High accuracy non-contact ultrasonic thickness gauging of aluminium sheet using electromagnetic acoustic transducers.

Aluminium sheet thickness has been calculated from ultrasonic data obtained using a send-receive, radially polarised electromagnetic acoustic transducer (EMAT). Sheets in the thickness range between 0.1 and 0.5 mm have been measured using this non-contact approach at a stand-off of up to 1.5 mm. Normal incidence shear waves generated and detected in the sheet and the resultant waveforms have been processed using transit time measurements and Fourier analysis. Two broad band EMAT systems have been used to perform the measurements with centre frequencies of approximately 5 MHz and frequency content up to 10 and 20 MHz respectively. The most accurate measurements of thickness on thin sheets have been made using Fourier analysis and have yielded measurements accurate to within 0.2% (or 0.4 microm) for 280 microm thick aluminium sheets. Discrete shear wave echoes can be observed for sheets down to a thickness of 250 microm using the higher frequency EMAT system. However temporal measurements of these signals yield lower accuracy results when compared to the Fourier analysis method which is capable of sub-micron accuracy.     

High frequency shear horizontal plate acoustic wave devices

It has been shown recently that shear horizontal acoustic waves propagating in piezoelectric plates whose thickness h is much less than the acoustic wavelength λ possess a number of attractive properties for use in sensor and signal processing applications. In order to exploit the potential benefits of these waves, however, one needs to fabricate devices on very thin plates. We have developed a suitable fabrication method which can be used to realize devices on such thin plates. In this method, the device is first fabricated on a plate of normal thickness (approximately 500 μm) and the substrate is then lapped from the back side to reduce the thickness. The technique has been utilized to realize devices on plates of thickness less than 70 μm. A shear horizontal plate acoustic wave (SH-PAW) delay line of fundamental resonant frequency greater than 25 MHz and insertion loss less than 7 dB has been realized on a 60 μm thick Y – cut, X – propagation lithium niobate substrate. The device also shows strong response near the third harmonic frequency of 75 MHz.     

Correlations between acoustic properties and bone density in bovine cancellous bone from 0.5 to 2 MHz

Correlations between acoustic properties and bone density were investigated in the 12 defatted bovine cancellous bone specimens in vitro. Speed of sound (SOS) and broadband ultrasonic attenuation (BUA) were measured in three different frequency bandwidths from 0.5 to 2 MHz using three matched pairs of transducers with the center frequencies of 1, 2.25, and 3.5 MHz. The relative orientation between ultrasonic beam and bone specimen was the mediolateral (ML) direction of the bovine tibia. SOS shows significant linear positive correlation with apparent density for all three pairs of transducers. However, BUA shows relatively weak correlation with apparent density. SOS and BUA are only weakly correlated with each other. The linear combination of SOS and BUA in a multiple regression model leads to a significant improvement in predicting apparent density. The correlations among SOS, BUA, and bone density can be effectively and clearly represented in the three-dimensional space by the multiple regression model. These results suggest that the frequency range up to 1.5 MHz and the multiple regression model in the three-dimensional space can be useful in the osteoporosis diagnosis.     

300MHZ scanning laser acoustic microscope

The knife—edge and harmonic technique in the Scanning LaserAcoustic Microscope is studied in this paper.The operating frequency of theSLAM can be increased from 100MHz to 300MHz by using the harmonic tech-nique.The acoustic images of some samples are obtained on our SLAM at300MHz.     

Analytical and numerical calculations of optimum design frequency for focused ultrasound therapy and acoustic radiation force

Focused ultrasound therapy relies on acoustic power absorption by tissue. The stronger the absorption the higher the temperature increase is. However, strong acoustic absorption also means faster attenuation and limited penetration depth. Hence, there is a trade-off between heat generation efficacy and penetration depth. In this paper, we formulated the acoustic power absorption as a function of frequency and attenuation coefficient, and defined two figures of merit to measure the power absorption: spatial peak of the acoustic power absorption density, and the acoustic power absorbed within the focal area. Then, we derived “rule of thumb” expressions for the optimum frequencies that maximized these figures of merit given the target depth and homogeneous tissue type. We also formulated a method to calculate the optimum frequency for inhomogeneous tissue given the tissue composition for situations where the tissue structure can be assumed to be made of parallel layers of homogeneous tissue. We checked the validity of the rules using linear acoustic field simulations. For a one-dimensional array of 4 cm acoustic aperture, and for a two-dimensional array of 4 × 4 cm² acoustic aperture, we found that the power absorbed within the focal area is maximized at 0.86 MHz, and 0.79 MHz, respectively, when the target depth is 4 cm in muscle tissue. The rules on the other hand predicted the optimum frequencies for acoustic power absorption as 0.9 MHz and 0.86 MHz, respectively for the 1D and 2D array case, which are within 6% and 9% of the field simulation results. Because radiation force generated by an acoustic wave in a lossy propagation medium is approximately proportional to the acoustic power absorption, these rules can be used to maximize acoustic radiation force generated in tissue as well.     

