Accurate depth-independent determination of acoustic backscatter coefficients with focused transducers

The accuracy of a method of data reduction for determining acoustic backscatter coefficients was tested using focused transducers and narrow-band pulses. Two phantoms with well-defined scattering properties were the bases of the tests, one having low attenuation and one with tissue-mimicking attenuation. The experimentally determined backscatter coefficients were found to be independent of transducer-to-scattering-volume distance and to agree very well with theoretical values, typically within 10%.     

A direct method for measuring the temperature coefficients of bulk acoustic velocities in crystals

A method for direct measurement of the temperature coefficients of the velocities of bulk acoustic waves propagating along arbitrary directions in crystal planes is described. It represents a specific version of the Schaefer-Bergmann acoustooptic diffraction in which the effect of the temperature is precisely compensated by biasing the acoustic frequency. The method is equally applicable to the determination of other kinds of velocity coefficients.     

Extensions of the field-emission fluctuation method for the determination of surface diffusion coefficients

The use of scanning tunneling microscopy and related techniques for the determination of surface diffusion coefficients of adsorbates is discussed. Three schemes, all extensions of the field-emission current fluctuation method are presented and analyzed. The first consists of determining single site correlation functions with a STM in its more or less normal operating mode. The second consists of retracting the tip approximately one tip radius so that a circular region of high field is created on the flat surface from which field emission and diffusion-induced current fluctuations can be obtained. The third considers creation of a long narrow region of high field by placing a very fine cylindrical wire parallel to and above the plane substrate. This last scheme allows determination of diffusion anisotropy. Detailed expressions for the current correlation functions for the three schemes and criteria for allowed vibrational amplitudes and drift are derived. It is concluded that all three schemes should be feasible but probably would differ in the range of diffusion coefficient values they would be able to handle.     

腐蚀声发射信号降噪方法

为了从复杂背景噪声中有效地检测出腐蚀声发射信号,采用短时分形维数和离散分数余弦变换相结合的降噪方法,利用声发射检测系统,对6% FeCl₃·6H₂O溶液中Q235钢板的全面腐蚀和局部腐蚀声发射信号进行了降噪处理。实验结果表明,腐蚀声发射信号分别加入白噪声、有色噪声和粉红噪声,在输入信噪比为0~15dB的条件下,此方法降噪效果与标准离散余弦、离散分数余弦变换方法相比,输出信噪比最高可提升8 dB。所述降噪方法对检测腐蚀声发射信号以及对金属剩余寿命的评价具有一定意义。      

On the estimation of backscatter coefficients using single-element focused transducers

The ultimate goal of quantitative ultrasound (QUS) imaging methods based on backscatter coefficient (BSC) estimates is to obtain system-independent structural information about samples. In the current study, three BSC estimation methods were compared and evaluated using the same backscattered pressure datasets in order to assess their consistency. BSC estimates were obtained from two phantoms with embedded glass spheres and compared to theoretical BSCs calculated using size distributions estimated using optical microscopy. Effective scatterer diameter and concentration estimates of the glass spheres were also obtained from the estimated BSCs. One estimation method needed to be compensated by more than an order of magnitude in amplitude in order to produce BSCs comparable to the other two methods. All calibration methods introduced different frequency-dependent effects, which could have noticeable effects on the bias of QUS estimates derived from experimental BSCs. Although in most cases the experimental QUS estimates obtained with all three methods were observed to differ by less than 10%, larger differences are expected depending on both the pressure focusing gain of the transducer (proportional to the ratio of the square of the aperture radius to the product of the wavelength and focal length) and ka range used in the estimation.     

