An experimental study of acoustoelectric transducers with nonuniform distribution of the piezoelectric coefficient

In this paper, an experimental test of a theoretical model published previously is presented that describes the behavior of an acoustoelectric transducer with a nonuniform distribution of the piezoelectric coefficient within its bulk. Results of this theoretical model are first reviewed. Uniform and nonuniform piezoelectric transducers were fabricated, following a procedure described herein. The receive transfer functions of the transducers were recorded experimentally, and a comparison is made with the theoretical transfer functions predicted by the model, which shows good agreement. The transmit transfer functions of the uniform and nonuniform transducers were also measured and are reported. Numerical calculations of the different transfer functions given by the theoretical model for a uniform transducer associated with different backing materials are also presented, and the results are shown to be equivalent to the results following from the Mason equivalent circuit. Comparisons with experimental results and with Mason's equivalent circuit verified the new theoretical model.     

The forward and backward projection of acoustic fields from axisymmetric ultrasonic radiators using impulse response and Hankel transform techniques

A generalized impulse response formulation to evaluate the harmonic pressure field of ultrasonic planar vibrators having axisymmetric nonuniform surface velocity distributions is presented. The harmonic pressure is expressed as a Fourier transform of a generalized impulse response which is a function of the spatially nonuniform velocity of the vibrator. A backward projection method is then developed to reconstruct the normal surface velocity of axisymmetric vibrators from harmonic field pressures using an angular spectrum or Hankel transform formulation. The numerical accuracy of the backward projection technique is evaluated using the impulse response formulation to evaluate the pressure fields for several velocity distributions on disk vibrators. Experiments were performed to reconstruct the velocity distributions over the surface of a uniformly driven piezoelectric ceramic disk and ceramic ring using farfield measurements of the complex pressure. The experimental results were in good agreement with theoretical results based on the electrode patterns of the transducers.     

Examination of vibration amplitude distribution of ultrasonic transducers using optical holography with a modulated reference beam

The theoretical base, experimental arrangement and results of the examination of the vibration amplitude distribution of ultrasonic transducers using an optical holography method with a reference beam whose frequency is shifted by the same amount as that of the vibrating object is presented. The thickness vibrations of transducers radiating into liquid in the frequency range 0.5 to 3 MHz have been examined.The minimum absolute value of displacement amplitude detected with this procedure is of the order 10^?9 m.     

Design of a self-calibrating simulated acoustic emission source

The use of conical piezoelectric transducers as point acoustic sources has been investigated. It has been shown that transducers based on a design originally developed at the National Institute for Standards and Technology in the USA can be used as point transmitters over the frequency range of interest in acoustic emission measurements (100 kHz to around 1 MHz). They should, therefore, be suitable for use in experiments to calibrate structures so that acoustic emission source strengths can be determined. It has also been shown that measurements of the response of the transmitting transducer backing can be used to assess the coupling efficiency, and hence to remove concerns about inconsistent coupling affecting the calibration measurements. The results indicate that the variation of the backing response with coupling is due to a shift in the resonance frequencies of the transducer with the mechanical load impedance. If other transducers can be shown to behave in a similar fashion this effect could be used to measure coupling in standard acoustic emission and ultrasonic transducers.     

基于飞秒光频梳的压电陶瓷闭环位移控制系统

利用飞秒光频梳、外腔可调谐半导体激光器和法布里-珀罗干涉仪建立了一套压电陶瓷亚纳米级闭环位移控制系统. 将可调谐半导体激光器锁定至光频梳, 通过精确调谐光频梳的重复频率, 实现了半导体激光器在其工作频率范围内的精密调谐. 利用Pound-Drever-Hall锁定技术将带有压电陶瓷的法布里-珀罗腔锁定至半导体激光器, 进而通过频率发生系统控制压电陶瓷产生亚纳米级分辨率的位移. 实验研究发现锁定至光频梳后可调谐半导体激光器1 s的Allan标准偏差为1.68×10^-12, 将其在30.9496 GHz范围内进行连续闭环调谐, 可获得压电陶瓷的位移行程约为4.8 μm; 以3.75 Hz的步长扫描光频梳的重复频率, 实现了压电陶瓷的450 pm闭环位移分辨率并测定了压电陶瓷的磁滞特性曲线. 该系统不存在非线性测量误差, 且激光频率及压电陶瓷位移均溯源至铷钟频率源. 关键词： 光频梳 压电陶瓷 法布里-珀罗腔 可调谐半导体激光器     

