided by the dynamic k-points over the static k-point design and conventional hard pulses was demonstrated via simulations. Images acquired with dynamic k-points showed systematic improvement of signal and contrast at 7T over regular TSE-especially in cerebellar and temporal lobe regions without the need of parallel transmission. Conclusion: Designing dynamic k-points for a 3D TSE sequence allows the acquisition of T-weighted brain images on a single-transmit system at ultra-high field with reduced dropout and only mild residual effects due to the B inhomogeneity. [ABSTRACT FROM AUTHOR]

A 2D mass-redistributed finite element method (MR-FEM) for pure acoustic problems was recently proposed to reduce the dispersion error. In this paper, the 3D MR-FEM is further developed to solve more complicated structural-acoustic interaction problems. The smoothed Galerkin weak form is adopted to formulate the discretized equations for the structure, and MR-FEM is applied in acoustic domain. The global equations of structural-acoustic interaction problems are then established by coupling the MR-FEM for the acoustic domain and the edge-based smoothed finite element method for the structure. The perfect balance between the mass matrix and stiffness matrix is able to improve the accuracy of the acoustic domain significantly. The gradient smoothing technique used in the structural domain can provide a proper softening effect to the 'overly-stiff' FEM model. A number of numerical examples have demonstrated the effectiveness of the mass-redistributed method with smoothed strain. [ABSTRACT FROM AUTHOR]

The conditions for observing photon echo signals in crystals with excitation and detection in different resonant transitions with a single common energy level are studied. Uncorrelated inhomogeneous broadening in different resonance transitions is shown to influence the formation of stimulated photon echo in three-level systems. Lowering the sample temperature makes it possible to increase the relaxation time, which is used in experiments for observing photon echo. Uncorrelated inhomogeneous broadening in different resonance transitions is temperature independent and affects the intensity of the response at low temperatures, as well. Observation of stimulated photon echo in solid three-level samples requires a correct choice of the time interval between the first and second exciting pulses, but is not related to the magnitude of the irreversible transverse relaxation of the system. [ABSTRACT FROM AUTHOR]

We consider information recording and reconstruction using a reversed stimulated echo hologram, when the recording medium is exposed to pulses of nonresonant electromagnetic standing waves. We show that the spatial intensity distribution in the stimulated echo hologram response depends on the strength of the electric fields in the nonresonant standing waves, which makes it possible to control the reconstructed image. [ABSTRACT FROM AUTHOR]

Many contemporary concert halls are so called surround or vineyard design, in which audience members gather around the orchestra. However, not so many articles on the acoustics of such halls have been published. This article presents the acoustical differences of vineyard and shoebox designs. A recently opened vineyard-type hall, Helsinki Music Centre, is used as a case study, and we discuss its perceptual acoustic qualities and investigate the reasons for subjective observations with objective room-acoustic measurements. In particular, we focus on factors affecting the perception of music dynamics. Results from the objective parameters and spatiotemporal analysis show that the sound field is dominated by the direct sound and early energy in the median plane. These findings are in agreement with the perceptual effects, and resemble the acoustic conditions of another vineyard hall, Berlin Philharmonie. A comparison to a classical shoebox hall reveals fundamental differences in the structure of the sound field. [ABSTRACT FROM AUTHOR]

The scientific approach to room acoustics is now well over a hundred years old. Yet two concert halls from the nineteenth century (the Musikvereinssaal, Vienna of 1870 and the Concertgebouw, Amsterdam of 1888) are still viewed as among the best in the world, as is the first product of a scientific approach ( Boston Symphony Hall, Massachusetts of 1900). Has a century of research and building new auditoria failed to make any progress? One is reminded of the situation with violins, modern violin makers fight to compete with the reputation of Stradivarius' violins built 300 years ago. With auditorium acoustics, the search has been long, with bursts of activity at certain times often in response to new experimental techniques. Considerable progress has been made but the search is not over. This article hopes to elaborate on the challenges involved. [ABSTRACT FROM AUTHOR]

The early lateral energy fraction (LF) is one of the most important descriptors of concert hall sound. This technical note investigates the performance and practicality of measuring LF with a 3-D microphone array system and compares this to two traditional dual microphone systems. A series of simulated sound fields consisting of the direct sound and a single reflection in an anechoic chamber were used to assess the performance of the three systems. The reflection was varied in angle and level, and the measured LF values were compared to the known or calculated values. The 3-D system was found to be the most accurate, with measured LF values within 0.4 just-noticeable-differences (JND) of the calculated values. The two dual microphone systems were mostly within one JND of the calculated values. [ABSTRACT FROM AUTHOR]