Frequency dependence of tissue attenuation measured by acoustic microscopy

Broadband scanning acoustic microscopy (SAM) has been used to investigate the mechanical properties of sections of tissue with a resolution of around 8 microns. The work reported here extends these results by reporting the frequency dependence of the attenuation coefficient from 100-500 MHz. A discussion of the theory of the measurements is presented. The scanning laser acoustic microscope (SLAM) is used to characterize similar tissue sections at 100 MHz. The data obtained with the two forms of acoustic microscopy are compared with results from the literature.     

Characterization of dense bovine cancellous bone tissue microstructure by ultrasonic backscattering using weak scattering models

A weak scattering model was proposed for the ultrasonic frequency-dependent backscatter in dense bovine cancellous bone, using two autocorrelation functions to describe the medium: one with discrete homogeneities (spherical distribution of equal spheres) and another, which considers tissue as an inhomogeneous continuum (densely populated medium). The inverse problem to estimate trabecular thickness of bone tissue has been addressed. A combination of the two autocorrelation functions was required to closely approximate the backscatter from bovine bone with various microarchitecture, given that the shape of trabeculae ranges from a rodlike to a platelike shape. Because of the large variation in trabecular thickness, both at an intraspecimen and an interspecimen level, thickness distributions for individual trabeculae for each bone specimen were obtained, and dominant trabecular sizes were determined. Comparison of backscatter measurements to theoretical predictions indicated that there were more than one dominant trabecular sizes that scatter sound for most specimens. Linear regression, performed between dominant trabecular thickness and estimated correlation length, showed significant linear correlation (R(2)=0.81). Attenuation due to scattering by a continuous distribution of scatterers was predicted to be linear over a frequency range from 0.3 to 0.9 MHz, suggesting a possibility that scattering may be a significant source of attenuation.     

Measurement of acoustic dispersion using both transmitted and reflected pulses

Traditional broadband transmission method for measuring acoustic dispersion requires the measurements of the sound speed in water, the thickness of the specimen, and the phase spectra of two transmitted ultrasound pulses. When the sound speed in the specimen is significantly different from that in water, the overall uncertainty of the dispersion measurement is generally dominated by the uncertainty of the thickness measurement. In this paper, a new water immersion method for measuring dispersion is proposed which eliminates the need for thickness measurement and the associated uncertainty. In addition to recording the two transmitted pulses, the new method requires recording two reflected pulses, one from the front surface and one from the back surface of the specimen. The phase velocity as well as the thickness of the specimen can be determined from the phase spectra of the four pulses. Theoretical analysis and experimental results from three specimens demonstrate the advantages of this new method.     

Conventional electromagnetic acoustic transducer development for optimum Lamb wave modes

Lamb waves are normally utilized for inspecting thin metal sheets. Wheel type probes using piezoelectric oscillators have generally been used as the sensors for Lamb waves. Recently, the electromagnetic acoustic transducer (EMAT) has been developed and is beginning to be used as a Lamb wave detector. We have developed a useful type of transducer for Lamb waves. The new EMAT consists of a meander coil with a narrow distance of 2.5 mm and has a symmetrical structure in the vertical direction for both surface sides. The new EMAT can generate Lamb waves with variable wavelengths corresponding to the frequency range from approximately 300 kHz to 2.5 MHz and multiple modes, and can also generate selected symmetrical and anti-symmetrical mode Lamb waves. It is demonstrated that the optimum Lamb wave mode could be produced by the appropriate positioning of the EMATs and controlling the phase (same or inversed) of the electrical signal driving the device. The described EMAT can be used to examine steel (or other material) sheets of different thickness. It is also shown that the S0 (0.3 MHz) mode Lamb wave is the most effective for the deepest (up to 6 mm) penetration.     

基于PC机的一种多功能声显微镜及可视化技术的研究

近来研制成功了一种基于ＰＣ机、我们称为“ＴＨＳＡＭＭ”型的多功能声显微镜。在ＰＣ机内插入采样率１ＧＳＰＳ的超高速Ａ／Ｄ卡和信号产生和接收卡,利用软件完成信号检测、处理、显示功能。仪器工作频率为１－１００ＭＨｚ;在检测样品时,同时多层显示Ａ、Ｂ、Ｃ扫描结果。根据声学理论、Ｍａｒｃｈｉｎｇ Ｃｕｂｅ等值面抽取方法和ＯｐｅｎＧＬ技术完成了三维数据的可视化处理,显示样品的内部三维结构。该系统已成功应用于多     

Lens multielement acoustic microscope in the mode for measuring the parameters of layered objects

