Non-linear effect of wall linings on sound attenuation in ducts

The equivalent surface source method is extended to the analysis of high intensity sound propagation in a duct whose wall is partially treated with a sound absorbing material. The propagation of sound in the gas is assumed to be linear, but the acoustic resistance of the sound absorbing material is assumed to be a function of the normal acoustic velocity. The problem is reduced to a non-linear integro-differential equation for the fluid particle displacement at the lined wall surface, which can be solved by a successive approximation method. Numerical examples show that the non-linear effect decreases or increases the peak sound attenuation rate of the lowest mode depending upon the linear component of the resistance. The dependence of the attenuation spectrum on modal phase difference of multi-mode incident waves is heavily affected by the non-linear effect. In the case of incident waves of multi-circumferential modes, different circumferential modes are generated by the non-linear effect.     

Sound absorption by an active resonator array near an impedance surface

Absorption of sound waves incident on a plane surface with arbitrary impedance by a planar active resonator array consisting of monopole or dipole resonators and positioned near the surface is considered. Appropriate tuning of active resonators ensures complete absorption of sound waves incident at a fixed angle in a broad frequency band. The effect of tuning errors on the efficiency of sound absorption by the systems under study is investigated. It is shown that, for rigid surfaces, a monopole resonator array yields a higher absorption efficiency, whereas for soft surfaces, a dipole resonator array is the more efficient one.     

Threshold and Lennard-Jones resonances and elastic lifetimes in the scattering of atoms from crystalline surfaces

In this paper we apply the GR method for solving the scattering equations of atoms from a hard corrugated surface, on accelerated particles above a hard corrugated surface and a hard corrugated surface with an attractive well. The solutions are given for the Rayleigh hypothesis that under the range of corrugation presented in this paper leads to the exact ones. Threshold resonances are studied observing that the appearance and disappearance of beams must be for a general theory with vertical tangent. The structure of the Lennard-Jones resonances given for the model mentioned above. For the first time it is stressed that Lennard-Jones resonances are not observed in metal surfaces in general, and, accordingly, they are unobserved in compact metallic surfaces. This is correlated with the fact that diffraction has not been observed. Both facts are due to the very weak corrugation of the gas-metal interaction potential. According to our results, the Lennard-Jones resonances in metals present greater difficulties to be observed experimentally. We also note that the absence of diffraction in compact metal surfaces is because they are almost plane and not because of the Debye-Waller effect. Finally, we have calculated the lifetimes of the atoms at the crystal surfaces. These are larger, the smaller the incident energy and the larger the corrugation. But the lifetimes are particularly large at resonance condition (10^?11 s).     

Surface excitation of hypersound in piezoelectric crystals by plane electromagnetic waves

The generation of hypersound at a free surface of a piezoelectric crystal by means of an incident plane electromagnetic wave is considered and the corresponding boundary problem is discussed in detail. The formula developed in this paper are quite general and can be applied to any piezoelectric crystal and any face orientation. As an important example, the excitation of sound waves at several quartz faces is treated numerically and the results are presented in diagrams showing directly the power conversion from the plane incident electromagnetic wave into the sound waves as function of the angle of incidence and of polarization directions.     

掩埋目标声散射特性及其实验

基于变分原理建立2D-FE弱形式解模型,针对轴对称掩埋目标进行散射远场的快速、高精度数值计算。入射波垂直于对称轴,给出:掩埋深度不变,以1°为间隔,改变掠射角所得到目标散射声压级相对于垂直海底照射获得结果的差值随频率、掠射角变化的表达式;掠射角不变,以0.1m为间隔,改变掩埋深度得到的散射声压级相对于浅掩埋条件下获得结果的差值随频率、深度变化的表达式。实验结果表明:在回波最强的正横方向,掩埋弹性目标表现出的共振散射特性与在自由场空间中具有相似性,表明了掩埋条件下目标内填充的透射和表面环绕波理论的适用性,验证了大掠射角入射得到散射声压级差值的变化规律。研究成果对宽带、高频入射声波探测更深掩埋目标提供思路。      

Stimulated Cherenkov radiation of a relativistic electron beam moving over a periodically corrugated surface (quasi-optical theory)