Application of an acoustic backscatter technique for characterizing the roughness of porous soil

An acoustic backscatter technique proposed by Oelze et al. [J. Acoust. Soc. Am. 109, 1826-1832 (2001)] was used to characterize the roughness of porous soil surfaces. Roughness estimation errors are minimized when the effective flow resistivity of the porous soil is high, e.g., above 300,000 mks Rayls/m. Four soil plots were constructed by roughening soil with farming implements. Three plots were sealed using Saran powder dissolved in methyl ethyl ketone (MEK) and then covered to prevent further weathering. A fourth plot was left in the open and exposed to rainfall, which also acted to seal the surface and further change the roughness. In sealing the surface the effective flow resistivity of the surface was increased above 300,000 mks Rayls/m, which is typical for weathered agricultural surfaces. The roughness power spectra of the soil surfaces were measured by acoustic backscatter and alternatively by a laser profiler. Regression analysis was used to approximate each roughness power spectrum versus roughness wave number with a best-fit line. The best-fit line was used to calculate the rms height and the correlation length of the rough surface by integrating the approximate roughness power spectrum over a range of roughness wave number values. The range of roughness wave number values defines the roughness length scales used in the statistical calculations. High-roughness wave numbers correspond to smaller length scales of roughness and low-roughness wave numbers correspond to larger length scales of roughness. Over certain ranges of roughness wave number values the statistics from the acoustic backscatter and laser profiler measurements is in good agreement. However, as the low-cutoff roughness wave number is decreased and the high-cutoff roughness wave number is increased, agreement between the laser and acoustic techniques diminishes.     

统计最优近场声全息重建精度和计算速度优化方法

分析了统计最优近场声全息(Statistically optimized nearfield acousticholography,简称SONAH)的重建过程发现采用SONAH重建噪声源表面法向振速时误差较大的原因在于正则化参数选取较小。在此基础上提出一种单元平面波优化选择方法,该方法保留了单元平面波中的全部传播波和部分倏逝波,去除了一些较高波数的倏逝波成分,保证了重建过程中正则化参数的准确选取。另外,采用单元平面波优化选择方法还可以降低SONAH中传递矩阵的阶数,从而解决SONAH的计算速度随着测量点数目的增加急剧变慢的问题。通过数值仿真和实验对所提出的单元平面波优化选择方法的有效性进行了验证,结果表明采用该方法后SONAH的计算速度和法向振速的重建精度都得到了较大提高。     

An improved acoustic method for the determination of the Boltzmann constant at LNE-INM/CNAM

There is currently great interest in the international metrology community for new accurate determinations of the Boltzmann constant k, with a view to a new definition of the unit of thermodynamic temperature, the kelvin. Indeed, k is related to the quantum of energy kT, where T is the thermodynamic temperature.The value of the Boltzmann constant can be obtained from measurements of the velocity of the sound in a noble gas. In the method described here, the experiment is performed in a closed spherical cavity. To obtain an accurate value for k, all the parameters of the experiment (gas purity, static pressure, temperature of the device, exact shape of the cavity, etc.) have to be carefully controlled. As correction terms have to be applied to the acoustic signals, the validity of the theoretical models from which they are derived is crucial.The new determination carried out at the LNE-INM/CNAM is based on the same principles as in the acoustic experiment of Moldover et al. at NIST in 1988, which led to the most accurate determination of the Boltzmann constant up to now. However, several fundamental modifications and improvements have been made in this new experiment to measure and control the parameters that set the measured value of k. To cite this article: L. Pitre et al., C. R. Physique 10 (2009).     

Vibration and acoustic data for a punch press

Mode shape, radiation ratio and damping data obtained from pure tone vibration shaker tests on a 170 kN mechanical C frame punch press are presented in this paper. The experimental results show the value of the radiation ratio to be very dependent upon the structure for frequencies below 1 kHz. Above this frequency the assumption of unity for the radiation ratio can be made. Values of the damping ratio are of the order of 0·01 for the modes which have been defined; however, data from blanking operations show that this value can increase by a factor of two to three.     

