Nonlinear parameter estimation in water-saturated sandy sediment with difference frequency acoustic wave

Difference frequency acoustic wave from nonlinear interaction of two primary acoustic waves at frequencies of 76 and 114 kHz was utilized with a parametric acoustic array theory to estimate the nonlinearity parameter of water-saturated sandy sediment. Such nonlinearity parameter can be used as background information for the nonlinear acoustic investigation of bottom or sub-bottom profiling in the ocean sandy sediments. Because of its lower attenuation the difference frequency acoustic wave method can be usefully applied to estimate the nonlinearity parameter of ocean sediment in the ocean as well as under laboratory conditions. The nonlinearity parameter β for the water-saturated sandy sediment used as a reference in this study was estimated as β = 80.5 ± 5.1 at the difference frequency of 38 kHz. It was agreed very well with that estimated at the difference frequency of 67 kHz, when two primary frequencies were 137 and 204 kHz. The estimated nonlinearity parameter of water-saturated sandy sediment in this study was also compared and analyzed with those estimated in previously published literatures. It was suggested that the difference frequency wave method used to estimate the nonlinearity parameter of water-saturated sandy sediment can be employed as a good method to estimate the nonlinearity parameters of fluid-like granular media.     

海底沉积物上的声反射

本文将海底沉积物视为一种紧密的湿颗粒介质。声波在其中传播时将出现两个膨胀波和一个旋转波。考虑到这些特征,再假设海水与沉积物的界面是平的,导出了平面声波在其上的反射系数的表达式。文中就松散颗粒介质的情况对反射系数进行了简化,得出了前人用唯象理论所获得的结果。文末将理论与实验结果进行了比较,两者的一致性较好。     

Self-action of seismic acoustic waves in the moist sandy soil

The effect of phase self-action of the longitudinal seismic acoustic wave observed in moist sandy soil is studied theoretically in framework of the phenomenological equation of state.     

Velocity dispersion in water-saturated granular sediment

Recent experiments in the Gulf of Mexico have yielded a wealth of information on the environmental conditions and geoacoustic response of a uniform sand stratum immediately beneath the seafloor. A comparison of p-wave velocities measured at low (125 Hz) and high (11-50 kHz) frequencies in this layer indicates that there is a significant amount of velocity dispersion that occurs in the interval between these extremes. This narrow-band dispersion, which is not consistent with the often-used assumption of a nearly constant-Q model, is in accordance with the predictions of the Biot theory. It results from viscous damping in the fluid as it moves relative to the skeletal frame. Other recent field data that support this conclusion are presented.     

Fundamental study on subharmonic imaging by irradiation of amplitude-modulated ultrasound waves

The second harmonic and subharmonic components, the frequencies of which are twice and one half the fundamental frequency, are included in echoes from contrast agents. An imaging method, which employs a second harmonic (second harmonic imaging), is widely used in medical diagnoses. On the other hand, subharmonic is expected to provide a higher contrast between biological tissues and blood flow because echo signals are generated only from blood containing the contrast agents. However, the subharmonic component echo signal power from contrast agents is relatively low. This has resulted in little progress in the field of subharmonic imaging. In this study, a new imaging method is proposed using amplitude-modulated waves as transmitted waves combined with the pulse inversion method to enhance subharmonic echo signals. Two optimal frequencies are set, including the modulated waves, F(1) and F(2), so that the subharmonic frequency of F(1) and the second harmonic frequency of F(2) may result in the same value. This allows a more powerful signal at the frequency band because the second harmonic and subharmonic components are integrated. Furthermore, a B-mode ultrasound image of an agar phantom that imitated biological tissue and showed the effectiveness of our method was reconstructed. As a result, the echo power of the subharmonic component was enhanced by approximately 11.8 dB more than the conventional method and the signal to noise ratio showed an improvement of 7.6 dB.     