The propagation of waves over periodically corrugated surfaces and their excitation by relativistic electron beams are investigated within the framework of a quasi-optical approach. The dispersion equation is derived for normal waves under the assumption of a small (in the scale of the period and wavelength) corrugation depth, based on which two limiting cases are identified. In the first limiting case, the wave frequency is far from the Bragg resonance, and the propagation of waves can be described in terms of the impedance approximation, in which the fundamental spatial harmonic slows down. In the second limiting case realized at frequencies close to the Bragg resonance, the field is represented as two counterpropagating quasi-optical wave beams coupled on a corrugated surface and forming a normal surface wave. When interacting with an electron beam, convective instability, which can be used to realize amplifier regimes, corresponds to the first case, and absolute one, which is applied in surface-wave oscillators, corresponds to the second case. The developed theory is used to determine basic characteristics of amplifier and oscillator schemes: the growth rates, the energy exchange efficiency, and the formation of a self-consistent spatial structure of the radiated field. The practical realization of relativistic submillimeter amplifiers and surface-wave oscillators is shown to hold promise.     

The effect of surface stress on the propagation of Lamb waves

This work investigates the possibility of the propagation of Lamb waves in thin solid layers with external traction free surfaces, in the presence of surface elasticity, inertia and residual stress. It is demonstrated that such waves do exist and that their characteristics can be quite different from their classical counterparts. The governing equations with non-classical boundary conditions involving the bulk and surface stress are solved exactly in the frequency-wavenumber domain. This solution is utilized to compute the Lamb wave modes for different layer thicknesses. An efficient strategy to capture all the modes of Lamb waves within a given frequency window is outlined. It is shown that the effect of surface elasticity and inertia becomes significant with increasing frequency and decreasing layer thickness, where the number of modes participating within a given frequency window is more than that permitted by the classical theory. Further, it is observed that the nature of the Lamb wave modes (in terms of negative dispersion) in the presence of surface stress is similar to what predicted by the nonlocal theory and microstructure based continuum theory.     

A wavepacket approach to gas-surface scattering: Application to surfaces with imperfections

A time dependent wavepacket approach is applied to diffractive scattering from surfaces with imperfections. This semiclassical method is based on Gaussian wavefunctions whose average positions and momenta are those of classical trajectories. The approach can be applied to arbitrary potentials. Positions, shapes and magnitudes of the diffraction peaks are obtained in a single calculation. In a first example the wavepacket method is applied to a stepped surface. The diffraction pattern is calculated for different incident scattering angles and for various regular and random distributions of steps with two different sizes. Second, the approach is used to study the scattering for an interaction potential modeling a corrugated surface with adsorbed atoms. The diffraction intensities are examined for several final scattering directions and again for regular and random arrangements of the adsorbed atoms.     

Anisotropic sound and shock waves in dipolar Bose-Einstein condensate

We study the propagation of anisotropic sound and shock waves in dipolar Bose-Einstein condensate in three dimensions (3D) as well as in quasi-two (2D, disk shape) and quasi-one (1D, cigar shape) dimensions using the mean-field approach. In 3D, the propagation of sound and shock waves are distinct in directions parallel and perpendicular to dipole axis with the appearance of instability above a critical value corresponding to attraction. Similar instability appears in 1D and not in 2D. The numerical anisotropic Mach angle agrees with theoretical prediction. The numerical sound velocity in all cases agrees with that calculated from Bogoliubov theory. A movie of the anisotropic wave propagation in a dipolar condensate is made available as supplementary material.     

On whistling of pipes with a corrugated segment: Experiment and theory

Corrugated pipes are commonly used because of their local rigidity combined with global flexibility. The flow through such a pipe can induce strong whistling tones, which is an environmental nuisance and can be a threat to the mechanical integrity of the system. This paper considers the use of a composite pipe: a shorter corrugated pipe segment embedded between smooth pipe segments. Such a pipe retains some flexibility, while the acoustical damping in the smooth pipe reduces whistling tones. Whistling is the result of coherent vortex shedding at the cavities in the wall. This vortex shedding is synchronized by longitudinal acoustic waves traveling along the pipe. The acoustic waves trigger the vortex shedding, which reinforces the acoustic field for a critical range of the Strouhal number values. A linear theory for plane wave propagation and the sound production is proposed, which allows a prediction of the Mach number at the threshold of whistling in such pipes. A semi-empirical approach is chosen to determine the sound source in this model. This source corresponds to a fluctuating force acting on the fluid as a consequence of the vortex shedding. The functional form of the Strouhal number dependency of the dimensionless sound source amplitude is based on numerical simulations. The magnitude of the source and the Strouhal number range in which it can drive whistling are determined by matching the model to results for a specific corrugated pipe segment length. This semi-empirical source model is then applied to composite pipes with different corrugated segment lengths. In addition, the effect of inlet acoustical convective losses due to flow separation is considered. The Mach number at the threshold of whistling is predicted within a factor 2.     

