Determination of acoustic impedances of multi matching layers for narrowband ultrasonic airborne transducers at frequencies <2.5 MHz - Application of a genetic algorithm

The effective ultrasonic energy radiation into the air of piezoelectric transducers requires using multilayer matching systems with accurately selected acoustic impedances and the thickness of particular layers. One major problem of ultrasonic transducers, radiating acoustic energy into air, is to find the proper acoustic impedances of one or more matching layers. This work aims at developing an original solution to the acoustic impedance mismatch between transducer and air. If the acoustic impedance defences between transducer and air be more, then finding best matching layer(s) is harder. Therefore we consider PZT (lead zirconate titanate piezo electric) transducer and air that has huge acoustic impedance deference. The vibration source energy (PZT), which is used to generate the incident wave, consumes a part of the mechanical energy and converts it to an electrical one in theoretical calculation. After calculating matching layers, we consider the energy source as layer to design a transducer. However, this part of the mechanical energy will be neglected during the mathematical work. This approximation is correct only if the transducer is open-circuit. Since the possibilities of choosing material with required acoustic impedance are limited (the counted values cannot always be realized and applied in practice) it is necessary to correct the differences between theoretical values and the possibilities of practical application of given acoustic impedances. Such a correction can be done by manipulating other parameters of matching layers (e.g. by changing their thickness). The efficiency of the energy transmission from the piezoceramic transducer through different layers with different thickness and different attenuation enabling a compensation of non-ideal real values by changing their thickness was computer analyzed (base on genetic algorithm). Firstly, three theoretical solutions were investigated. Namely, Chebyshev, Desilets and Souquet theories. However, the obtained acoustic impedances do not necessarily correspond to a nowadays available material. Consequently, the values of the acoustic impedances are switched to the nearest values in a large material database. The switched values of the acoustic impedances do not generally give efficient transmission coefficients. Therefore, we proposed, in a second step, the use of a genetic algorithm (GA) to select the best acoustic impedances for matching layers from the material database for a narrow band ultrasonic transducer that work at frequency below the 2.5 MHz by considering attenuation. However this bank is rich, the results get better. So the accuracy of the propose method increase by using a lot of materials with exact data for acoustic impedance and their attenuation, especially in high frequency. This yields highly more efficient transmission coefficient. In fact by using increasing number of layer we can increase our chance to find the best sets of materials with valuable both in acoustic impedance and low attenuation. Precisely, the transmission coefficient is almost equal to unity for the all studied cases. Finally the effect of thickness on transmission coefficient is investigated for different layers. The results showed that the transmission coefficient for air media is a function of thickness and sensitive to it even for small variation in thickness. In fact, the sensitivity increases when the differences of acoustic impedances to be high (difference between PZT and air).     

套管-水泥界面微间隙厚度的低频超声反演方法

研究了应用低频超声垂直入射反射波实现套管-水泥界面流体微间隙薄层厚度的反演方法。应用灵敏度函数比较分析了分层系统反射谱各阶谐振频率处的相位跳变量较之各阶谐振频率作为反演特征量的优势。通过目标函数分析,选取分层系统3~10阶谐振频率处相位跳变量作为特征量,利用BP神经网络实验获得0.1~1.0 mm内不同间隙层厚度的反演结果。反演结果与真实值的平均相对误差为4.6%,最小反演厚度仅为波长的18%。所选取的相位特征量也可用于其它层状高声阻抗介质下的薄层厚度反演。      

超薄原子蒸汽的透射与反射光谱

通过数值分析研究了超薄气体的透射与反射光谱特性与其厚度大小的树应关系。研究表明,透射光谱随薄膜厚度的增大而展宽,光谱呈现对称性,而在反射的情况下,这一对称性由薄膜厚度为1／4波长的奇数或偶数倍决定。     

Reflection coefficient of ultrasonic waves as a measure of interface stresses in high pressure multi-layer cylinders

Abstract

Contact stresses in high pressure multi-layer cylinders are sometimes measured using ultrasonic technique. As a measure of the contact stresses the coefficient of reflection of ultrasonic waves from the contact interface between two adjacent outer layers is applied. The coefficient of reflection is an involved function of several parameters. Experimental and theoretical investigations show that this function may change non-monotonously with frequency of ultrasonic waves which may lead to an ambiguity of the assumed criterion. In order to check this function for higher frequencies, measurements of the reflection coefficient from a contact interface between plane steel samples were made for the frequencies up to 190 MHz.     

