利用声衰减反演气泡群分布的方法研究

通过测量含气泡水的声衰减反演气泡群参数是获取水中气泡分布的重要方法,但是经典方法忽略了较高浓度气泡水中的强频散特性和气泡振动参数的改变,导致反演较高浓度气泡群分布时会产生巨大误差.为解决这个问题,本文基于等效媒质理论建立起了声衰减和相速度的联系,并考虑了含气泡水平均量对气泡阻尼系数和共振频率的影响.在此基础上,通过将反...     

超声造影剂微气泡的包膜黏弹特性的定量表征研究

包膜黏弹特性显著影响微气泡超声造影剂的诊断及治疗应用效果. 本文结合原子力显微镜技术及声衰减特性测量提出了一种对微气泡造影剂包膜黏弹特性定量表征的新方法. 首先采用原子力显微镜技术进行机械特性分析得到包膜微气泡的有效硬度及体弹性模量; 然后测量声衰减特性, 基于微气泡动力学理论, 计算包膜微气泡的体黏度系数. 为验证方法的有效性, 实验制备了直径为1-5 μm的白蛋白包膜微气泡造影剂, 原子力显微镜测量的有效硬度和体弹性模量分别为0.149±0.012 N/m和8.31±0.667 MPa, 并与粒径无关. 声衰减特性测量和动力学理论拟合的包膜微气泡的体黏度系数为0.374±0.003 Pa·s. 该方法可推广至其他种类包膜微气泡的黏弹特性表征, 对超声造影剂的制备及其诊断和治疗应用有积极意义.     

原油中声衰减和声速的温度特性的测量

原油是一种混合物。它的一些性质随温度而变，如原油的声衰减和声速都是温度的函数．本文使用一个换能器发射声波，另一个换能器接收声波，在实验室内测得12t－80℃之间的声波幅度值，利用插值算法，求出该温度范围内原油声衰减的温度特性曲线；利用线性拟合算法，求出原油声速的温度特性曲线。结果表明：随着温度的升高，原油的声衰减和声速都呈下降趋势，而且测量结果与换能器的灵敏度无关．     

混合弹性颗粒体系声衰减数值模拟

采用概率统计方法——蒙特卡罗方法,建立一种预测液固两相体系中混合弹性球形颗粒声衰减的理论模型。在单颗粒声散射和吸收的基础上,将连续超声波抽象离散化为大量独立的声子,追踪声散射过程,通过统计接收器探测声子数最终确定声衰减系数。采用数值方法对单一球形颗粒的液固两相体系中声衰减进行预测和比较,确定了该方法的可行性后将该方法推广到混合颗粒体系中,对玻璃微珠/铁粉构成的混合颗粒及多分散混合颗粒体系进行数值研究。结果表明:在体积浓度低于10%时,蒙特卡罗法预测得玻璃微珠或铁粉颗粒声衰减和ECAH,Lloyd和Berry,Waterman等建立的模型结果吻合。对于混合颗粒构成的两相体系,算例中,随着体积浓度增大到10%,声衰减系数随混合颗粒数目比的变化呈现出了非线性的变化,同时颗粒物性也会影响不同组分颗粒对声衰减的贡献,算例中铁粉颗粒比玻璃微珠对声衰减的影响更大。     

人体血管壁超声传输衰减特性的研究

本文利用超声脉冲反射法，对人体血管壁声衰减特性进行了离体测量，给出了人体血管壁声能量衰减参数的测量值。这对超声在医学领域的应用以及超声连体非介入血栓消溶有着十分重要的意义。     

水中上浮气泡动态特性研究

针对水中的上浮气泡,计入表面张力的作用,然后将气泡边界层内黏性效应分为法向和切向两部分,其中附加法向黏性应力通过Young-Laplace关系考虑;附加切向黏性力是基于黏性耗散能量等效原理,引入黏性修正压力代替.首先建立了轴对称和三维上浮气泡边界元模型,将数值结果与理论值和实验值进行对比分析,有良好的符合度,验证了数值模型的有效性;然后针对毫米量级上浮气泡的平衡速度与形态,讨论了气泡初始条件、表面张力和黏性对气泡上浮过程中动力学行为的影响;最后,提出了一种处理三维上浮气泡融合的数值方法,计算结果与实验现象符合良好,并且能够反映气泡融合后的复杂现象细节.     