Scattering properties of an individual metallic nano-spheroid by the incident polarized light wave

The scattering properties of a metallic nano-spheroid under the illumination of different polarized light waves are investigated using 3D boundary element method. The influences of different geometrical sizes of the nano-spheroid and incident directions of the illuminating light wave on the scattering spectrum are studied for different incident polarized light waves. The results show that the metallic nano-spheroid has two intrinsic resonant modes, corresponding to different polarization states and resonant wavelengths. The scattering enhancement, the resonant wavelength, and the location of the enhanced optical field are strongly dependent on the polarization properties of the illuminating light waves, and they can be modulated by appropriately choosing the polarization directions of the incident light wave.     

Study on echo suppression effect and coloration due to periodic-type diffusers

An echo is one of the obstacles in halls and theatres. In order to suppress this undesirable echo, walls and ceilings have been often given absorptive surfaces which absorb sound energy or corrugated surfaces which provide wave diffusion. While it is not difficult to predict the degree of echo suppression effects in the case of absorptive surfaces, it is generally difficult to do that in the case of corrugated surfaces. In addition, if the surfaces have periodicity, unintended effects called coloration can distort tonal characteristics of the sound field. Thus, many acoustic designers would hesitate to employ this kind of periodic-type diffusers. However, there are some cases where it is necessary to suppress echoes without any energy loss. Periodic-type diffusers have the advantages of easy estimation of diffusion properties, simple method of design, and ready availability. In this study, a subjective experiment with simulated stimuli was carried out to clarify the echo suppression effect of the periodic-type diffusers from the viewpoint of reflected energy level. Although coloration interfered subjective judgments in some cases of the experiment, it is shown that the periodic-type diffusers have echo suppression effects if the coloration does not occur. This study is also devoted to discussion about causes and features of the coloration.     

Enhanced retroreflectance effects in the reflection of light from randomly rough surfaces

A review is made of theoretical and experimental work on retroreflection enhancements in the diffuse component of light elastically reflected from randomly rough surfaces. These effects are seen as a narrow peak in the angular distribution of the intensity of diffusely reflected light which is centered about the direction for reflected light motion antiparallel to the original incident beam. This peak is observed in the scattering of light from many different types of rough surfaces and has been studied in fields as diverse as solid state physics, astronomy, geophysics, meterology and radar. Work covering all of these fields will be presented in this review.

Retroreflection enhancements arise both from shadow casting properties of surface irregularities and from the phase coherence of retroreflected light. These mechanisms can act to create retroreflection enhancements from rough surfaces of dielectric and/or metallic compositions and of surface disorders characterizable on length scales which are large, comparable to or small compared to the wavelength of the scattered light.

Specific discussions will be presented of three types of enhanced retroreflectance: (1) A treatment of the optical glory and Heiligenschein phenomena which are concerned with the meterological and geophysical study of light reflected from clouds and terraine will be given. (2) The theory of the opposition effect, encountered in astronomy as an enhanced retroreflection in the light scattered from atmosphereless planets and space debris, will be used to provide a theoretical basis to understand shadowing effects. (3) A recently discovered phenomenon of enhanced retroreflection from weakly rough metallic mirrors, associated with the Anderson localization of surface waves, is also presented. This last phenomenon and its relationship to the study of the Anderson localization of surface waves will be emphasized throughout our discussions.

Similar enhancement effects in the scattering of acoustic waves from rough surfaces and a brief outline of some recent work on optical backscattering enhancements due to the Anderson localization of bulk polariton modes, is also presented.     

Enhanced anisotropy in Paratellurite for inhomogeneous waves and its possible importance in the future development of acousto-optic devices

The anisotropic feature of most crystals, involves a direction dependent wave velocity for each of the possible modes. Paratellurite (Tellurium dioxide) is extraordinary because, for one of the propagation modes, i.e. the quasi shear horizontal (QSH) mode, the anisotropy is exceptional. This results, on the one hand in a very strong directional dependent sound velocity and on the other hand, in a low wave velocity in certain directions, resulting in a high figure of merit for the acousto-optical interaction. In the case of inhomogeneous waves, the slowness surfaces change their shape and magnitude, for all crystals. However, for paratellurite, this effect is again extraordinary. As soon as a relatively small inhomogeneity is considered, the sound velocity for the QSH mode becomes really exceptionally anisotropic, resulting in a slowness surface that is almost spherical, covered by pins. The velocity corresponding to those 'pins', is much lower than in the case of homogeneous plane waves, which is very promising for the future development of acousto-optic cells involving an even higher figure of merit.     