A holographic method for the determination of all linear electrooptic coefficients applied to Ce-doped strontium-barium-niobate

Using c- and a-cut plates of strontium-barium-niobate doped with 0.66 mol.% cerium all nonzero components of the linear electrooptic tensor r₁₃, r₄₂, r₃₃ including the sign are determined from angular dependence of the two-beam coupling coefficient Γ using transmission and reflection geometry of holographic recording. We found: r₁₃=56 pm/V, r₄₂=38 pm/V, and r₃₃=333 pm/V. In addition we calculated the effective trap density N_eff to be 2.2×10¹⁷ cm^-3. The advantages of this method as well as the values obtained are discussed. Received: 19 November 1998 / Revised version: 15 January 1999 / Published online: 7 April 1999     

各向异性扩散滤波远探测声波测井图像降噪方法*

声波远探测测井技术近年来在复杂油气藏的构造识别和储层评价中发挥着重要作用。该技术利用来自井外的反射波对井旁地质构造进行准确成像,但由于反射波具有幅度低、受幅度强的井孔直达波干扰等特点,实际数据提取到的反射波信噪比往往比较低,需要对反射波进行降噪处理。非线性各向异性扩散滤波能够在滤除图像噪声的同时保留图像边缘及细节等信息,在地震数据处理和医学图像去噪中都有广泛应用。该文从各向异性扩散滤波的基本原理入手,将提取到的井外反射波信号当作图像,采用不同扩散张量进行处理,通过含噪声的模拟数据处理验证了该方法的处理效果并建立起适合于远探测测井的数据处理流程,实际远探测数据处理结果进一步表明其具有较好的应用前景。     

Assessment of accuracy of the structure-factor-size-estimator method in determining red blood cell aggregate size from ultrasound spectral backscatter coefficient

A computer simulation study to produce ultrasonic backscatter coefficients (BSCs) from red blood cell (RBC) clusters is discussed. The simulation algorithm is suitable for generating non-overlapping, isotropic, and fairly identical RBC clusters. RBCs were stacked following the hexagonal close packing (HCP) structure to form a compact spherical aggregate. Such an aggregate was repeated and placed randomly under non-overlapping condition in the three-dimensional space to mimic an aggregated blood sample. BSCs were computed between 750 KHz and 200 MHz for samples of various cluster sizes at different hematocrits. Magnitudes of BSCs increased with mean aggregate sizes at low frequencies (<20 MHz). The accuracy of the structure-factor-size-estimator (SFSE) method in determining mean aggregate size and packing factor was also examined. A good correlation (R(2) ≥ 0.94) between the mean size of aggregates predicted by the SFSE and true size was found for each hematocrit. This study shows that for spherical aggregates there exists a region for each hematocrit where SFSE works most accurately. Typically, error of SFSE in estimating mean cluster size was <20% for dimensions between 14 and 17 μm at 40% hematocrit. This study suggests that the theoretical framework of SFSE is valid under the assumption of isotropic aggregates.     

The application of a transient testing method to the determination of acoustic properties of unknown systems

A transient testing technique for the determination of matrix parameters of acoustic systems has been developed and presented earlier in two papers by the authors. This paper deals with the application of this rapid testing method to the determination of several acoustic properties of unknown systems. Theoretical expressions for acoustic impedances and reflection factors are presented as functions of measured pressure ratios and known matrix parameters. Excellent results obtained for simple test cases show that the procedure is fast and accurate over at least a 70 dB range. Moreover, the method lends itself to applications with steady mean flow in the system and with either transient or random input acoustic excitation.     

Small-slope simulation of acoustic backscatter from a physical model of an elastic ocean bottom

An underwater acoustic experiment with a two-dimensional rough interface, milled from a slab of PVC, was performed at a tank facility. The purpose was to verify the predictions of numerical models of acoustic rough surface scattering, using a manufactured physical model of an ocean bottom that featured shear effects, nonhomogeneous roughness statistics, and root-mean-square roughness amplitude on the order of the acoustic wavelength. Predictions of the received time series and interface scattering strength in the 100-300 kHz band were obtained from the Bottom Reverberation from Inhomogeneities and Surfaces-Small-Slope Approximation (BORIS-SSA) numerical scattering model. The predictions were made using direct measurements of scattering model inputs-specifically, the geoacoustic properties from laboratory analysis of material samples and the grid of surface heights from a touch-trigger probe. BORIS-SSA predictions for the amplitude of the received time series were shown to be accurate with a root-mean-square residual error of about 1 dB, while errors for the scattering strength prediction were higher (2-3.5 dB). The work is part of an ongoing effort to use physical models to examine a variety of acoustic scattering and propagation phenomena involving the ocean bottom.     

