Classical problems of nonlinear acoustics and wave theory

Focusing of a vibration field in finite sized elastic systems using time reversal of signals from a point’s wideband source is theoretically analyzed. It is shown that the result of space and time focusing for a classical reversal algorithm in limited systems with small losses depends of the positional relationship of the source and the signal receiver. A focusing algorithm eliminating this dependence and increasing the efficiency of the focusing of the vibration field is proposed. Theoretical estimates are confirmed with the results of numerical experiments with the models of elastic bodies.     

All-optical probing of the nonlinear acoustics of a crack

Experiments with an all-optical method for the study of the nonlinear acoustics of cracks in solids are reported. Nonlinear acoustic waves are initiated by the absorption of radiation from a pair of laser beams intensity modulated at two different frequencies. The detection of acoustic waves at mixed frequencies, absent in the frequency spectrum of the heating lasers, by optical interferometry or deflectometry provides unambiguous evidence of the elastic nonlinearity of the crack. The high contrast in crack imaging achieved by remote optical monitoring of the nonlinear acoustic processes is due to the strong dependence of the efficiency of optoacoustic conversion on the state of the crack. The highest acoustic nonlinearity is observed in the transitional state of the crack, which is intermediate between the open and the closed ones.     

近场声全息技术应用有关物理问题研究

近场声全息(NAH)技术,用于大型结构或水声基阵性能参数测量、校准,要有一套高精度声场全息扫描测量接收基阵及同步测量和控制系统装置,以及适应的处理、分析、计算软件。研究实践表明:有关这套测控、分析系统硬、软件的设计指标、参数和使用要求,都涉及一些声学物理问题。为确保全息场量值测量可靠、场重建精度高,对声源的辐射声信号波数谱和近场声压分布,进行了必要的物理分析、计算。得到要保留的高波数分量、全息面的形式和尺度及其离声源的间距、全息面上测点间距等诸参数,与声源形状、尺度、频率等量之间定量关系并给出要求、判据的参考。     

The acoustics of the caves

This work shows the acoustic measurements realized at three underground places: the caves of Pertosa or of the Angel, the caves of Castelcivita and the caves of Castellana. These places are used for some years to represent music and theater performances, whose success is also due to the mystery and the charm that distinguish them, based on the natural setting of the caves. The places of the caverns where the shows are played were characterized analyzing the monaural acoustic parameters (T₃₀, EDT, C₈₀, D₅₀ and Rasti) obtained with the impulse response resulting from blowing up some balloons inside them. The values of monaural acoustic parameters measured in the caves can be compared with the parameters of the concert halls currently in use.     

最概然分布、微观状态数及其相关问题

通过对一个简单模型的研究可知,最概然分布包含的微观状态数和全部可能微观状态数的比值在热力学极限下为零.当一个热力学体系处于平衡态时,最概然分布及其附近相对于实验可以达到的精度来说的极其狭窄的区域内的分布包括了体系几乎全部可能的微观状态数.     

The wave equation with viscoelastic attenuation and its application in problems of shallow-sea acoustics

A suitable tool for the simulation of low frequency acoustic pulse signals propagating in a shallow sea is the numerical integration of the nonstationary wave equation. The main feature of such simulation problems is that in this case the sound waves propagate in the geoacoustic waveguide formed by the upper layers of the bottom and the water column. By this reason, the correct dependence of the attenuation of sound waves in the bottom on their frequency must be taken into account. In this paper we obtain an integro-differential equation for the sound waves in the viscoelastic fluid, which allows to simulate the arbitrary dependence of acoustic wave attenuation on frequency in the time domain computations. The procedure of numerical solution of this equation based on its approximation by a system of differential equations is then considered and the methods of artificial limitation of computational domain are described. We also construct a simple finite-difference scheme for the proposed equation suitable for the numerical solution of nonstationary problems arising in the shallow-sea acoustics.     

Inverse problem of nonlinear acoustics: Synthesizing intense signals to intensify the thermal and radiation action of ultrasound

Inverse problems of nonlinear acoustics have important applied significance. On the one hand, they are necessary for nonlinear diagnostics of media, materials, manufactured articles, building units, and biological and geological structures. On the other hand, they are needed for creating devices that ensure optimal action of acoustic radiation on a target. However, despite the many promising applications, this direction remains underdeveloped, especially for strongly distorted high-intensity waves containing shock fronts. An example of such an inverse problem is synthesis of the spatiotemporal structure of a field in a radiating system that ensures the highest possible energy density in the focal region. This problem is also related to the urgent problems of localizing wave energy and the theory of strongly nonlinear waves. Below we analyze some quite general and simple inverse nonlinear problems.     