An acoustic microscope with a cylindrical lens and ultrasound transducer have been considered, as well as the method based on it for the measuring of longitudinal and transverse wave velocities, the thickness and density of the investigated layer. A theoretical model of the microscope has been constructed, and the relation between the spatiotemporal output signal of the transducer and the angular dependence of the sample reflection coefficient has been found. It has been shown that the velocities of body waves and the thickness can be determined by the delays of ultrasound responses reflected from the layer boundaries measured by the transducer elements, and the density, by the amplitudes of these responses. The method was tested experimentally using a 20-element transducer with a central frequency of 15 MHz and a period of 0.8 mm. The example of a duralumin plate has shown that the error in measuring the thickness and velocity of longitudinal waves error does not exceed 1%; the velocity of transverse waves, 2%; and the density can be estimated with an accuracy of about 5%.     

Coating thickness affects surface stress measurement of brush electro-plating nickel coating using Rayleigh wave approach

A surface ultrasonic wave approach was presented for measuring surface stress of brush electro-plating nickel coating specimen, and the influence of coating thickness on surface stress measurement was discussed. In this research, two Rayleigh wave transducers with 5 MHz frequency were employed to collect Rayleigh wave signals of coating specimen with different static tensile stresses and different coating thickness. The difference in time of flight between two Rayleigh wave signals was determined based on normalized cross correlation function. The influence of stress on propagation velocity of Rayleigh wave and the relationship between the difference in time of flight and tensile stress that corresponded to different coating thickness were discussed. Results indicate that inhomogeneous deformation of coating affects the relationship between the difference in time of flight and tensile stress, velocity of Rayleigh wave propagating in coating specimen increases with coating thickness increasing, and the variation rate reduces of difference in time of flight with tensile stress increasing as coating thickness increases.     

Transfer function method for investigating the complex modulus of acoustic materials: Spring-like specimen

The complex modulus of acoustic materials has to be known as a function of frequency. Among many methods for investigating the complex modulus, it is advantageous to use the transfer function method for detailed frequency analysis. In this method, a cylindrical or prismatic specimen is excited into longitudinal vibration at one end, the other end being loaded by a mass. The complex modulus can be calculated after having measured the transfer function: i.e., the vibration amplitudes of the specimen ends and the phase angle between them. In this paper the transfer function and its measurability are investigated theoretically and experimentally in that frequency range where the specimen can essentially be modelled by lumped parameter mechanical elements. The role of the measurement errors is analyzed and it is shown that the smaller the loss factor the higher the measurement accuracy that is needed. Furthermore, it is shown that disregarding the longitudinal wave motion of the specimen at higher frequencies leads to an apparent increase of the dynamic modulus and to an apparent decreases of the loss factor. This effect may be compensated up to a certain frequency by the lateral wave motion of the specimen. Experimental results supporting the theoretical predictions are presented.     

Mechanical characterization of sintered piezo-electric ceramic material using scanning acoustic microscope

Lead Zirconate Titanate (PZT) is a piezo-electric ceramic material that needs to be characterized for its potential use in microelectronics. Energy dispersive X-ray analysis (EDX) is conducted to determine the chemical composition of the PZT ceramics. The scanning electron microscope (SEM) is performed to study the surface morphology, grain structure and grain boundaries. The SEM image helps us to understand the surface wave propagation and scattering phenomena by the PZT and the reason for its anisotropy and inhomogeneity due to the grain structure. In this paper scanning acoustic microscopy at 100 MHz excitation frequency is conducted for determining mechanical properties of PZT. Earlier works reported only the longitudinal wave speed in PZT while in this paper longitudinal, shear and surface acoustic wave speeds of sintered PZT are measured from its acoustic material signature (AMS) curves, also known as V(z) curves. AMS or V(z) curve is the variation of the output voltage as a function of the distance between the acoustic lens focal point and the reflecting surface. The average velocities of longitudinal, shear and surface acoustic waves in a PZT specimen are determined from its V(z) curve generated at 100 MHz excitation frequency and found to be over 5000 m/s, over 3000 m/s and between 2500 and 3000 m/s, respectively. From these velocities all elastic constants of the specimen are obtained.     

Miniaturization of surface acoustic waves rotary motor

This paper presents the experimental study of a miniaturized surface acoustic waves (SAWs) rotary motor and the theoretical calculation. After the first success in SAW rotary motor operating at 9.85 MHz, a smaller rotary motor is designed. With the operating frequency of 30 MHz and the driving voltage of 120 V(p-p), the motor can rotate at a speed of 270 rpm.     