纳米金表面修饰与表面等离子体共振传感器的相互作用研究

为了分析纳米金表面修饰对表面等离子体共振(SPR)的放大作用,以及其对传感器本身的影响,首先,基于色散介质的吸收理论,通过建立波长型SPR生物传感器四层膜结构的数学模型,理论分析了传感器表面所吸附纳米金对传感器的影响:纳米金的表面修饰,改变了表面等离子体传感器中棱镜表面各介质层内电磁场的能量分布,削弱了金属膜在共振吸收中的作用,从而使SPR曲线的半波宽度增加,最小反射系数增大,金膜的最优膜厚度也随之改变.其次,通过不同厚度的金膜外吸附纳米金的对比试验,验证了此理论.金膜厚45nm、表面修饰10nm纳米金颗 关键词： 表面等离子体共振 生物传感器 纳米金 金属膜     

Influence of the thickness of matching layers on narrow band transmitter ultrasonic airborne transducers with frequencies <100 kHz: Application of a genetic algorithm

The acoustic impedances of matching layers and their thicknesses are the most important and influential parameters in the performance of airborne ultrasonic transducers. In this paper, the optimum thicknesses of the matching layers of the narrow band transmitter ultrasonic transducer regarding transmission coefficient were determined by individual calculations using a genetic algorithm. The genetic algorithm was chosen because it is a powerful tool in the optimization domain. The results show that the permitted thickness variation is 0.0005% for one matching layer, and this value can be increased to 0.0031%, which corresponds to the permitted thickness variation for five matching layers. Approximately 55% enhancement in the transmission coefficient is theoretically possible, and 42% enhancement was observed experimentally when the genetic algorithm was applied to calculate the matching layer thicknesses in place of the quarter wavelength equation that is conventionally used for the determination of layer thickness. To verify our approach, the effect of the thickness variation on the transmission coefficient has been investigated experimentally for three, four and five matching layers. The experimental results displayed good agreement with the theoretical predictions.     

Peculiarities of Acoustic Wave Reflection from a Boundary or Layer of a Two-Phase Medium

A mathematical model is presented for determining the oblique incidence of an acoustic wave at both a boundary and layer of a gas–drop mixture or a bubbly liquid of finite thickness. The basic wave reflection and transmission patterns are established for the incidence of a low-frequency acoustic wave at an interface between a pure gas and a gas–drop mixture, as well as between a pure and bubbly liquid. A range of varying volume fractions for a drop is determined, for which the zero value of the reflection coefficient is possible for low frequencies at oblique incidence. It is shown that the reflection coefficient will never be zero at angles of incidence above 24.5° from a gas–drop mixture at a pure gas boundary; however, when a wave is incident from a pure gas at a gas–drop mixture boundary, a zero reflection coefficient is possible for nonzero angles of incidence and the volume fraction of inclusions. The results of calculating reflection of an acoustic wave from a two-phase layer of a medium with a finite thickness are presented. It is established that the minimum reflection coefficient is possible depending on the perturbation frequency for a certain range of angles of incidence for the boundary or the layer of the gas–drop mixture, which is governed mainly by difference in densities between it and the pure gas.     

阶梯圆环压电超声换能器径向振动性能的理论研究*

提出了一种阶梯圆环径向振动压电超声换能器,根据力电类比原理建立了阶梯圆环及阶梯圆环换能器的径向振动等效电路,推导了阶梯圆环的径向共振频率方程和位移放大系数,在此基础上进一步推导了换能器的径向共振和反共振频率方程。通过理论推导和有限元仿真模拟分析了阶梯圆环压电超声换能器的径向振动性能。结果表明,增大阶梯圆环中内外环的径向厚度之比K1或减小轴向厚度之比K2,阶梯圆环的一阶径向共振频率减小,二阶径向共振频率增大;在二阶径向共振模式下,K1、K2值在一定范围内阶梯圆环可实现由内向外的径向位移振幅放大;随着压电陶瓷圆环的内半径增大,阶梯圆环压电超声换能器的一阶、二阶径向共振和反共振频率减小,二阶径向共振下的有效机电转换系数趋于零;增大阶梯圆环内环的外半径,换能器的一阶径向共振和反共振频率减小,二阶径向共振和反共振频率先增大后减小,理论推导与仿真模拟结果符合良好。本文研究结果为阶梯圆环压电超声换能器的工程应用提供理论参考。     

Influence of the thickness of multilayer matching systems on the transfer function of ultrasonic airborne transducer

The effective ultrasonic energy radiation into the air of piezoelectric transducers requires using multilayer matching systems with accurately selected acoustic impedances and the thickness of particular layers. This problem is of particular importance in the case of ultrasonic transducers working at a frequency above 1 MHz. Because the possibilities of choosing material with required acoustic impedance are limited (the counted values cannot always be realised and applied in practice) it is necessary to correct the differences between theoretical values and the possibilities of practical application of given acoustic impedances. Such a correction can be done by manipulating other parameters of matching layers (e.g. by changing their thickness). The efficiency of the energy transmission from the piezoceramic transducer through different layers with different thickness enabling a compensation of non-ideal real values by changing their thickness was computer analysed. The result of this analysis is the conclusion that from the technological point of view a layer with defined thickness is easier and faster to produce than elaboration of a new material with required acoustic parameter.     