计入地面附加衰减的声屏障插入损失估算方法*

基于理论推导和计算,给出了公路声屏障声学设计中,在考虑地面附加衰减情况下计算插入损失的方法。该方法综合考虑了有限长线声源无限长声屏障绕射声衰减量、有限长线声源地面衰减量及遮蔽角对插入损失的影响。通过与《声屏障声学设计和测量规范》（HJ/T90-2004）的计算结果的对比,验证了本文所给方法的精确性及可行性,并对规范所给地面衰减修正量进行了商榷。最后,给出了当预测点位于有限长路段中央法线上时,通过计算线声源地面衰减量得到计算插入损失所需参数值,再计算插入损失的简便方法。本研究为存在地面附加衰减情况下有限长声屏障插入损失计算提供了一个新的参考方法。     

卤化银微晶中光电子衰减谱的特性

光电子在卤化银材料潜影形成过程中发挥着重要的作用 ,光电子衰减行为在很大程度上决定于卤化银的晶体结构。采用光学与微波双共振技术 ,测量了自由光电子和浅俘获光电子的衰减谱 ,得到了卤化银中电子陷阱的密度和深度分布。以自由光电子寿命为纽带 ,通过分析掺杂卤化银晶体中电子陷阱的分布情况 ,可以确定浅电子陷阱掺杂剂的最佳掺杂浓度。     

旋涡声散射特性的尺度效应数值研究

以声波为主要表现形式的膨胀过程和以旋涡为主要表现形式的剪切过程之间的非线性耦合问题一直以来都是流体力学的研究热点.尤其是旋涡对声波的散射问题,具有重要的科学意义与工程应用背景.本文通过线性紧致格式直接数值求解二维欧拉方程,获得了平面声波穿过均熵Taylor涡的散射特性.与之前经典文献中的标准算例比较,结果极其吻合,直接...     

旋涡声散射的空间尺度特性数值研究

旋涡对声波的散射问题是声波在复杂流场中传播的基本问题,在声源定位、声目标识别及探测、远场噪声预测等方面具有重要的学术研究价值和工程应用价值,如飞行器的尾涡识别、探测及测距,湍流剪切流中声目标预测,声学风洞试验中声学测量和声源定位等.声波穿过旋涡时会产生非线性散射现象,其物理机理主要与声波波长和旋涡半径的长度尺度比相关....     

In vitro measurement of attenuation and nonlinear scattering from echogenic liposomes

Echogenic liposomes (ELIP) are an excellent candidate for concurrent imaging and drug delivery applications. They combine the advantages of liposomes-biocompatibility and ability to encapsulate both hydrophobic and hydrophilic drugs-with strong reflections of ultrasound. The objective of this study is to perform a detailed in vitro acoustic characterization - including nonlinear scattering that has not been studied before - along with an investigation of the primary mechanism of echogenicity. Both components are critical for developing viable clinical applications of ELIP. Mannitol, a cryoprotectant, added during the preparation of ELIP is commonly believed to be critical in making them echogenic. Accordingly, here ELIP prepared with varying amount of mannitol concentration are investigated for their pressure dependent linear and non-linear scattered responses. The average diameter of these liposomes is measured to be 125-185 nm. But they have a broad size distribution including liposomes with diameters over a micro-meter as observed by TEM and AFM. These larger liposomes are critical for the overall echogenicity. Attenuation through liposomal solution is measured with four different transducers (central frequencies 2.25, 3.5, 5, 10 MHz). Measured attenuation increases linearly with liposome concentration indicating absence of acoustic interactions between liposomes. Due to the broad size distribution, the attenuation shows a flat response without a distinct peak in the range of frequencies (1-12 MHz) investigated. A 15-20 dB enhancement with 1.67 μg/ml of lipids is observed both for the scattered fundamental and the second harmonic responses at 3.5 MHz excitation frequency and 50-800 kPa amplitude. It demonstrates the efficacy of ELIP for fundamental as well as harmonic ultrasound imaging. The scattered response however does not show any distinct subharmonic peak for the acoustic excitation parameters studied. Small amount of mannitol proves critical for echogenicity. However, mannitol concentration above 100 mM shows no effect.     