Relatively undistorted waves and waveforms in nonlinear acoustics: Exact solutions of equations

We demonstrate the physical conditionality and efficiency of using relatively undistorted waves as an anzats for determining exact solutions to equations of nonlinear acoustics.     

Quantitative EPR: Some of the most difficult problems

Many of the errors in EPR concentration measurements result from the inability of an analyst to use the reference sample that is identical to the sample to be analyzed in a number of substantial respects. Generally, it is not correct to compare directly the intensities of the signals from two samples that contain different paramagnetic centers, or are different in matrix material, dimensions, or position in the cavity. It is necessary to evaluate the resulting systematic errors and correct them. The paper is devoted to the analysis of the following sources of errors which frequently occur in such situations: nonuniformity of the cavity fields; different extent of microwave field perturbation by samples with different dielectric constants; inequality of the transition probabilities corresponding to the used spectral lines of the samples under comparison. Theoretical and semiempirical calculations enable one to determine the magnitudes of the errors from these sources and to obtain the appropriate correction factors in many specific cases.     

Variational approaches to the problems of plasma stability and of nonlinear plasma dynamics

A variational method is developed in order to investigate the nonlinear dynamics and stability of plasma using hydrodynamic plasma models, namely, the one-fluid, Hall, and electron MHD models. The key idea of the method is to adequately take into account variational symmetries and the associated conservation laws inherent in these hydrodynamic models. This approach is applied to derive variational criteria for the stability of a steadily moving plasma and to propose a variational method of the adiabatic separation of fast and slow motions, which makes it possible to simplify (reduce) the basic hydrodynamic models.     

The use of extra-terrestrial oceans to test ocean acoustics students

The existence of extra-terrestrial oceans offers the opportunities to set examination questions for which students in underwater acoustics do not already know the answers. The limited set of scenarios in Earth's oceans that can be presented to students as tractable examination questions means that, rather than properly assessing the individual scenario, students can rely on knowledge from previous examples in assessing, for example, which terms in equations are large and small, and what numerical values the answers are likely to take. The habit of adapting previous solutions with which the student is comfortable, to new scenarios, is not a safe approach to learn, as it ill equips the future scientist or engineer to identify and tackle problems which contain serious departures from their experience.     

Investigation of noise-shielding efficiency with the method of sequences of maximum length in application to the problems of aviation acoustics

A study of the phenomenon of diffraction of acoustic waves in application to the task of noise shielding by the method of maximum length sequences has been carried out. Rectangular plates and an aircraft model of integrated layout are used as the screens. In the study of noise shielding by aircraft model, the theorem of reciprocity is used. A comparison of experimental results with calculations performed in the framework of the geometrical theory of diffraction (GTD) is performed. On the basis of calculations, the identification of the contributions from different areas of the shielding surface in the full acoustic field is carried out. For the aircraft model, the shielding factor is calculated depending on the frequency.     

Enhancing the dynamic range of targeted energy transfer in acoustics using several nonlinear membrane absorbers

In order to enhance the robustness and the energy range of efficiency of targeted energy transfer (TET) phenomena in acoustics, we discuss in this paper about the use of multiple nonlinear membrane absorbers (called nonlinear energy sinks or NES) placed in parallel. We show this way, mainly thanks to an experimental set-up with two membranes, that the different absorbers have additional effects that extend the efficiency and the possibilities of observation of TET. More precisely, we present the different behavior of the system under sinusoidal forcing and free oscillations, characterizing the phenomena for all input energies. The frequency responses are also presented, showing successive clipping of the original resonance peak of the system, and strongly modulated regimes (SMR). A model is finally used to generalize these results to more than two NES and to simulate the case of several very similar membranes in parallel which shows how to extend the existence zone of TET.     

A nonlinear signal-based control method and its applications to input identification for nonlinear SIMO problems

The problem of control system design can be conceptualised as identifying an input signal to a plant (the system to be controlled) so that the corresponding output matches that of a pre-defined reference signal. Primarily, this problem is solved via well-known techniques based upon the principle of feedback design, an essential component for ensuring stability and robustness of the controlled system. However, feedforward design techniques also have a large part to play, whereby (in the absence of feedback control and assuming that the plant is stable) a model of the plant dynamics can be used to modify the reference signal so that the resultant feedforward input signal generates a plant output signal that is sufficiently close to the original reference signal.     

通信声学的发展及研究现状

人类作为信息交流的主要信息产生源和信息接收槽,语言声学、心理声学和生理声学在信息技术中发挥着越来越重要的作用。作为声学与信息技术相互渗透的产物-通信声学,近十几年来随着数字信号处理技术的发展,其研究和应用领域得到了很大的扩展。本文在简述通信声学产生和发展的必然性和必要性之后,主要介绍了通信声学目前的研究领域及其应用。