非均匀组织医学超声发射声场仿真

为了分析医学超声在非均匀组织中的分布特性,建立了超声发射声场的计算模型。采用交错网格有限差分法对耦合超声非线性方程进行数值求解,获得了声速、密度及非线性参数非均匀分布情况下人体腹壁组织内的超声声场分布数据。同均匀介质相比:当声速均匀而密度非均匀时,声束仍聚焦良好,焦点处声能下降了1.8 dB;当密度均匀而声速非均匀时,声束发散严重,焦点处声能下降了3.8 dB,下降程度与非均匀组织接近。组织声速在空间分布的非均匀性是导致聚焦声束能量分布畸变的主要原因。     

球形腔聚焦换能器的非线性声场建模

球形腔聚焦换能器是一种特殊形式的聚焦换能器。为理论证实球形腔聚焦换能器能突破传统超声聚焦在聚焦精度和聚焦增益上的限制,采用Westervelt非线性方程并结合时域有限差分法,建立了球形腔聚焦换能器的非线性声场的数值模型。数值计算了直径为120 mm的0.6 MHz球形腔聚焦换能器的非线性声场,并与传统球壳形聚焦换能器进行了对比。当激励声压为100 kPa时,球形腔聚焦换能器与同尺寸壳形聚焦换能器相比,焦点正声压增益提高约8.5倍,且焦域精度更高,-6 dB聚焦区域在z方向减小约20倍,达到次波长尺度。研究表明球形腔聚焦换能器在高强度聚焦超声精细治疗上具有潜在的应用前景。     

Simulation of sound field in a tissue medium generated by a concave spherically annular transducer

The concave spherically annular transducer is regarded as a negative and a positive concave spherical transducer, and the spheroidal beam equation is used to simulate the linear and nonlinear sound field in a tissue medium generated by this transducer. It is found that the acoustic focus of the ring does not coincide with the acoustic focus of its central part. If the width of the ring increases, its acoustic focus will move toward the geometric focus and the amplitudes of nonlinear harmonics will increase obviously. If there are several coaxial rings placed on the concave spherical surface, more than one peak will appear along the axial direction for the fundamental, and high harmonics focus better. The distribution of sound field will change with the number and the excited signals of rings, so it maybe is a potential approach to treat locally big tumors.     

Simulation study of the superharmonic field generated from a phase array transducer

Ultrasound imaging is the most widely used noninvasive medical imaging modality. Its latest elite concept is Superharmonic Imaging which is the most talked-about future of medical diagnostic ultrasound.In this paper,the computational and analytical study for superharmonic field generation from phased array transducer of 16×16 elements is presented. For this,the model preferred,includes the calculation for diffraction effect in frequency domain and nonlinear effect in time domain.The attenuation is included along with the diffraction in frequency domain as well.The comparative analysis of superharmonic field is also carried out with simulated fundamental and second harmonic fields by the same model.Similarly,the comparison with the results from the literature is also reported.     

Sound field of an electromagnetic acoustic transducer

The characteristic of radiated sound field excitated within a non-ferromagnetic aluminum by an electromagnetic acoustic transducer consisting of circle-spiral coil and vertical bias magnetic field is studied.This article presents an improved force source model according to the principle of electromagnetic induction.Then the analytic displacement solution of sound field produced by that force source model is deduced.The tangential and normal directivities of shear and longitudinal sounds are calculated numerically and compared with experimental results.Experimental results conform well to the theoretical prediction,which not only shows that the electromagnetic acoustic transducer of this conformation has a good directivity of vertical shear-wave radiation,but also confirms the given force source model is effective to describe the actual physical behavior of the probe.Theoretical and experimental study lays a foundation for optimization design and engineering application of the electromagnetic acoustic probe.     

Acousto-optic interaction in a non-homogeneous acoustic field excited by a wedge-shaped transducer

The article is devoted to theoretical analysis of light diffraction in a non-homogeneous acoustic field created by a wedge-shaped piezoelectric transducer. Relationships describing electrical, acoustic and acousto-optic properties of the acousto-optic cell are derived in the approximation of a small thickness of the piezoelectric plate. Principal characteristics of acousto-optic interaction are examined such as dependences of the light diffraction efficiency on the phase mismatch and the acoustic wave amplitude. It is shown that the acoustic field has a complicated amplitude-phase structure changing with the acoustic frequency. It is demonstrated that the diffraction efficiency in the Bragg regime can approach 100% in spite of a noticeable phase mismatch. The appropriate optimal values of ultrasound power and incidence angles of light are found.     