Room acoustical parameters are audio features, usually extracted from monaural or binaural measurements of room acoustical environments, and used to predict different aspects of the 'room acoustical impression'. The paper takes a closer look at the nature of this perceptional construct and at different approaches to develop a psychological measuring instrument for the multidimensional perceptional profile of room acoustical environments. Even after several decades of research, there is no satisfactory solution available for this purpose. The reasons for this lie in methodological deficits with respect to test development and item analysis as well as in a much too small sample of stimuli used in previous studies. Prospects for progress are opened up by state-of-the-art technologies for room acoustical simulation and auralization, which may be used to provide a large and representative sample of room acoustical environments as well as an authentic presentation in experimental studies. The fundamental perceptional components delivered by this approach will, most likely, not be predictable by traditional room acoustical parameters, but require advanced measurement techniques based on spherical arrays of transducers for both source and receiver characteristics, as well as new auditory models for feature extraction. The physical and psychological aspects of the problem are, in any case, inextricably linked with each other. [ABSTRACT FROM AUTHOR]

The seismic performance assessment of existing masonry buildings involves many uncertainties, whose impact can be reduced to some extent by using non-destructive in-situ tests of such buildings, at least when destructive in-situ tests, which can provide more reliable results, cannot be performed. In this paper the extent of the potential beneficial effects achievable by calibration of a structural model of a building to its experimentally estimated vibration periods has been investigated. This was done by performing measurements of ambient and forced vibrations on an old two-storey masonry building, and by then assessing its seismic performance using a simplified nonlinear method. The results of numerical investigations revealed that the natural vibration periods of such buildings can be reproduced with sufficient accuracy, although it is possible that they will be overestimated or underestimated by analysts by up to around 40 %. This means that the accuracy of the prediction of the intermediate results of the seismic performance assessment of any particular building can be significantly increased by calibration of the structural model. Additionally, the beneficial effects of such calibration were observed even in the case of the final outcome of the nonlinear analysis, which is expressed through the near-collapse limit state capacity in terms of the peak ground acceleration. [ABSTRACT FROM AUTHOR]

Structural acoustic control problems are considered. The main aim is to reduce a pressure/noise in an acoustic cavity by an appropriate activation of piezoceramic devices. The physical model is comprised of an acoustic chamber with flexible (elastic) walls to which piezoceramic devices are attached. The devices play the role of actuators and sensors. The mathematical description of the model is governed by a coupled system of partial differential equations (PDE's) involving the wave equation coupled with the dynamic shell equation (modeling the wall). The goal of this paper is to present new results on optimal control problems with “smart” controls. The control algorithm is constructed in a feedback form via a solution of a suitable Riccati type equation. The main technical/mathematical difficulty of the problem is related to the fact that the control operators are unbounded. This leads, in general, to the unbounded gain operators and optimal synthesis of the control function which is defined in a very “weak” sense only. However, for the problem at hand, we show that in spite of the unboundedness of the control operators, the feedback gains are bounded and the optimal synthesis is fully meaningful. This is due to the “regularizing” effects of shell dynamics which are partially propagated into the “hyperbolic” component of the structure. [ABSTRACT FROM AUTHOR]

The scattering of flexural waves by two chains of mechanical resonators characterized by effective admittances is considered. In the first (lower) chain, the effective admittance is represented by a susceptance, whereas, in the second (upper) chain, the resonators are characterized by a complex admittance. The spatial periods of the chains are identical. A plane harmonic flexural wave is obliquely incident on the upper chain, and the scattered field formed by the chains is expressed as a superposition of homogeneous and inhomogeneous Bragg spectra. Intense scattering of the incident wave only occurs in the case of mutual compensation of the resonator susceptance and the radiation admittance. A pair of chains with periods not exceeding the half-wavelength of the flexural wave represents an effective insulator for this wave. In the half-space behind the first chain, the zeroth spectral component of the scattered field completely cancels the resonance-frequency incident flexural wave. Let the second chain be positioned at one of the displacement antinodes of the total field formed by the incident field and the zeroth scattered spectrum. Then, if the active components of the effective admittance of resonators belonging to the second chain are identical to the radiation admittance, the incident flexural wave is completely absorbed by the resonators. [ABSTRACT FROM AUTHOR]

This paper studies a structural acoustic model consisting of an interior acoustic wave equation with variable coefficients and a coupled Kirchhoff plate equation with a curved middle surface. By the Riemannian geometry approach and the multiplier technique, we establish exact controllability of the hybrid system under verifiable assumptions on the geometry of the interior domain and the interface boundary with two controls: One is a Neumann boundary control exerted on the wave equation, and the other acts on the interior of the plate equation. Furthermore, if the control for the plate equation is active alone, we prove that the hybrid system with partial Robin boundary condition of the wave equation is exactly controllable with the plate component and approximately controllable with the wave component. [ABSTRACT FROM AUTHOR]