气泡体积分数对沙质沉积物低频声学特性的影响

由于有机物质分解等原因,实际的海底沉积物中存在气泡,气泡的存在会显著影响沉积物低频段的声学特性,因此研究气泡对沉积物低频段声速的影响机理具有重要意义.考虑到外场环境的不可控性,在室内水池中搭建了大尺度含气非饱和沙质沉积物声学特性获取平台,在有界空间中应用多水听器反演方法首次获取了含气非饱和沙质沉积物300—3000 Hz频段内的声速数据(79—142 m/s),并同时利用双水听器法获取了同一频段的数据(112—121 m/s).在声波频率远低于沉积物中最大气泡的共振频率时,根据等效介质理论,将孔隙水和气泡等效为一种均匀流体,改进了水饱和等效密度流体近似模型.模型揭示了气泡对沉积物低频段声学特性的影响规律,理论上解释了沉积物中声速的降低.通过分析模型预报声速对模型参数的敏感性,根据测量得到的声速分布反演得到了沉积物不同区域的气泡体积分数,气泡体积分数从1.07%变化到2.81%.改进的模型为沉积物中气泡体积分数估计提供了一种新方法.     

Experimental study of nonlinear dust acoustic solitary waves in a dusty plasma

The excitation and propagation of finite-amplitude low-frequency solitary waves are investigated in an argon plasma impregnated with kaolin dust particles. A nonlinear longitudinal dust acoustic solitary wave is excited by pulse modulating the discharge voltage with a negative potential. It is found that the velocity of the solitary wave increases and the width decreases with the increase of the modulating voltage, but the product of the solitary wave amplitude and the square of the width remains nearly constant. The experimental findings are compared with analytic soliton solutions of a model Korteveg-de Vries equation.     

Experimental study of the formation of nonlinear acoustic waves in focused beams

The shock wave formation in focused beams produced by spherical hydroacoustic transducers with different apertures and an operating frequency of 3 MHz, as well as in weakly divergent high-intensity beams of the same frequency, is studied experimentally. The profiles of the received signals are analyzed for different receiving points in the acoustic beam and for different combinations of nonlinear and diffraction effects. It is found that the distortion of the initial waveform (i.e., of the compression and rarefaction phases) is asymmetric. The asymmetry of the wave profile in a focused beam is more pronounced than that in a quasi-plane wave while the asymmetric distortion of the high-frequency carrier causes an asymmetric distortion of the pulse envelope. The angular characteristics of the difference-frequency waves produced by parametric sound radiators are compared using both focused and weakly divergent beams of pump waves. The experiments also show that the appearance of a bubbly phase screen in the region before the point of the shock formation either shifts this point to greater distances or makes the discontinuity formation impossible. Results illustrating the changes that occur in the shock wave characteristics when the bubbly phase screen is placed in the region of the fully developed shock are presented.     

声波在水-含气沉积物界面的反射

含气泡海洋沉积物的声学特性是海底探测的重要问题。为了研究气泡存在对水-含气沉积物界面声反射系数的影响,本文基于气泡振动修正的Biot波动方程推导了气泡存在修正的Biot弹性模量,并结合水-沉积物界面的“开孔”边界条件推导了声波从水入射到水-含气沉积物界面的反射系数。数值分析表明气泡的振动导致反射系数呈现显著的频率特性。在气泡共振频率附近,由于气泡的共振引发的强散射和强衰减,使得反射系数很大,无论以何种角度入射,声波都很难进入含气泡的沉积物。本文研究结果表明,气泡半径、含量、声波频率以及入射角度都是影响水-含气沉积物界面反射系数的主要因素。     

Ion acoustic subharmonic excitation in a plasma

Ion acoustic subharmonic excitation in a plasma, with ion-neutral collision frequency greater than the frequency of excitation, is theoretically investigated. Two-fluid theory with source term is used to describe the system. The system exhibits either subharmonic excitation of orders 1/2 and 1/3, or subharmonic excitation of orders 1/3, 1/4 and 1/5. The resonance frequency range and the amplitude of second harmonic for each case is calculated. A comparison with experimental data can be used to obtain the values of the parameters describing the source term.     

Harmonic and subharmonic acoustic wave generation in finite structures

The generation of harmonic and subharmonic vibrations is considered in a finite monodimensional structure, as it is produced by the nonlinear acoustic characteristics of the medium. The equation of motion is considered, where a general function of the displacement and its derivatives acts as the forcing term for (sub)harmonic generation and a series of 'selection rules' is found, depending on the sample constrains. The localization of the nonlinear term is also considered that mimics the presence of defects or cracks in the structure, together with the spatial distribution of subharmonic modes. Experimental evidence is given relative to the power law dependence of the harmonic modes vs. the fundamental mode displacement amplitude, and subharmonic mode distribution with hysteretic effects is also reported in a cylindrical sample of piezoelectric material.     