Theoretical and phantom based investigation of the impact of sound speed and backscatter variations on attenuation slope estimation

Quantitative ultrasound features such as the attenuation slope, sound speed and scatterer size, have been utilized to evaluate pathological variations in soft tissues such as the liver and breast. However, the impact of variations in the sound speed and backscatter due to underlying fat content or fibrotic changes, on the attenuation slope has not been addressed. Both numerical and acoustically uniform tissue-mimicking experimental phantoms are used to demonstrate the impact of sound speed variations on attenuation slope using clinical real-time ultrasound scanners equipped with linear array transducers. Radiofrequency data at center frequencies of 4 and 5 MHz are acquired for the experimental and numerical phantoms respectively. Numerical phantom sound speeds between 1480 and 1600 m/s in increments of 20 m/s for attenuation coefficients of 0.3, 0.4, 0.5, 0.6, and 0.7 dB/cm/MHz are simulated. Variations in the attenuation slope when the backscatter intensity of the sample is equal, 3 dB higher, and 3 dB lower than the reference is also evaluated. The sound speed for the experimental tissue-mimicking phantoms were 1500, 1540, 1560 and 1580 m/s respectively, with an attenuation coefficient of 0.5 dB/cm/MHz. Radiofrequency data is processed using three different attenuation estimation algorithms, i.e. the reference phantom, centroid downshift, and a hybrid method. In both numerical and experimental phantoms our results indicate a bias in attenuation slope estimates when the reference phantom sound speed is higher (overestimation) or lower (underestimation) than that of the sample. This bias is introduced via a small spectral shift in the normalized power spectra of the reference and sample with different sound speeds. The hybrid method provides the best estimation performance, especially for sample attenuation coefficient values lower than that of the reference phantom. The performance of all the methods deteriorates when the attenuation coefficient of the reference phantom is lower than that of the sample. In addition, the hybrid method is the least sensitive to sample backscatter intensity variations.     

Evaluation of ultrasonic attenuation without invoking the diffraction correction separately

A method is presented for the ultrasonic attenuation measurement as a function of frequency using a broad band transducer. The diffraction correction has been introduced in the expression such that an algorithm may be developed for evaluation of attenuation directly. This eliminates the introduction of the diffraction correction curve separately. Results are presented for various types of aluminium alloys. The method is suitable for samples placed a distance further than three times the near field and which have a size much larger than the transducer.     

A novel power spectrum calculation method using phase-compensation and weighted averaging for the estimation of ultrasound attenuation

An estimation of ultrasound attenuation in soft tissues is critical in the quantitative ultrasound analysis since it is not only related to the estimations of other ultrasound parameters, such as speed of sound, integrated scatterers, or scatterer size, but also provides pathological information of the scanned tissue. However, estimation performances of ultrasound attenuation are intimately tied to the accurate extraction of spectral information from the backscattered radiofrequency (RF) signals. In this paper, we propose two novel techniques for calculating a block power spectrum from the backscattered ultrasound signals. These are based on the phase-compensation of each RF segment using the normalized cross-correlation to minimize estimation errors due to phase variations, and the weighted averaging technique to maximize the signal-to-noise ratio (SNR). The simulation results with uniform numerical phantoms demonstrate that the proposed method estimates local attenuation coefficients within 1.57% of the actual values while the conventional methods estimate those within 2.96%. The proposed method is especially effective when we deal with the signal reflected from the deeper depth where the SNR level is lower or when the gated window contains a small number of signal samples. Experimental results, performed at 5 MHz, were obtained with a one-dimensional 128 elements array, using the tissue-mimicking phantoms also show that the proposed method provides better estimation results (within 3.04% of the actual value) with smaller estimation variances compared to the conventional methods (within 5.93%) for all cases considered.     

Nonlinear oscillation and acoustic scattering of bubbles

The scattered acoustic pressure and scattered cross section of bubbles is studied using the scattered theory of bubbles. The nonlinear oscillations of bubbles and the scattering acoustic fields of a spherical bubble cluster are numerically simulated based on the bubble dynamic and fluid dynamic. The influences of the interaction between bubbles on scattering acoustic field of bubbles are researched. The results of numerical simulation show that the oscillation phases of bubbles are delayed to a certain extent at different positions in the bubble cluster, but the radii of bubbles during oscillation do not differ too much at different positions. Furthermore, directivity of the acoustic scattering of bubbles is obvious. The scattered acoustic pressures of bubbles are different at the different positions inside and outside of the bubble cluster. The scattering acoustic fields of a spherical bubble cluster depend on the driving pressure amplitude, driving frequency, the equilibrium radii of bubbles, bubble number and the radius of the spherical bubble cluster. These theoretical predictions provide a further understanding of physics behind ultrasonic technique and should be useful for guiding ultrasonic application.     