Thickness Determination for a Two-Layered Composite of a Film and a Plate by Low-Frequency Ultrasound

We present an ultrasonic method for determining the thickness of a composite consisting of a soft thin film attached to a hard plate substrate, by resonance spectra in the low frequency region, The interrogating waves can be incident only to the two-layered composite from the substrate side. The reflection spectra are obtained by FFT analysis of the compressive pulsed echoes from the composite, and the thicknesses of the film and the substrate are simultaneously inversed by the simulated annealing method from the resonant frequencies knowing other acoustical parameters in prior. The sensitivity of the method to individual thickness, its convergence and stability against experimental noises are studied, Experiment with interrogating wavelength 4 times larger than the film thickness in a sample of a polymer film （0.054mm） on an aluminium plate （6.24mm） verifies the validity of the method. The average relative errors in the measurement of the thicknesses of the film and the substrate are found to be -4.1% and -0.62%, respectively.     

Frequency-dependent reflection of elastic wave from thin bed in porous media

The reflection of elastic wave from thin bed in porous media is important for oil and gas reservoir seismic exploration.The equations for calculating frequency-dependent reflection amplitude versus incident angle(FDAVA) from thin bed in porous media are obtained based on porous media theory. Some conclusions are obtained from numerical analysis, specifically, slow compression wave may be ignored when considering boundary conditions in most situations; the dispersion of reflection from thin bed is much higher than that from thick layer and is periodic in frequency domain, which is affected by the thickness of thin bed, velocity, and incident angle; the reflection amplitude envelope in frequency domain decays exponentially, which is affected by the thickness of thin bed, attenuation, and incident angle; the reflection amplitude increases with thickness of thin bed increasing, and then it decreases when the thickness reaches to a quarter of wavelength.     

Permalloy-FeMn exchange-biased multilayers grown on flexible substrates for microwave applications

Permalloy-FeMn multilayers deposited onto flexible substrates oriented for wide-band absorber applications were fabricated using RF sputtering deposition. The ferromagnetic resonance (FMR) frequency was tuned by changing the thickness of the Permalloy layers. Plural FMR frequencies appeared in the multilayer film due to the difference in exchange couple energies at their interfaces. A multilayer thin film with varying thickness of Permalloy layers was also fabricated with the properties of a wide-band absorber. Its range of 1-4 GHz (the absorption width where the reflection loss is less than 10 dB) appears promising for future applications.     

Behavior of rigid-porous layers at high levels of continuous acoustic excitation: theory and experiment

A model for the propagation of high amplitude continuous sound through hard-backed rigid-porous layers has been developed which allows for Forchheimer's correction to Darcy's law. The nonlinearity associated with this is shown to be particularly important in the range of frequencies around layer resonance. The model is based on the introduction of particle velocity dependent flow resistivity into the equivalent fluid model expression for complex tortuosity. Thermal effects are accounted for by means of a linear complex compressibility function. The model has been used to derive analytical expressions for surface impedance and reflection coefficient as a function of incident pressure amplitude. Depending on the material parameters, sample thickness, and frequency range the model predicts either growth or decrease of reflection coefficient with sound amplitude. Good agreement between model predictions and data for three rigid-porous materials is demonstrated.     

Reflection of light from a plane-parallel layer modulated by ultrasound

The features of Bragg reflection of light from a plane-parallel layer perturbed by a standing ultrasonic wave are investigated. It is shown that, with an increase in the thickness of the modulated layer, the reflection coefficient of the light wave diffracted by the ultrasound reaches the maximum value at smaller intensities of the ultrasound and is determined by the relation between the refractive indices of the adjoining media. The possibility of efficient acousto-optical diffraction of light in the Bragg reflection regime at superlattices of photoelasticity in the field of low-frequency ultrasonic waves with a moderate level of acoustic power is established.     

Interaction of ultrasonic wave with void-containing layer in solid

Using a general mode for sound reflection from multilayered media,we present in this paper the expressions for sound reflection and transmissioncoefficients on void-containing interface layer in solid and derive the character-istic equation for symmetric and antisymmetric modes of the interface wavesalong the layer.The method for evaluating the effective elastic moduli ofvoid-containing solid is also introduced.The numerical calculation given in thispaper shows the influences of the void volume concentration and layer thick-ness on the sound reflection coefficients and interface wave velocity,providing atheoretical basis for inverse of the mechanical properties of void-containinglayer based on the ultrasonic measurement.     