Modeling of functionally graded piezoelectric ultrasonic transducers

The application of functionally graded material (FGM) concept to piezoelectric transducers allows the design of composite transducers without interfaces, due to the continuous change of property values. Thus, large improvements can be achieved, as reduction of stress concentration, increasing of bonding strength, and bandwidth. This work proposes to design and to model FGM piezoelectric transducers and to compare their performance with non-FGM ones. Analytical and finite element (FE) modeling of FGM piezoelectric transducers radiating a plane pressure wave in fluid medium are developed and their results are compared. The ANSYS software is used for the FE modeling. The analytical model is based on FGM-equivalent acoustic transmission-line model, which is implemented using MATLAB software. Two cases are considered: (i) the transducer emits a pressure wave in water and it is composed of a graded piezoceramic disk, and backing and matching layers made of homogeneous materials; (ii) the transducer has no backing and matching layer; in this case, no external load is simulated. Time and frequency pressure responses are obtained through a transient analysis. The material properties are graded along thickness direction. Linear and exponential gradation functions are implemented to illustrate the influence of gradation on the transducer pressure response, electrical impedance, and resonance frequencies.     

Acoustic impedance matching of medical ultrasound transducers

Ultrasonic transducers for pulse-echo systems are studied both theoretically and experimentally. For the theoretical calculations the Mason model for thickness-mode disc transducers with and without backing and matching layers is used. By building several of the theoretically investigated transducer configurations it is shown that theory and experiment agree well. Thus the properties of a transducer can be predicted to a good approximation before its experimental realization. To find transducers with good sensitivity and short pulses, the pulse shape and frequency response for the following classes of transducers were studied both theoretically and experimentally: transducers with backing only, transducers with heavy backing and front matching layers, and air-backed transducers with front matching layers.     

Elastic wave field computation in multilayered nonplanar solid structures: a mesh-free semianalytical approach

Multilayered solid structures made of isotropic, transversely isotropic, or general anisotropic materials are frequently used in aerospace, mechanical, and civil structures. Ultrasonic fields developed in such structures by finite size transducers simulating actual experiments in laboratories or in the field have not been rigorously studied. Several attempts to compute the ultrasonic field inside solid media have been made based on approximate paraxial methods like the classical ray tracing and multi-Gaussian beam models. These approximate methods have several limitations. A new semianalytical method is adopted in this article to model elastic wave field in multilayered solid structures with planar or nonplanar interfaces generated by finite size transducers. A general formulation good for both isotropic and anisotropic solids is presented in this article. A variety of conditions have been incorporated in the formulation including irregularities at the interfaces. The method presented here requires frequency domain displacement and stress Green's functions. Due to the presence of different materials in the problem geometry various elastodynamic Green's functions for different materials are used in the formulation. Expressions of displacement and stress Green's functions for isotropic and anisotropic solids as well as for the fluid media are presented. Computed results are verified by checking the stress and displacement continuity conditions across the interface of two different solids of a bimetal plate and investigating if the results for a corrugated plate with very small corrugation match with the flat plate results.     

Finite element three-dimensional analysis of the vibrational behaviour of the Langevin-type transducer

The vibrational behaviour of the Langevin transducer is usually analysed using 1D Mason model which is valid when the lateral dimensions of the transducer are smaller than a quarter wavelength at the fundamental longitudinal resonance. In this work a 3D finite element analysis of the Langevin transducer's behaviour operating in the underwater sonar range of frequencies (30-140 kHz) is presented. Several samples with total length greater, comparable to, and smaller than the diameter of the transducer are analysed. For each sample, the resonance frequencies in the observed frequency range are computed and compared with those obtained experimentally from the measurements carried out using several in-house built prototypes. For the most important aspect ratios the resonance displacement distributions are presented and discussed. The results obtained permit to gain insight into the coupling phenomenon between thickness-extensional and radial modes and suggest that in practical applications transducers with diameters comparable to or greater than total length of the active stack can also be used. The trade-off between the efficiency and power handling ability for different designs is also discussed.     