电磁声换能器的辐射声场研究

分析了一种圆环形螺线圈、垂直偏置磁场结构的电磁声探头在非铁磁性介质铝中的辐射声场特性。首先根据电磁感应原理对探头进行了物理和数学建模,将表面力源近似成水平和竖直分布的两种,且水平分量远大于竖直分量,推导出该种力源产生的声场的理论解析解。分别数值计算出了横波和纵波的切向和法向指向性,并和实验结果进行对比。实验结果和理论计算符合较好,为此种结构的电磁声换能器的实际工程应用提供了理论依据。      

复频聚焦超声换能器水中焦区商场特性的研究

本文对新研制的一种复频聚焦超声换能器的辐射声场进行了实验研究,测定了辐射压在轴向上的分布曲线,进而确定了焦区位置,这与理论所得结果符合较好,测定了辐射声压在焦平面上的分布曲线,并对焦区声场进行了定位,最后对换能器在水中焦区辐射声的频谱进行了研究,不仅观测到了两个源波,而且还观测到了和频波,差频波以及倍频波,证实了声散射声效应的存在。     

Quantitative Schlieren measurements of a high energy electromagnetic transducer acoustic shock field

Schlieren photography has been used to analyse quantitatively the acoustic field of an electromagnetic acoustic transducer (EMAT). By measuring the angle through which the rays are refracted it is possible to compute the refractive index gradient and thus determine both the absolute and complex pressure related structures of the images. Using this method, planar and focused shock transients generated by the EMAT have been evaluated and compared with transducer derived pressure measurements. The peak pressure in the unfocused shock was found to be 3.2 MPa and 4.6 MPa for the Schlieren and piezoelectric transducer measurements respectively. Corresponding values for the focused shock-wave agreed to within experimental error at about 19 MPa.     

Transient acoustic field radiated by a thickness—shear mode transducer

The acoustic field generated by a thickness—shear mode transducer is theoretically analysed by an approximate model to find the origin of the longitudinal pulse in the signal received by the same transducer. It is believed to arise from the edge longitudinal wave.     

Semi-analytical computation of the acoustic field of a segment of a cylindrically concave transducer in lossless and attenuating media

Conventional ultrasound transducers used for medical diagnosis generally consist of linearly aligned rectangular apertures with elements that are focused in one plane. While traditional beamforming is easily accomplished with such transducers, the development of quantitative, physics-based imaging methods, such as tomography, requires an accurate, and computationally efficient, model of the field radiated by the transducer. The field can be expressed in terms of the Helmholtz-Kirchhoff integral; however, its direct numerical evaluation is a computationally intensive task. Here, a fast semianalytical method based on Stepanishen's spatial impulse response formulation [J. Acoust. Soc. Am. 49, 1627-1638 (1971)] is developed to compute the acoustic field of a rectangular element of cylindrically concave transducers in a homogeneous medium. The pressure field, for, lossless and attenuating media, is expressed as a superposition of Bessel functions, which can be evaluated rapidly. In particular, the coefficients of the Bessel series are frequency independent and need only be evaluated once for a given transducer. A speed up of two orders of magnitude is obtained compared to an optimized direct numerical integration. The numerical results are compared with Field II and the Fresnel approximation.     

The influence of a nonuniform acoustic field on small-scale processes at a heterogeneous boundary

An experiment is conducted on estimating the velocity of a Schlichting boundary flow arising when a focused field falls on a rigid boundary in a liquid. The velocity of a small-scale Schlichting flow is determined by an indirect method from the characteristics of the cocurrent Rayleigh flow using the particle image velocimetry method. The velocity of the Schlichting flow attained in experiments gives us the possibility of significantly accelerating mass-transfer processes at a heterogeneous boundary, which is confirmed by experimental results on acoustic intensification of rapid growth of salt monocrystals conducted under strictly controlled laboratory conditions.     

任意阵形水声换能器阵辐射声场计算

本文用边界元的方法计算了阵形任意的水声发射换能器共形阵的辐射声场。在详细研究了边界元方法基本理论的基础上，采用Pro／ENGINEER软件与ANSYS软件相结合来建立换能器阵的边界元模型，再用SYSNOISE软件计算其辐射声场和辐射阻抗。这种声场计算方法可用于发射换能器阵设计阶段对设计结果进行预报。     

Semicylindrical acoustic transducer from a dielectric elastomer film with compliant electrodes