Optimization of acoustic scattering from dual-frequency driven microbubbles at the difference frequency

The second harmonic radiation of acoustically driven bubbles is a useful discriminant for their presence in clinical ultrasound applications. It is useful because the scatter from a bubble at a frequency different from the driving can have a contrast-to-tissue ratio better than at the drive frequency. In this work a technique is developed to optimize the scattering from a microbubble at a frequency different from the driving. This is accomplished by adjusting the relative phase and amplitudes of the components of a dual-frequency incident ultrasound wave form. The investigation is focused primarily on the example of dual-mode driving at frequencies of 1 MHz and 3 MHz, with the scattering optimized at 2 MHz. Bubble radii of primary interest are 0.5 to 2 microm and driving amplitudes to 0.5 atm. Bubbles in this size range are sensitive to modulation of driving. It is shown that an optimal forcing scheme can increase the target response eightfold or more. This suggests new applications in imaging and in bubble detection.     

环形ZnO薄膜谐振器的横模抑制与测试分析

采用Tiersten方程研究了环形ZnO薄膜谐振器中横模寄生问题, 获得了环(圆)形薄膜谐振器的横模振动方程, 求得横模位移场解和频率色散方程; 然后采用电磁学模式合成理论进行分析, 发现环形薄膜谐振器横模频率与环形电极的内外径之比a/b有关, 振动模式可由圆形薄膜谐振器横模模式合成得到, 通过控制a/b能够抑制横模模式数和调控基膜频率. 采用外差激光干涉仪和网络矢量分析仪测量并比较了同批次的圆形和环形薄膜谐振器的上电极横模振动图样和电阻抗曲线. 振动图样显示环形薄膜谐振器振动模式可由半径为a和半径为b的圆形薄膜谐振器振动模式合成, 仅存在节圆数大于0的横模振动, 等于0的横模模式被抑制; 电阻抗曲线显示当a/b为0.436时, 环形薄膜谐振器的基频(约1217 MHz)和圆形的(0, 1)模式频率相等. 测量数据验证了模式合成理论的分析结果正确性, 为薄膜谐振器的横模抑制研究提供了理论基础和新方法.     

Transfer function method for investigating the complex modulus of acoustic materials: Rod-like specimen

The complex Young's modulus of acoustic materials as a function of frequency is generally investigated by using a cylindrical or prismatic specimen of the material excited into longitudinal harmonic vibration at one end, the other end being loaded by a mass. The transfer function method is the most advantageous to use for the investigation, which involves the measurement of the vibration amplitudes of the specimen ends and the phase angle between them. In this paper, the transfer function method is analyzed theoretically and experimentally in that frequency range where the specimen can essentially be modelled by a longitudinally vibrating rod (rod-like specimen). The analysis includes the measurability of the transfer function, the role of the measurement errors, the frequency range of the method and the maximum dynamic strain of the specimen. A special application of the method is developed for investigating the complex modulus of low loss materials at the extremes of the absolute value of the transfer function, without measuring the phase angle. Experimental results obtained by both methods are presented.     

Measurements of the longitudinal wave speed in thin materials using a wideband PVDF transducer

A flat transducer was constructed, using a 9-microm-thick PVDF (polyvinylidene fluoride) film for generation and detection of high-frequency ultrasonic waves, and used for measurements of the phase velocity of longitudinal waves traveling along the thickness direction in a very thin material. The transducer has a useful wideband frequency characteristic extending from 10 MHz to over 150 MHz. Measurements of the phase velocity of the longitudinal waves are carried out using a 0.212-mm-thick glass slide and a 0.102-mm-thick stainless-steel shim, using water as a coupling medium. The thickness limit for this measurement appears to be approximately 20 microm. The phase velocity of the longitudinal mode is obtained as a function of frequency in the frequency domain by using a modified sampled continuous wave (cw) technique. It can also be measured in the time domain by using a broadband pulse of short duration.     

Internal deformation of a uniform elastic solid by acoustic radiation force

Tissue elasticity estimation is a growing area of ultrasound research. One proposed approach would apply acoustic radiation force to displace tissue and use ultrasonic motion tracking techniques to measure the resultant displacement. Such a technique might allow noninvasive imaging of tissue elastic properties. The potential of this method will be limited by the magnitude of displacements which can be generated at reasonable acoustic intensity levels. This paper presents methods for estimating the internal displacements induced in an elastic solid by acoustic radiation force. These methods predict displacements on the order of 400 microns in the human vitreous body, 0.008 micron in human breast, and 0.020 micron in human liver at an acoustic intensity of 1.0 W/cm2 (in water) and an operating frequency of 10 MHz. While the displacement generated in the vitreous should be readily detectable using ultrasonic methods, the displacements generated in the breast and liver will be much more difficult to detect. Methods are also developed for predicting the time dependent temperature increases associated with attenuated acoustic fields in the absence of perfusion. These results indicate promise for radiation force imaging in the vitreous, but potential difficulties in applying these techniques in other parts of the body.