经典局域共振型水声超材料的吸隔声性能研究*

基于水声超材料吸声机理和多层平行介质平面波理论,建立局域共振型水声超材料结构,通过COMSOL进行建模计算,研究该结构的吸声性能机理,此外为了验证钢背衬的隔声性能,在该水声超材料结构基础上添加一层0.005m厚的钢背衬进行仿真对比。研究结果表明,在频段为200Hz-4000Hz时,水声超材料声学性能较好,吸声性能整体较优,且添加钢背衬的水声超材料隔声性能较优,甚至在某频率点达到15dB的隔声差值;此外通过位移场图进一步揭示水声超材料的吸声机理,发现水声超材料结构的位移场和钢背衬都对吸声性能会产生影响,钢背衬通过影响共振吸收来影响吸声性能,而位移场则通过位移幅度大小影响吸声性能。     

Visualized measurement of the electric field in large aperture LiNbO3 crystal by digital holographic interferometry

The electric field distribution in Li Nb O3 crystal under different electrode shape is presented by using the digital holographic interferometry. Three configurations of phase modulator including the rectangular electrode type,single-triangle electrode type, and dual-triangle electrode type are performed in this experiment. The nonuniform electric field distribution in these phase modulators are observed and the electric field increases with voltage increasing. The digital holographic interferometry with high electro-optic effect improves the measurement precision. The digital holographic interferometry provides an effective way for studying the electric field distribution. Such in situ quantitative analysis of electric field distribution is a key to optimizing electrode shape.     

On the problem of coupling of ultrasonic receivers to layered structures for process control

Different layers in the construction of ultrasonic receiving transducers have been identified as a significant factor in the frequency response of these transducers. The discussion has been extended to the reception of signals through the layers of a process vessel-wall to determine whether the resulting system can be made independent of frequency over a significant spectral range and whether a system can be identified that is essentially independent, in its spectral response, to the thickness of one of the layers (the couplant to the vessel). While the present results are limited to a frequency range of 0–2 MHz, they can readily be extended to other frequency ranges. These computations have as variables the backing material, the transducer material, a wear plate, one or two quarter wavelength layers, the couplant and the structure of the vessel wall. The effect of thin bonding layers for the quarter wavelength plates is also briefly considered. As may have been expected each extra layer introduced into the system produces its own series of resonances. The case of a number of quarter wavelength layers can give a flat frequency response over almost a megahertz of bandwidth but great care needs to be taken with the bonding layers. A system in which the thickness of the couplant had little effect on the overall spectral response was not found.     

Insulator-based dielectrophoresis in viscous media—Simulation of particle and droplet velocity

The velocity of micro-particles in a nonuniform electric field was examined as a function of electrical potential and particle size to illustrate the possible application of dielectrophoresis (DEP) as a new separation technique in viscous media. A new comprehensive model is presented that combines the effects of DEP and electrohydrodynamic forces on particle motion. The current model simulation takes into account the possible significant influence of electrohydrodynamic effects depending on the particle size, electrode distance, and voltage applied during DEP particle separation. The heat generated as a consequence of high electric field strength leads to density gradients in the liquid, thus inducing buoyancy forces that cause fluid convective motion.Experimental velocity measurements using two materials having extreme properties, i.e. polyethylene (PE) particles (diameter range 100–2000 μm) and water droplets (diameter range 25–275 μm), both suspended in a viscous medium (silicone oil), correspond with the proposed theoretical predictions. The comprehensive model presented was applied to insulator-based DEP in a direct current (dc) electric field, but it is expected to allow predictions of various similar systems.     

High frequency acoustic microscopy with Fresnel zoom lens

The acoustic field distributions and the convergent beams generated by the planar-structure Fresnel zone transducers on solid surface are investigated. Because only 0 and 180 degree phase transducers are used, an imaging system with the Fresnel zoom lens could work at very high frequency, which overcomes the frequency limit of the traditional phased array acoustic imaging system. Simulation results are given to illustrate the acoustic field distributions along the focal axis and the whole plane as well. Based on the principle of scanning of the focus with the change of frequency for the excited signal, an experimental imaging system is also built. Acoustic Fresnel zone transducers are fabricated at center frequency of 400 MHz. Measurements and detections of the known hole flaws at different depths of the fused quartz sample are presented to show that the imaging system with Fresnel zoom lens could move its focus by only changing the frequency of the excited signal.     