A semicylindrical acoustic transducer was constructed using a dielectric elastomer film with compliant electrodes that is an electroactive polymer composed of a polyurethane elastomer base and polyethylene dioxythiophene/polystyrene sulfonate electrodes. The use of this dielectric elastomer is advantageous because polyurethane is a common material that keeps its shape without any rigid frame. Because the dielectric elastomer films are essentially incompressible, electric-field-induced thickness changes are usually translated into much larger changes of the film area and side length. Here it is proposed that this change in side length can be utilized for sound generation when the film is bent into a semicylindrical shape. Accordingly, a semicylindrical acoustic transducer was fabricated using a film of thickness of 300 μm and its acoustic characteristics were investigated. The transducer can be operated at low applied voltages by reducing the film thickness, as long as the film is thick enough to generate sufficient force to overcome sound radiation impedance. The second harmonic distortion of the transducer was also investigated as a function of the ratio of the direct current bias voltage to the alternating current audio signal amplitude.     

低频功率超声换能器辐射声场的激光干涉检测

基于媒质的压光效应,它的折射率n是声压的函数,于是利用激光干涉技术及理论上或实验上得到的声压相对分布,可以绝对测定声场中光路上各点的声压.本文对激光干涉测量声压的原理、方法作简单介绍,给出了单面活塞辐射源中场的声压分布的理论表达式,对一增幅杆超声换能器辐射声场中的声压进行了激光干涉实验测定,并与用 PVDF换能器实测的声压分布及声压值进行比较.     

柱面分层流体饱和孔隙地层中的声波测井波场模拟

因钻井作业时泥浆侵入等因素,井外介质沿径向常呈现非均匀性。本文将地层视为柱面分层孔隙介质,采用传递矩阵法模拟声波测井波场。注意到慢纵波波数的实部和虚部都很大,当井外含大厚度层且频率较高时,易因计算大自变量的Hankel函数而溢出,我们在波场表达式中引入了归一化的Hankel函数。论文将前人模拟关于柱面分层固体弹性介质声波测井的广义反射/透射系数计算方法推广到了分层孔隙介质情况,导出了波幅系数的计算公式,并针对含侵入带的柱面分层孔隙地层,计算了声波测井全波波形。     

Effects of the transducer dynamics in underwater acoustic communication

The inter-carrier interference (ICI) of Orthogonal Frequency Division Multiplexing (OFDM) symbols, which is caused by the transducer’s dynamics, has been analysed and simulated in this paper. The small Cyclic Prefix (CP) method is proposed to overcome the ICI caused by the transducer in order to maintain the orthogonality between sub-carriers without much loss of the energy and the bandwidth efficiency. The simulation results show that the small CP whose length is longer than the transient response of the transducer can eliminate the ICI problem.     

Simulation procedure of unconfined seepage with an inner seepage face in a heterogeneous field

An inner seepage face phenomenon is given and a numerical simulation procedure has been developed.It may appear at the interface of two materials when an unconfined seepage flows from a porous media to a coarser porous media with a higher permeability.Inaccuracy and divergent problems may arise both in a saturated-only and in a variably saturated analysis while an inner seepage face is not simulated with a special procedure.The position of the seepage face is determined during the nonlinear iteration process and the flux of the inner seepage face nodes is transferred to the downstream side nodes.Validity and efficiency of the procedure are illustrated by the simulation of two dimensional steady state seepage examples of heterogeneous zoned dams which is usually used to validate algorithms.An analysis of a three-dimensional earth core rockfill dam is also presented here.The procedure can also be applied to general transient seepage problems.     

波浪回声信号仿真

本文基于波浪观测谱的反变换,即采用线性迭加（傅利叶重构）法及解卷积滤法实现波浪仿真,在此基础上,进一步确定波浪脉中回声到达时间波浪声散射强度（平均回声级）及起伏规律,并由８０９８单片机硬件实现。     

1-3型二维柱形凹面线聚焦换能器声场特性

设计并研制了柱面结构的1-3型复合材料凹面线聚焦换能器,在提高换能器带宽的同时,可以实现声场的线聚焦。将换能器内部PZT柱作为独立声源,应用瑞利积分和叠加原理,推导出了柱形凹面换能器总声场的理论表达式。通过仿真计算分析了换能器在聚焦线上的声场特点以及相关参数对聚焦性能的影响。对换能器参数做出合理设计,使换能器在实现线聚焦的同时,声场在聚焦线上的起伏较小,从而设计并制作出聚焦性能良好的线聚焦凹面换能器探头。实验测试结果表明采用本文方法计算得到的换能器声场与实测的声场分布基本符合,柱形凹面换能器在其几何焦点附近范围内均可实现聚焦,并在侧向上形成清晰的聚焦线,其聚焦线长度为换能器的侧向结构长度,在聚焦线上声场分布起伏较小。