C. Bruneel

Acoustical image reconstruction in parallel‐processing analog electronic systems

Ultrafast cardiac‐valve ultrasonic tomography requires parallel multichannel processing of received echoes. In parallel processing the level of secondary ’’ghost’’ images due to spatial undersampling is much higher than in slower series processors which use a selective field insonification. The paper describes a 20‐channel moving‐focus parallel‐processing analog electronic system, which is realized in our laboratory. It is shown analytically that the secondary‐image level is reduced and remains unchanged when the receiver angular aperture (aperture relative to distance) is limited and kept constant during the whole observation time.

The infinite planar baffles problem in acoustic radiation and its experimental verification

In acoustics, the analysis of pressure distribution in a field of surface sources is most frequently performed using the assumption that the source constitutes a part of an infinite planar rigid baffle (Rayleigh’s approximation). However, in many practical cases of ultrasonic echography, assumption of a soft pressure‐release baffle (the Rayleigh‐Sommerfeld approximation) or of free‐field conditions (the Kirchhoff approximation) seemed to be better matched to real conditions. A theoretical survey of the planar baffles problem is given in this paper, and its practical aspects in acoustic source radiation are discussed. Some experiments, showing the influence of different boundary conditions onto radiation patterns, verify the theoretical predictions.

Anomalous behavior in the radiation pattern of piezoelectric transducers induced by parasitic Lamb wave generation

In order to improve ultrasonic image quality, transducers, similar to those currently used in nondestructive testing and medical imaging applications, have been studied extensively. Attention has been given to single isolated transducers, for which anomalous radiation patterns have been obtained, and to transducers integrated in an array structure, where additional problems occur due to coupling effects between nearest neighbors. An experimental study of these phenomena has made it possible in both cases to find the origin of the perturbations with respect to the theoretical situations which occur in the radiation pattern. Solutions for increasing these perturbations are also suggested.

Acoustic shear wave propagation in Paratellurite with reduced spreading

An original configuration of propagation of acoustic shear wave in Paratellurite slightly off the [110] axis in the (001) plane is described. It allows greatly reducing the spreading of the acoustic energy. The main characteristics of an acousto‐optic interaction with this acoustic shear wave are then discussed. An experimental validation based on the schlieren method is finally proposed. It shows the effective reduction of the acoustic beam diffraction of the shear wave propagating with a 5° angular tilt. It is compared with the acoustic diffraction obtained for a shear wave propagating in TeO2 along the [110] axis without angular tilt.

Application of the finite element method to two‐dimensional radiation problems

Acoustic fields radiated by vibrating elastic bodies immersed in an infinite fluid domain are, in general, quite difficult to compute. This paper demonstrates in the two‐dimensional (2‐D) case that the radiated near field can be easily obtained using the finite element method if dipolar damping elements are attached to the mesh external circular boundary. These elements are specifically designed to absorb completely the first two components of the asymptotic expansion of the radiated field. Then, the paper provides a new extrapolation method to compute far‐field pressures from near‐field pressures, using the 2‐D Helmholtz equation and its solution obeying the Sommerfeld radiation condition. These developments are valid for any radiation problem in 2D. Finally, two test examples are described, the oscillating cylinder of order m and a finite width planar source mounted in a rigid or a soft baffle. This approach is the generalization to 2‐D problems of a previously described approach devoted to axisymmetrical and three‐dimensional (3‐D) problems [R. Bossut et al., J. Acoust. Soc. Am. 86, 1234–1244 (1989)]. It has been implemented in the ATILA code. It is well suited to the modeling of high‐frequency transducers for imaging and nondestructive testing.

Radiation from finite phased and focused linear array including interaction

Resultant pressure of linear arrays, consisting of N elementary transducers, are in general computed with the help of simplified assumptions for the displacement field of the transducer radiating surface. Moreover, interactions between neighboring elements are not included in most of the numerical approaches. In this paper, a new calculation scheme is proposed to compute the resultant pressure of such arrays, including interactions. For this purpose, using the finite element method (FEM), the far-field directivity pattern of a part of the array is computed using dipolar dampers and a previously described extrapolation algorithm [J. Assaad et al., J. Acoust. Soc. Am. 94, 562–573 (1993)]. This part is constituted of an active elementary transducer (electrically driven) mounted between 2Q passives (electrically grounded) neighboring transducers. Then, the resultant pressure of a finite phased and focused array can be obtained by summing up the far-field directivity patterns of the 2Q+1 transducers sets weigh...

Electromechanical coupling coefficients and far‐field radiation patterns of lithium niobate bars (Y‐cut) used in high‐frequency acoustical imaging and nondestructive testing

For high‐frequency applications in ultrasonic imaging and nondestructive testing, lithium niobate (LiNbO3) can be used when cut under the shape of long bars with proper orientation and width to thickness ratio W/T. However, due to a strong material anisotropy, the modeling of such elementary bar transducers, for the sake of optimization, is quite difficult. In this paper, the use of the finite element method, with the help of the ATILA code, is proposed to compute resonance frequencies, coupling coefficients, electrical impedances and in‐water far‐field directivity patterns. (YZw)36° and (YXl)36° cuts with different W/T ratios have been analyzed and numerical results have been carefully and successfully compared with measurements. Main results concern the (YZw)36° cut for which symmetrical directivity patterns are obtained. In this case, a quite constant value is found for the thickness mode coupling coefficient ke in the ranges W/T<0.8 (ke≊50%) and 1.2

Measurement of the thickness of thin layers by ultrasonic interferometry

An ultrasonic interferometer working in a pulsed mode is described in this paper. It allows for the measurement of coating thicknesses as thin as 5 μm with a 5% precision over different substrates at a very high rate (up to 1000 times per second). The optimal conditions for this interferometric measurement are defined theoretically and the probe characteristics have been optimized technologically. This, together with the design of a very large frequency bandwidth (from 90 to 510 MHz) electronic setup, leads to interesting performances. The advantages of the system for achieving thickness measurements are discussed and comparisons are made with other systems.

Optimization of a transmission acoustic microscope

The acoustic field focused by a spherical solid‐water interface has been computed and the results have been used to optimize the geometry of a transmission acoustic microscope. An experimental study has been performed, which enables the sampling of the focused acoustic field, as may be realized by using the Fourier‐transform‐operation concept for an ideal lens.

Combining two radar techniques to implement a collision avoidance system

Many microwave collision avoidance radars have been tested using FMCW (frequency modulated continuous wave) radar and pulsed radar working around 10, 35 and 60 GHz. The aim of this paper is to describe an efficient radar system, termed bimodal, obtained by the combination of these two systems. The FMCW sensor is used for close detection and easier speed calculation, and the pulsed one for large distances and multi-target separation. The bimodal system has been used to improve the performances of the two sensors by retaining the advantages of each. The new radar is able to detect obstacles between 2 and 150 m with low emitted power. The whole system is controlled by a decision program unit. An accuracy of m has been achieved.