Edge resonance and zero group velocity Lamb modes in a free elastic plate

The local resonances of a free isotropic elastic plate are investigated using laser ultrasonic techniques. Experimental results are interpreted in terms of zero group velocity Lamb modes and edge mode. At a distance from the edge larger than the plate thickness a sharp resonance is observed at the frequency where the group velocity of the first symmetrical Lamb mode vanishes. Close to the edge of the plate, the resonance due to the edge mode dominates. Both zero group velocity and edge resonances appear at the theoretically predicted frequencies. These frequencies do not vary with the distance from the edge of the plate and the transition between the two modes of vibration, at about the plate thickness, is abrupt. Using a laser excitation on the edge, the amplitude profile of the normal displacement at the edge resonance frequency was determined.     

海面冰层对声波的反射和散射特性

北极海面冰层复杂多变,其对声波的反射和散射严重影响冰下水声信道的传输特性,建立海面冰层的声波反射和散射模型对冰下水声通信研究具有重要意义.假设海面冰层为多层固体弹性介质且冰-水界面粗糙,满足微扰边界条件,导出声波从海水介质入射到海面冰层时相干反射系数满足的线性方程组.对相干反射系数随声波频率、掠射角、冰层厚度的变化进行数值分析.进一步引入根据散射声场功率谱密度计算散射系数的方法,改变掠射角,对冰层厚度、散射掠角对散射系数的影响进行研究.     

Image recording on photothermoplastic media with different thicknesses of the thermoplastic layer

The quality of images recorded on photothermoplastic media (PTPMs) with various thicknesses of the thermoplastic layer is investigated. It is found that the potential of a corona-forming electrode initiating deformations on a PTPM surface and the diffraction efficiency, corresponding to the additional maxima in its dependence on the spatial frequency, increase with decreasing thickness of the thermoplastic layer, in contrast to the diffraction efficiency at the resonance frequency. It is shown that the frequencies corresponding to additional maxima in the dependence of the diffraction efficiency on the spatial frequency are preferable for application as the carrier frequency for raster-scan recording and as the center carrier frequency for holographic image recording on PTPMs in order to reach a higher diffraction efficiency, image contrast, and signal-to-noise ratio than those attainable at the resonance frequency. An anomalous relation between the resolution and sensitivity of PTPMs is found under the suggested recording conditions and carrier frequencies. It is shown that both the resolution and the sensitivity of PTPMs increase with decreasing thickness of the thermoplastic layer.     

Multilayer neutron monochromator-polarizer based on iron

In this paper, a multilayer neutron monochromator-polarizer, consisting of 490 alternating layers of iron and niobium, is proposed. Each layer of iron or niobium has a thickness of 25 Å. The use of thin layers in the structure period improves the quality of the monochromator-polarizer and yields a neutron beam reflected from this structure with high wavelength resolution and a small fraction of minor nonmonochromatic contributions to the intensity of the reflected beam. The fact that the layer thicknesses are the same in the period suppresses even-order Bragg peaks. The parameters of the first-order Bragg peak for eight model neutron monochromators-polarizers based on iron and cobalt are given. It follows from the calculated curve of the coefficient of neutron reflection from the Fe/Nb structure that the first-order Bragg peak has a half width of 1.1%, a relative reflection coefficient of 0.997, and a maximum polarizing efficiency of unity. We also present the first experimental curves of the coefficient of neutron reflection from this structure, depending on the momentum transfer for both spin components of the beam.     

Model and experimental analysis of oblique incident ultrasound in a tissue layer using doublet mechanics theory

The fundamental framework of doublet mechanics (DM) is used to analyze high-frequency ultrasound wave propagation in materials with discrete microstructure. Ultrasonic reflection coefficients were measured from a thin layer of tissue embedded between two glass substrates at oblique incidence. Theoretical calculations for the reflection coefficients of a multi-layered system at oblique angles are performed using both DM theory and the classical continuum mechanics theory (CCM). For example, at the frequency of 10 MHz at incident angle 8° in sample with 30 μm thickness, the discrepancy in the magnitude of the reflection coefficient between experimental results and theoretical prediction is 15.8% for DM but 79.0% for CCM; similar results at other frequencies and incident angle in the samples with 30 and 60 μm thickness have also been obtained, which demonstrates that the DM theory can better describe the wave propagation in tissue. The influence of the incident angles and tissue thickness are also discussed in this paper.