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Based on the analytic expressions for the radiated field of a circular concave piston given by Hasegawa et al., an integral for calculation of the radiation force on a plane absorbing target in a spherically focused field is derived. A general relation between acoustic power P and normal radiation force Fn is obtained under the condition of kr ≫ 1. Numerical computation is carried out by using the symbolic computation program for practically focused sources and absorbing circular targets. The results show that, for a given source, there is a range of target positions where the radiation force is independent of the target’s position under the assumption that the contribution of the acoustic field behind the target to the radiation force can be neglected. The experiments are carried out and confirm that there is a range of target positions where the measured radiation force is basically independent of the target’s position even at high acoustic power (up to 700 W). It is believed that when the radiation force method is used to measure the acoustic power radiated from a focused source, the size of the target must be selected in such a way that no observable sound can be found in the region behind the target.

In this paper, the multiple scattering of interacting encapsulated microbubbles in suspensions is calculated using two novel approaches – Kargl’s effective medium approach and Ye and Ding’s approach of 2nd-order correction. Two types of contrast agents with bubbles of different sizes and concentrations are chosen for our numerical simulations. One is Albunex ® , which is depicted by Church and has a size range of several microns, and the other is sodium laureate solution (before fractionation) described by Soetanto and Chan and has an average size of 35.5 μm. The numerical results from these two approaches are compared with that from the linear approximation. It is found that the multiple scattering effects on attenuation and dispersion of sound in suspensions are evident in the cases of high bubble volume fractions, basically greater than the order of 1 × 10 −4 , and much more obvious for larger bubbles with average size of tens of microns.

A time-frequency analysis method is proposed to analyze the shear-horizontal (SH) waves propagating in an isotropic plate bordered with one elastic solid layer. To examine the validity of this approach, the SH wave signals simulated by the finite element modeling method are analyzed by the reassigned short-time Fourier transform. The extracted dispersion data are in good agreement with results derived from dispersion equations. Results indicate that an increase in the loading layer thickness can cause a decrease in the group velocity of the SH0 mode and the cut-off frequency of higher modes, implying a possibility for the evaluation of the loading layer thickness by using this method. The limitations of this method are also discussed in this paper.

A theory on acoustic streaming generated by Lamb waves propagating in a thin composite membrane was developed. Using this theory, the mass-transport velocity in water, which is loaded on a 4-μm-thick membrane, due to Lamb waves at 4.7 MHz was calculated. The results are in good agreement with the experimental values measured by Moroney et al. [Appl. Phys. Lett. 59, 774 (1991)].

General dispersion relations for the antisymmetrical (A) and symmetrical (S) Lamb-wave-like modes of a thin plate symmetrically bordered by two thin layers have been derived using a perturbation approach. The derived dispersion relations were applied to treat the effects of thin layers (liquid layers, polymer layers or solid layers), which were in direct contact with the plate on the propagation of Lamb waves. The analytical expressions of the first-order approximation of the change of wave-numbers and the attenuation coefficients for non-solid layers were obtained. It has been shown that for non-solid-layer loading, the A0 mode is more sensitive than the S0 mode. For the A0 mode, the sensitivity of mass loading for any layers is approximately equal to 1 m s for two layers, where ms is the mass per unit area of the plate.

The effective nonlinearity parameter B/A of water containing a three-dimensional ensemble of randomly distributed uniform-sized trapped cylindrical bubbles was measured. The experimental results indicate that the dramatic enhancement of the effective nonlinear parameter B/A is due to the nonlinear resonance oscillation of the trapped bubbles. The measured effective B/A for the system is of the magnitude of 10(4) to 10(5).

Based on the analytic expressions given by T. Hasegawa, et al.. (J.Acoust. Soc. Am. 79(4) 927-931 1986) an integral for calculation of the radiation force on a plane absorbing target in spherically focused field is derived. For focused fields a general relation between acoustic power P and normal radiation force Fn, P=GFn is obtained from the integral. Numerical computation of the integral is carried out for the practical HIFU source and absorbing circular target, through which the value of calibration constant G can be obtained The experiments are conducted for the same source and target used in the calculation. For the given source and targets, the calculations and experiments show that there is a range of target's position where the radiation force is independent of target's position.