水声材料横波声速和衰减系数参量源法测量系统

提出了测量10-100 kHz频段水声材料平均横波声速和衰减系数的测量系统,系统具有低频、小尺寸、窄波束的特点。测量装置应用截断参量源作为声源,结合了精密的坐标系统。在对平板声压透射系数的平面波模型进行理论修正和实验研究的基础上,测量平板样品的透射系数(插入损失)的频谱和角谱,并采用曲线拟合方法来估计样品在测量频段的平均横波声速和衰减系数。在2 m×1 m×1.5 m消声水槽中,对一些典型样品(尺寸500 mm×600 mm)进行了测量实验,结果表明,水声材料样品在声波不同入射角时的声学性能有较大差异,不能用声波垂直入射时的声学性能数据代替;横波声速和衰减系数是评定水声材料声学性能的重要参数,尤其在声波斜入射情况下;在研究材料构件或复杂器件的声学性能时它们也是必不可少和不能不考虑的。实验还验证了测量方法和系统的可行性,也表明仅在这一频段的测量还远不能满足水声工程的实际需求,有必要将测量方法应用扩展到更低或更高的频段。     

Tests of the empirical potential structure refinement method and a new method of application to neutron diffraction data on water

Empirical potential structure refinement (EPSR) is a method for developing a structural model of a liquid for which diffraction measurements are available. The EPSR technique involves refining a starting interatomic potential energy function in a way that produces the best possible agreement between the simulated and measured site-site partial structure factors. Here a series of test simulations are performed to establish how well the EPSR method can recover the interatomic potential for a single component fluid of Lennard-Jones particles, and for a binary fluid consisting of charged atoms interacting at short range by a Lennard-Jones potential. Special attention is given to the problem of developing an accurate interatomic potential for water using these procedures. An alternative method for perturbing the starting potential is used to obtain the best possible fit to the diffraction data. The resulting parametrization of the water potential is in contrast to many existing effective potentials for water, and indicates that water molecules in the liquid at ambient conditions are highly polarized, as has been suggested in recent ‘first-principles’ simulations of water. Three-body correlation functions and spatial density functions derived from the EPSR simulations show excellent agreement with those obtained with the model potential simulations. However, the potentials extracted by EPSR are found to depend on the constraints applied to the hardness of the core potential and the energy and pressure of the simulation, even when the fits to the data are equally good. It is concluded that performing EPSR on diffraction data can be used as a good test for interatomic potentials and to derive reliable many-body structures in the liquid state, but cannot on its own be used to derive a reliable set of site-site pair potentials for a particular system.     

Eigenfunction analysis of stochastic backscatter for characterization of acoustic aberration in medical ultrasound imaging

Presented here is a characterization of aberration in medical ultrasound imaging. The characterization is optimal in the sense of maximizing the expected energy in a modified beamformer output of the received acoustic backscatter. Aberration correction based on this characterization takes the form of an aberration correction filter. The situation considered is frequently found in applications when imaging organs through a body wall: aberration is introduced in a layer close to the transducer, and acoustic backscatter from a scattering region behind the body wall is measured at the transducer surface. The scattering region consists of scatterers randomly distributed with very short correlation length compared to the acoustic wavelength of the transmit pulse. The scatterer distribution is therefore assumed to be delta correlated. This paper shows how maximizing the expected energy in a modified beamformer output signal naturally leads to eigenfunctions of a Fredholm integral operator, where the associated kernel function is a spatial correlation function of the received stochastic signal. Aberration characterization and aberration correction are presented for simulated data constructed to mimic aberration introduced by the abdominal wall. The results compare well with what is obtainable using data from a simulated point source.