Improvement of the lateral resolution of finite-size hydrophones by deconvolution

In various fields of ultrasound applications, frequencies well above 10 MHz are used. As a consequence of this, ultrasound sensors, especially the piezoelectric hydrophones presently available which are used for the characterization of the respective fields, can no longer be considered as point receivers. By means of numerical deconvolution, the adverse averaging effect caused by the finite sensor size can be revoked. The efficiency of the deconvolution process is dealt with for both numerical simulations and experimental investigations. Best results were obtained using a reconstruction filter consisting of a combination of a Wiener filter, a pruning filter and an additional low-pass filter.     

On-line delta(18)O measurement of organic and inorganic substances

A method for the automated sample conversion and on-line oxygen isotope ratio (delta(18)O) determination for organic and inorganic substances is presented. The samples are pyrolytically decomposed at 1400 degrees C in the presence of nickelized graphite. With the system presented organic as well as inorganic samples such as nitrates, sulphates and phosphates of 50-100 &mgr;g O can be analyzed for their delta(18)O values with a standard deviation usually better than 0.5 per thousand. Additionally, carbon isotope ratios of organic substances and nitrogen isotope ratios of inorganic nitrogenous compounds are available in the same sample run. Data for international and some inter-laboratory reference materials are presented to show the accuracy and reliability of the method. The effect of some additives on the CO yield was checked for substances which do not pyrolyze completely. Copyright 1999 John Wiley & Sons, Ltd.     

Core-Shell-Structures in Multiphase Dielectric Ceramics

Developments in the field of multilayer capacitors (MLC) still require investigations of the micro-structure-property relationships of the dielectric materials. Multiphase ceramic dielectrics with “core-shell” micro-structure obviously are best suited for a “flat”, i.e. nearly constant temperature behavior of the dielectric constant (DC), which is of the order of 2000—4000. A general demand is that the DC-reduction within the temperature range —55 °C to +125 °C should be less than 15% (X7R classification). In addition to this, the sintering temperature should not exceed 1120 °C. The kind of structure, which is finally obtained is influenced by several factors: the composition of the precursor materials, the mixing procedure, the temperature-time regime during the firing process, and eventually the final annealing treatment. Using barium titanate as core material and mixing it with different perovskite compounds we adopted two different methods of production of the dielectrics, here investigated. They resulted in two different structure forming mechanisms, leading to different micro-structures.     

Stochastic aspects in the theory of spectral-line broadening. I. Collision time statistics andN-perturber limit

An atom in a gas or plasma experiences a random potential which gives rise to the so-called pressure broadening. The corresponding line shape is obtained in the usual two-level model by a trace operation from the Fourier transform of 〈T(t, 0)〉, the average of the time-development operator. Under certain technical assumptions it is rigorously shown by probabilistic techniques that 〈T(t, 0)〉 falls off faster thant ^?3+ε for any ε > 0, giving a continuous Fourier transform and line shape. An alternative expression is derived for the latter which explicitly displays its positivity and which is a limit over increasing perturber numbers. The latter generalizes results of von Waldenfels. Part I is preparatory for Part II, where a noncommutative cluster expansion is applied to the line-shape problem. Several open questions are pointed out which merit a rigorous investigation.     

Weak-noise in nondissipative periodic systems

A class of stochastic differential equations is considered which describe dynamical systems without systematic relaxation terms but subjected to external noise. A stationary distribution is derived in the weak-noise limit and criteria are given in how far this yields an approximation for finite noise stationary distributions. Concerning time dependent behaviour it is shown how reduced statistical information may be drawn from period broadening. As example the harmonic oscillator and a richer non-linear model are discussed.     

Reconstruction of ultrasonic fields by deconvolving the hydrophone aperture effects. I. Theory and simulation

In many ultrasound applications-especially in the area of medical diagnostics-conventional sensors and particularly piezoelectric hydrophones do not comply with the requirements for "point detectors". It therefore suggests itself to develop methods, which allow ultrasonic fields to be reconstructed from the spatially averaged measurement values with the necessary spatial resolution. In this publication, this task will be treated as an inverse problem. It will be shown by simulations that it is possible with the aid of reconstruction methods to invert the aperture effects of the sensor. Three different reconstruction methods are tested: (1) Wiener's method furnishes the best approximation of the field to be reconstructed in the sense of the minimum mean square error. (2) The power spectral equalization method is based on equalization between the spatial power spectral densities of the field to be reconstructed and that of its estimated value. (3) With the maximum a-posteriori method, the maximum a-posteriori probability density function for the reconstructed field is searched. The results of this paper show that the quality of reconstruction depends on the signal-to-noise ratio (SNR) and the spatial frequency bandwidth of the sound field investigated. Good quality reconstructions were obtained at correlation coefficients larger than 0.995 and an SNR of at least 40 dB. In general, the quality of reconstruction decreases with decreasing SNR and is unacceptable at SNR = 20 dB. The quality also decreases with decreasing axial distance from the source that is with increasing spectral frequency bandwidth.