柱状分层固体媒质的声散射

本文评述了各向同性和横向各向同性柱状分层固体声散射理论和实验研究进展介绍了描述圆柱状界面薄层特性的弹簧模型，也讨论了该领域中有待进一步研究的一些问题。     

Acoustic absorption in aqueous potassium phosphate

Measurements of acoustic absorption and velocity as a function of frequency and concentration in KH₂PO₄–K₂HPO₄ buffers at 4°C and pH 5-7 are reported. The dependence of the observed acoustic relaxation parameters on concentration is consistent with that to be expected from perturbation of a monomer-dimer equilibrium with an equilibrium constant [for 2H₂PO ₄ ^– (H₂PO₄)₂ ^2–] of 0.21 M^–1, a bimolecular rate constant of 5×10⁸ M^–1-sec^–1 and a standard volume change of –5 cm³ mole. The equilibrium constant for H₂PO ₄ ^– + HPO₄ ^2–H₃(PO₄)₂ ^3– is estimated to be 0.7 M^–1.     

Evaluation of parallelism by ultrasonic echo method

The determination of parallelism in a transparent plate is usually done either by the movement of a stylus or by an optical method, such as interference fringes. For opaque objects, the only method appears to be the use of stylus. The authors have developed an acoustic method to evaluate the parallelism between faces of any block even if it is opaque to light. It is based on the echoes received from the two surfaces when incident upon by a short duration high frequency ultrasonic pulse. The paper describes the basic principle and also the methodologies for evaluation in laboratory and in industry. The method requires essentially only one side to be accessible. Theoretical and experimental results show excellent agreement up to 1° of arc. The uncertainty of measurements of results is ±0.1°. It is expected that the new method would find applications in several industries.     

Transmission of ultrasound through a single layer of bubbles

We investigate, both experimentally and theoretically, the effect of coupling between resonant scatterers on the transmission coefficient of a model system of isotropic scatterers. The model system consists of a monodisperse layer of bubbles, which exhibit a strong monopole scattering resonance at low ultrasonic frequencies. The layer was a true 2D structure obtained by injecting very monodisperse bubbles (with radius a ∼ 100 μm) into a yield-stress polymer gel. Even for a layer with a low concentration of bubbles (areal fraction, n a ² , of 10-20%, where n is the number of bubbles per unit area), the ultrasonic transmission was found to be significantly reduced by the presence of bubbles (-20 to -50 dB) and showed a sharp minimum at a particular frequency. Interestingly, this frequency did not correspond to the resonance frequency of the individual, isolated bubbles, but depended markedly on the concentration. This frequency shift is an indication of strong coupling between the bubbles. We propose a simple model, based on a self-consistent relation, which takes into account the coupling between the bubbles and gives good agreement with the measured transmission coefficient.     

横观各向同性材料中激光超声谱有限元数值模拟

研究了横观各向同性材料中激光超声波的传播特征.基于谱有限元方法,建立了横观各向同性材料中激光超声的数值模型.利用谱有限元方法模拟脉冲激光作用于材料上产生超声波及其传播的过程.讨论了横观各向同性薄板的各向异性及各向同性平面内超声波的传播特征,并分析了材料厚度的变化对产生超声波模态的影响.     

Ultrasonic study on binary liquid mixtures between some bromoalkanes and hydrocarbons

Densities and ultrasound velocities for the binary mixtures of 1-bromobutane+benzene and 1,4-dimethylbenzene and of 1-bromopentane+cyclohexane and benzene have been measured at 308.15 K. Adiabatic compressibilities (beta(ad)), and Wada's constants (W) have also been evaluated as a function of composition. The ultrasound velocities decrease, attains a minimum and then increase with increase in mole fractions of hydrocarbons in the binary mixtures except in the case of 1-bromopentane+benzene binary mixtures where the variation is just the reverse. Dependence of adiabatic compressibilities with mole fractions of hydrocarbons is sigmoid. The non-ideal behaviour of the systems studied is explained on the basis of dipole-induced dipole interactions.     

The origin of the redshift in Brillouin spectra of silica films containing tin nanoparticles

The abrupt change of velocity in surface acoustic waves in thin films of amorphous SiO_x containing nanometre scale -Sn crystals is shown to be directly associated with the size-dependent melting of the nanoparticles, confirming preliminary experiments. High resolution thin film powder diffraction using synchrotron radiation shows that the abrupt redshift in the Brillouin spectra satellites occurs at the same temperature as the melting of the nanoparticles, evident for the loss of the Bragg peaks. Effective medium theory is used to explain the origin of the anomaly. A central peak in the Brillouin spectrum, the intensity of which shows a maximum at the melting temperature, can be interpreted in terms of overdamped fluctuations in the dielectric function. The melting temperature as a function of particle size is in agreement with theoretical predictions. No evidence for strain could be found on the X-ray diffraction profiles; the a- and c-axis thermal expansion coefficients are the same as those in bulk tin. Received 30 March 2000 and Received in final form 24 July 2000