扬声器锥壳的全频段强迫振动解

解析和数值研究了扬声器锥壳全频段的轴对称强迫振动。给出了典型低频段、典型转点频段和典型高频段的显式位移解析解、特征频率方程和轴向导纳表达式。解析结果与数值计算和实验结果结果非常吻合。在典型低频段,振动完全是纵波型的。在典型转点频段,全域的纵波运动和转点外侧域的横波运动共存,谐振和反谐振频率方程相应呈现出无矩解和弯曲解的耦合特性。在典型高频段,全域的纵波运动和横波运动互相独立,相应出现2组独立的纵波和横波固有频率。     

Detection of cracks at rivet holes using guided waves

Guided Lamb waves can be used for a fast inspection of large areas, e.g. the detection of cracks at rivet holes in the fuselage of airplanes. When the guided wave hits a discontinuity like a hole, a typical scattered displacement field is obtained. A change of the scattered field indicates the development of a crack. In the experiments, the first anti-symmetric mode A0 of Lamb waves in plates is excited selectively by means of a piezoelectric transducer. The used frequency range is below the cut-off frequencies of higher wave modes. The scattered field around undamaged and damaged holes is measured on a grid around the hole with a heterodyne laser interferometer. Two types of damage are introduced: a notch cut with a very fine saw blade, and a fatigue grown crack. A significant change in the scattered field due to the defect is seen. Good agreement of the experimental results with theoretical calculations is obtained. The wave propagation is studied using Mindlin's theory of plates. The scattered field is calculated analytically and using finite difference methods (FDMs).     

Theoretical analysis of an ultrasonic interferometer for precise measurements at high pressures

Expressions making it possible to calculate the transmission coefficient of an ultrasonic interferometer, its Q factor and errors of interferometric measurements are presented. These expressions are obtained on the basis of a one-dimensional model which takes into account the non-ideal reflection of acoustic waves from the transducers, diffraction losses and electromechanical properties of transducers. Fixed path ultrasonic interferometers which are widely used for high precision measurements of ultrasound absorption and velocity in liquids, include air-backed transducers. Air-backing limits the application of these interferometers for the measurements under the high pressures. The influence of non-gaseous backing of transducers on the characteristics of an interferometer is theoretically analysed by means of suggested expressions. Frequency dependences of main parameters of the interferometer are obtained. The possibility of high precision measurements of velocity and absorption of ultrasound in liquids under the high pressures by means of fixed path interferometers with liquid-backed transducers is shown.     

Spatial correlation in the ambient core noise field of a turbofan engine

An acoustic transfer function relating combustion noise and turbine exit noise in the presence of enclosed ambient core noise is investigated using a dynamic system model and an acoustic system model for the particular turbofan engine studied and for a range of operating conditions. Measurements of cross-spectra magnitude and phase between the combustor and turbine exit and auto-spectra at the turbine exit and combustor are used to show the presence of indirect and direct combustion noise over the frequency range of 0-400 Hz. The procedure used evaluates the ratio of direct to indirect combustion noise. The procedure used also evaluates the post-combustion residence time in the combustor which is a factor in the formation of thermal NO(x) and soot in this region. These measurements are masked by the ambient core noise sound field in this frequency range which is observable since the transducers are situated within an acoustic wavelength of one another. An ambient core noise field model based on one and two dimensional spatial correlation functions is used to replicate the spatially correlated response of the pair of transducers. The spatial correlation function increases measured attenuation due to destructive interference and masks the true attenuation of the turbine.     

Diffraction phenomena in the field of a circular piezoelectric transducer viewed in the time domain

Transient diffraction phenomena in the field of a piston transducer have been explained theoretically in terms of the direct and edge wave contributions to the piston response. Previous experimental studies by other workers have verified this concept for single cycles of 4 MHz radiation. Faster transients are difficult to study using thin piezoelectric transducers due to transducer resonance. Thick piezoelectric transducers of very low resonant frequency can be used to generate and receive step function acoustic transients with rise times of 20 ns or less and the problems of transducer resonance can be avoided. Experimental results are presented which show that when such a step function is propagated between two thick transducers the time course of the received wave can be explained in terms of the direct wave/edge wave model in conjunction with the theory of transient reverberations in piezoelectric elements.     

Characteristics of the reverberation signal at a vertical array with tonal insonification of a shallow-water basin

The angular and spectral characteristics of reverberation signals on a fixed path are studied by using vertical receiving arrays with a tonal insonification of the basin. The choice of the frequency range F ～ 200–300 Hz is caused, first, by the low propagation loss and the availability of high-power acoustic transducers for this range and, second, by the possibility to study the phenomena of sound scattering by the sea surface for comparable wavelengths of surface and acoustic waves. The data of experiments on two paths are considered: the first path is in the Barents Sea with propagation conditions corresponding to a uniform waveguide with sea depths H ～ 110–120 m, whereas the second path is in the White Sea with propagation conditions corresponding to a nonuniform waveguide where the sea depth varies from 30 to 250 m.