The dispersive characteristics of higher order mode Lamb waves (HOMLW) excited by interdigital transducers (IDT) are measured and analyzed, which are necessary for designing micro-sensor in ultrahigh frequency (UHF). A measurement system is set up, in which dispersive characteristics of HOMLW are obtained by the method of transform between frequency and time domains. The characteristics of amplitude-frequency and phase-frequency of Lamb wave are auto-measured by the system. By IFFT, the pulse response of the IDT device was obtained. Different modes were separated in time domain and dispersive curve of each mode is calculated by FFT. The best mode is chosen to design the micro-sensor in UHF. The phase velocity of HOMLW is greater than the surface wave (SAW) velocity and an oscillator in higher frequency can be made, so the absolute sensitivity of micro-sensor can be increased. In this paper, the dispersive characteristics of HOMLW excited by an IDT in a 127.86 degrees rotated Y-cut, X propagating lithium niobate plate is analyzed. An oscillator using a(13) mode is made, the phase velocity of which is measured about 19,652 m/s when h/lambda=0.94 (h=plate thickness, lambda=wavelength).

— The phenomenon of image distortions caused by the multiple scattering (MS) effects of encapsulated microbubbles in ultrasoniqc imaging was experimentally found in previous studies ( Soetanto and Chan 2000a ), but its mechanism has not been fully understood. To study the MS effects of microbubbles in contrast imaging, two approaches are employed in this article—the effective medium approach initialized by Kargl (2002) , which includes all the high-order rescattering of free bubbles, and the classic lowest-order approximation approach of Commander and Prosperetti (1989), which ignores the higher-order rescattering between bubbles. In this work, they are modified to model encapsulated microbubble suspensions, and the discrepancies in attenuation coefficients calculated by these two approaches, i.e., the higher-order rescattering of bubbles are defined as the measure of the MS effects of microbubbles. The intrinsic relations between the MS effects of microbubbles in suspensions and physical properties of the microbubbles, such as the bubble concentrations, sizes, and the shell thicknesses etc., are simulated and discussed. It is found that in suspensions for identical microbubbles >12 μm in size, the MS effects come to be significant when the bubble concentrations exceed 1 × 105 bubbles/mL. The MS effects of microbubbles with broad size spectrums are examined by simulating Soetanto and Chan’s experiments. Also, the MS effects of UCAs in current ultrasonic imaging practice are discussed. The STARs and extinction cross-sections of different-sized individual encapsulated microbubbles are calculated for further investigations on the mechanism of the MS effects of UCAs. (E-mail: zhudu@nju.edu.cn )

A sensor which measures physical parameters of a liquid with high sensitivity and accuracy is in great demand in medicine, biology, chemistry and environmental science [1]. Recently, resurgent interest in Lamb waves was initiated in their applications of multisensors [2]. The principle of these Lamb-Wave Sensors is that the propagation velocity of Lamb waves changes when a plate is bordered with a liquid. In this paper, we report the results of our recent theoretical investigations of the effects of bordering liquid layers on the propagation of Lamb waves in a solid plate.

The differences of the sensitivities of liquid- and solid-sensing of Lamb wave sensors are discussed. It is shown that the sensitivity of S/sub 0/ mode in liquid sensing is much smaller than that in solid sensing, and also much smaller than that of the A/sub 0/ mode.

The principles of the detection of thin layers of frost and ice formation on an aircraft will be discussed. The theoretical and experimental results of the influence of liquid layers on the propagation of Lamb waves in a plate of finite thickness will be presented. The dispersion equations of Lamb waves in a plate bordered with layers of liquids were derived. Numerical solutions of the equations show the phase velocity of Lamb waves changes with the thickness of the liquid layers. In the vicinity where the thickness of the liquid is equal to a quarter of a wavelength, there is no real solution of the equations. The experiment qualitatively agrees with the theory. [Work supported by Hercules Aerospace Co., Simmons Precision Aircraft System Div.]