声表面波检测铝板表面微缺陷深度实验研究

为了探究声表面波与不同深度微裂纹缺陷相互作用的关系,将脉冲激光作用于一系列不同缺陷的试件铝板上进行线光源激励,激发激光超声波。用超声传感器接收在铝板中传播的激光超声信号,通过数字荧光示波器采集激光超声在铝板中的传播数据。对采集到的反射波数据进行分离谱分离过程得到的铝板中激光超声的时域分布和透射波数据进行频域分析。实验发现:缺陷深度影响着反射回波两峰值特征点到达时间差,两者之间近似线性关系,也影响着透射波的截止频率且二者呈现递减关系。     

Broadband measurements of acoustic attenuation in water-saturated sands

A method for the measurement of acoustic attenuation in water-saturated granular media as a continuous function of frequency is described. Results for a range of water-saturated sands are presented together with acoustic velocity data and discussed in relation with porosity and grain size variations.     

Evidence of dispersion in an artificial water-saturated sand sediment

A laboratory experiment was conducted to measure the speed of sound in an artificial water-saturated granular sediment composed of cleaned and sorted medium-grained sand and degassed distilled water. The experiment was conducted within a range of frequencies where dispersion is predicted by a number of existing models. Between 2 and 4 kHz, the sound speed was inferred from measurements of the resonance frequencies of a thin-walled cylindrical container filled with the material. An elastic waveguide model was used to account for the effect of the finite impedance of the walls, although this effect was found to be small. From 20 to 300 kHz, the sound speed was obtained directly from time-of-flight measurements within the sediment. Dispersion in close agreement with the Williams effective density fluid model [K. L. Williams, J. Acoust. Soc. Am. 110, 2276-2281 (2001)] was observed.     

Self-demodulation of acoustic pulses in partially water-saturated river sand

Laboratory experiments on self-demodulation of high-frequency acoustic pulses in partially water-saturated river sand are reported. Based on experimentally revealed dependences of demodulated pulse parameters (propagation velocity, amplitude, and duration) on the static pressure and on the amplitude of the primary pulses, an equation of state for this medium is proposed. Parameters of this equation are derived from the comparison of theoretical predictions and experimental results.     

Experimental study of propagation of instability waves in a submerged jet under transverse acoustic excitation

An experimental study was conducted on the specific features of instability wave propagation in the mixing layer of a turbulent jet when the jet is excited by an external acoustic wave. We used the technique of conditional phase averaging of data obtained by particle image velocimetry using the reference signal of a microphone placed near the jet. The influence of the excitation frequency on the characteristics of large-scale structures in the mixing layer was investigated. It is shown that the propagation patterns of the instability waves agree well with previously obtained data on the localization of acoustic sources in turbulent jets.     

Effect of the sediment layer on the excitation of acoustic modes and lateral waves in shallow sea

The simplest model of a shallow sea in the form of an isovelocity water layer and a fluid sediment layer overlying a homogeneous elastic halfspace is used to investigate the effect of the thickness of the sediment layer and the sound velocity in it on the behavior of the frequency dependences of the amplitudes of trapped and leaky modes and shear and longitudinal lateral waves that are excited by an acoustic point source in a shallow-water oceanic waveguide.     

Gas-driven subharmonic waves in a vibrated two-phase granular material

Vibrated powders exhibit striking phenomena: subharmonic waves, oscillons, convection, heaping, and even bubbling. We demonstrate novel rectangular profile subharmonic waves for vibrated granular material, that occur uniquely in the two-phase case of grains, and a fluid, such as air. These waves differ substantially from those for the gas-free case, exhibit different dispersion relations, and occur for specific shaking parameters and air pressure, understandable with gas-particle flow models. These waves occur when the gas diffusively penetrates the granular layer in a time comparable to the shaker period. As the pressure is lowered towards P =0, the granular-gas system exhibits a Knudsen regime. This instability provides an opportunity to quantitatively test models of two-phase flow.     

Localized subharmonic waves in a circularly vibrated granular bed

Localized period-doubling waves arise in circularly shaken granular beds contained in an annular channel. These solitary wave packets are accompanied by a locally increased particle density. The width and velocity of the granular wave pulse are measured as a function of the bed height. A continuum model for the material distribution, based on the measured granular transport velocity as a function of the bed thickness, captures the essence of the experimental findings.