Experimental study on the Stokes effect in disordered birefringent microstructure fibers

In this paper, a 120-fs pulse transmission experiment is carried out using disordered birefringent microstructure fibers with cladding ventages. Through this experiment, it is found for the first time that remarkable Stokes and anti-Stokes waves can also be produced when the central wavelength of the incident pulse is in the normal dispersion regime of the microstructure fiber. The generation of the two waves can be explained by the four-wave mixing phase matching theory. Properties of the two waves under the action of femtosecond laser pulses with different parameters are studied. The results show that the central wavelength of anti-Stokes waves and Stokes waves produced under the two orthogonal polarization states shift by 63 nm and 160 nm, respectively. The strengths and central positions of the two waves in birefringent fibers can be controlled by adjusting the phase match condition and the polarization directions of incident pulses.     

Sound generation due to the interaction of surface waves

Two opposite gravity-capillary waves of equal frequency give rise to the formation of a standing wave on the ocean surface and, thus, in the nonlinear approximation, generate a sound wave of twofold frequency with an amplitude proportional to the squared height of the surface wave [1]. This effect, being caused by the nonlinear interaction of opposite surface waves, can give rise to the radiation of sound waves in both ocean and atmosphere [2]. Opposite waves can appear in the ocean as a result of different ocean-atmosphere interactions and, in particular, as a result of the blocking of capillary waves on the slope of a gravity wave.     

室内粗糙表面声散射体掠入射角度散射估计

针对经典边界元方法对掠入射角度下的散射估计精度较低的问题,发展了一种边界无网格模型.此模型将散射体视为具有无限延伸的形式,避免了经典边界元法中薄板形式所导致的声压差问题,而且更加符合实际房间中散射体的存在形式;模型利用无网格算法实现数值仿真,可对任意表面形状特别是曲面散射体具有更高的仿真精度。利用边界无网格模型计算了不同形状散射体的散射系数及散射声场,并将结果与解析方法、测量实验进行了对比.对比结果表明,边界无网格模型可以准确预测散射体的散射性质,特别是对掠入射角度的估计要优于经典边界元法.研究结论可应用于室内声学散射体特征预测及优化设计,对提高声场扩散及室内音质水平具有重要意义.      

The representation of 3D gaussian beams by means of inhomogeneous waves

There are different methods to mathematically represent a bounded beam. Perhaps the most famous method is the classical Fourier method that consists of the superposition of pure homogeneous plane waves all traveling in different directions and having an amplitude that can be found by the Fourier transform of the required profile. This method works perfectly for 2D as well as for 3D bounded beams. However, some researchers prefer the inhomogeneous wave theory to represent a bounded beam because some phenomena, e.g. the Schoch effect, are explained by this method by means of concepts that agree better with intuition. There are several papers dealing with this method for 2D gaussian beams. Until now, it has never been considered possible to represent 3D gaussian beams as well. The present paper shows a method to overcome this shortcoming and presents different sorts of 3D gaussian beams that are built up by means of inhomogeneous plane waves.     

Velocity of a SAW propagating in a 2D phononic crystal

We have studied the propagation of a surface acoustic waves (SAW), in a structure constituted by a 2D phononic film (a few micrometers thick and having lattice constants of a few hundreds of micrometers in the two directions of the propagation plane) deposited onto a homogeneous semi-infinite substrate. First, we have calculated the dispersion relations of the acoustic modes by using a plane waves expansion method. We found that the surface branch exhibits both the folding effect and a band gap for the propagation along some particular directions. This is a very interesting result which demonstrates that the effects related to the existence of the band gap (sound velocity dispersion, diffraction, refraction, ultrasound tunneling, etc.) can all appear, even if the thickness of the phononic film is much less than the penetration depth of the SAW. Then, we used an all-optical technique to monitor the spectral content of the SAW propagating along the GammaX direction in the reduced Brillouin zone. We show that a wave with frequency in the stop band, is destructively diffracted after it propagates through less than ten periods. Finally, we report on measurements of the Rayleigh wave phase velocity and we show that the transit time is independent of the distance traveled inside the phononic crystal, suggesting that tunneling trough the sample is involved.