Effect of viscosity on ultrasound wave reflection from a solid/liquid interface

A study of the simulated reflection of a wideband ultrasound shear wave from the solid/viscous fluid interface is presented. Various parameters affecting reflection factors including the material properties of the solid, fluid properties like density and viscosity, and the operating frequency are discussed. Simulated ultrasonic response waveforms are compared with the experimentally obtained data for NIST traceable calibration standards of viscosity. A good agreement was observed between the simulated and experimental waveforms at various viscosities and for different solid substrates.     

Determination of biomass concentration by measurement of ultrasonic attenuation

Experimental results are presented that demonstrate that ultrasonic attenuation can be used to estimate the biomass concentration of a biological suspension. The attenuation approach avoids the temperature sensitivity of established ultrasound velocity test methods, which have hindered their application in process environments. An empirical model is presented for the estimation of the biomass of Microcystis aeruginosa suspensions based on acoustic attenuation, temperature, and frequency, and is compared with experimental results.     

Acoustic measurement of suspensions of clay and silt particles using single frequency attenuation and backscatter

The use of ultrasonic acoustic technology to measure the concentration of fine suspended sediments has the potential to greatly increase the temporal and spatial resolution of sediment measurements while reducing the need for personnel to be present at gauging stations during storm events. The conversion of high-frequency attenuation and backscatter amplitudes to suspended silt and clay concentration has received relatively little attention in the literature. In order to improve the state of knowledge, a laboratory investigation was undertaken by the National Center for Physical Acoustics in cooperation with the USDA-ARS National Sedimentation Laboratory. In these experiments, two immersion transducers were used to measure attenuation and backscatter from 20 MHz acoustic signals propagated through suspended clay (smectite and kaolinite) and silt particles. The resulting data includes attenuation values for a wide range of concentrations (0.3–14 g/L) and particle sizes (0.03–14 μm diameter). Attenuation curves for each particle were compared to the theoretical attenuation curves developed by Urick (1948) and Sheng and Hay (1988) for scattering as presented by Landers (2010) [5,11,12]. In addition, it was found that the backscatter signal could be used to discriminate between suspensions dominated by clay or silt.     

Exvivo ultrasound attenuation coefficient for human cervical and uterine tissue from 5 to 10 MHz

Attenuation estimation and imaging in the cervix has been utilized to evaluate the onset of cervical ripening during pregnancy. This feature has also been utilized for the acoustic characterization of leiomyomas and myometrial tissue. In this paper, we present direct narrowband substitution measurement values of the variation in the ultrasonic attenuation coefficient in ex vivo human uterine and cervical tissue, in the 5-10 MHz frequency range. At 5 MHz, the attenuation coefficient values are similar for the different orientations of uterine tissue with values of 4.1-4.2 dB/cm, 5.1 dB/cm for the leiomyoma, and 6.3 dB/cm for the cervix. As the frequency increases, the attenuation coefficient values increase and are also spread out, with a value of approximately 12.6 dB/cm for the uterus (both parallel and perpendicular), 16.0 for the leiomyoma, and 26.8 dB/cm for the cervix at 10 MHz. The attenuation coefficient measured increases monotonically over the frequency range measured following a power law.     

Propagation of acoustic wave in viscoelastic medium permeated with air bubbles

Based on the modification of the radial pulsation equation of an individual bubble, an effective medium method (EMM) is presented for studying propagation of linear and nonlinear longitudinal acoustic waves in viscoelastic medium permeated with air bubbles. A classical theory developed previously by Gaunaurd (Gaunaurd GC and \"{U}berall H, {\em J. Acoust. Soc. Am}., 1978; 63: 1699--1711) is employed to verify the EMM under linear approximation by comparing the dynamic (i.e. frequency-dependent) effective parameters, and an excellent agreement is obtained. The propagation of longitudinal waves is hereby studied in detail. The results illustrate that the nonlinear pulsation of bubbles serves as the source of second harmonic wave and the sound energy has the tendency to be transferred to second harmonic wave. Therefore the sound attenuation and acoustic nonlinearity of the viscoelastic matrix are remarkably enhanced due to the system's resonance induced by the existence of bubbles.