A finite element model for rigid porous absorbing materials

An eight node isoparametric finite element is used to represent a rigid porous absorbing material. Tests on an assembly of these elements for a one dimensional model gave good agreement with an exact solution for the input impedance. Results from a two dimensional model show the effects of transverse propagating modes on the input impedance and indicate that for an absorbent with finite dimensions extended reaction is important.     

Transient acoustic wave propagation in rigid porous media: a time-domain approach

Wave propagation of acoustic waves in porous media is considered. The medium is assumed to have a rigid frame, so that the propagation takes place in the air which fills the material. The Euler equation and the constitutive relation are generalized to take into account the dispersive nature of these media. It is shown that the connection between the fractional calculus and the behavior of materials with memory allows time-domain wave equations, the coefficients of which are no longer frequency dependent, to be worked out. These equations are suited for direct and inverse scattering problems, and lead to the complete determination of the porous medium parameters.     

Validity of the limp model for porous materials: a criterion based on the Biot theory

The validity of using the limp model for porous materials is addressed in this paper. The limp model is derived from the poroelastic Biot model assuming that the frame has no bulk stiffness. Being an equivalent fluid model accounting for the motion of the frame, it has fewer limitations than the usual equivalent fluid model assuming a rigid frame. A criterion is proposed to identify the porous materials for which the limp model can be used. It relies on a new parameter, the frame stiffness influence (FSI), based on porous material properties. The critical values of FSI under which the limp model can be used are determined using a one-dimensional analytical modeling for two boundary sets: absorption of a porous layer backed by a rigid wall and radiation of a vibrating plate covered by a porous layer. Compared with other criteria, the criterion associated with FSI provides information in a wider frequency range and can be used for configurations that include vibrating plates.     

梯度飞片材料的波阻抗分布设计与优化

建立起了梯度飞片击靶的理论模型，对具有不同波阻抗分布特征的W-Mo-Ti体系梯度飞片的 击靶过程进行了数值模拟计算.结果表明，梯度飞片对靶产生的压缩是逐步进行的，靶板的 自由面速度也是逐渐提高的，准等熵压缩过程的加载速率远低于传统冲击压缩.为获得较好 的准等熵压缩效果，梯度飞片的波阻抗分布应遵循随厚度变化的二次至三次幂函数关系，并 且靶板厚度必须与梯度飞片的击靶速度等相匹配. 关键词： 梯度飞片材料 波阻抗分布 准等熵压缩     

Wideband characterization of the complex wave number and characteristic impedance of sound absorbers

Several methods for measuring the complex wave number and the characteristic impedance of sound absorbers have been proposed in the literature. These methods can be classified into single frequency and wideband methods. In this paper, the main existing methods are revisited and discussed. An alternative method which is not well known or discussed in the literature while exhibiting great potential is also discussed. This method is essentially an improvement of the wideband method described by Iwase et al., rewritten so that the setup is more ISO 10534-2 standard-compliant. Glass wool, melamine foam and acoustical/thermal insulator wool are used to compare the main existing wideband non-iterative methods with this alternative method. It is found that, in the middle and high frequency ranges the alternative method yields results that are comparable in accuracy to the classical two-cavity method and the four-microphone transfer-matrix method. However, in the low frequency range, the alternative method appears to be more accurate than the other methods, especially when measuring the complex wave number.     

Comments on the limp frame equivalent fluid model for porous media

In this letter, the low and high frequency limits of the effective density characterizing a limp frame porous medium are investigated. These theoretical limits are compared to the ones found for a classical rigid frame porous medium, and to experimental measurements. While the high frequency asymptotic behaviors of both limp and rigid effective densities are usually only slightly different, their low frequency behaviors are significantly different. Compared to experimental measurements performed on a limp frame fibrous layer, only the limp frame effective density yields good correlations over the whole frequency range.     

A mode matching approach for modeling two dimensional porous grating with infinitely rigid or soft inclusions

This work investigates the acoustical properties of a multilayer porous material in which periodic inclusions are embedded. The material is assumed to be backed by a rigid wall. Most of the studies performed in this field used the multipole method and are limited to circular shape inclusions. Here, a mode matching approach, more convenient for a layered system, is adopted. The inclusions can be in the form of rigid scatterers of an arbitrary shape, in the form of an air-filled cavity or in the form of a porous medium with contrasting properties. The computational approach is validated on simple geometries against other numerical schemes and with experimental results obtained in an anechoic room on a rigid grating embedded in a porous material made of 2 mm glass beads. The method is used to study the acoustic absorption behavior of this class of materials in the low frequency range and at a range of angles of incidence.     

Effective medium approach for calculations of optical anisotropy in porous materials

We report on a generalized approach for the calculation of optical properties of various porous semiconductors. The presented methodology provides a simple method for predicting the type and value of optical anisotropy in different materials. Specifically, the cases of electrochemically etched mesoporous Si on (110)-oriented substrate and electrochemically-etched porous InP and GaAs materials on (100) substrates are considered. The optical anisotropy of mesoporous Si is explained and the dependence of the optical birefringence of this material on various material parameters is obtained. The optical anisotropy of porous InP and GaAs with crystallographic pores is predicted based on the presented model. PACS 78.20.-e; 78.20.Bh; 78.20.Ci; 78.20.Fm; 78.30.Fs;78.55.Mb     

Law describing the approach to saturation of porous anisotropic materials

A law is obtained for the approach to saturation with respect to magnetization for anisotropic magnetic materials with inhomogeneous saturation magnetization. In the calculation, the spin-wave formalism is used. The effect of an inhomogeneous exchange interaction on the approach to saturation is taken into account. The relations obtained are generalizations of the already known Neel and Holstein-Primakov formulas.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 32–38, August, 1979.     

A zero-point energy approach for estimating the photon mass

An upper bound for the photon rest-mass is calculated on the basis of zero-point energy considerations within the context of fluctuations in the quantum foam, our results being compared with experimental data. Furthermore, some aspects related to energy spectrum are discussed.     

Vibration characteristic analysis of buried pipes using the wave propagation approach

A theoretical analysis for the free vibration of simply supported buried pipes has been investigated using the wave propagation approach. The pipe modeled as a thin cylindrical shell of linear homogeneous isotropic elastic material buried in a linear isotropic homogeneous elastic medium of infinite extent. The vibrations of the pipe are examined by using Flüggle shell equation. The natural frequencies are obtained for the pipes surrounded by vacuo or elastic medium. The results are compared with those available in the literature and agreement is found with them. It is found that the free vibration frequency of the pipe does not appear for some of the axial or circular vibration modes and the real natural frequencies of the pipe are significantly dependent on the rigidity of the surrounding medium.     

Measuring static thermal permeability and inertial factor of rigid porous materials (L)

An acoustic method based on sound transmission is proposed for deducing the static thermal permeability and the inertial factor of porous materials having a rigid frame at low frequencies. The static thermal permeability of porous material is a geometrical parameter equal to the inverse trapping constant of the solid frame [Lafarge et al., J. Acoust. Soc. Am. 102, 1995 (1997)] and is an important characteristic of the porous material. The inertial factor [Norris., J. Wave Mat. Interact. 1, 365 (1986)] describes the fluid structure interactions in the low frequency range (1-3 kHz). The proposed method is based on a temporal model of the direct and inverse scattering problems for the propagation of transient audible frequency waves in a homogeneous isotropic slab of porous material having a rigid frame. The static thermal permeability and the inertial factor are determined from the solution of the inverse problem. The minimization between experiment and theory is made in the time domain. Tests are performed using industrial plastic foams. Experimental and theoretical data are in good agreement. Furthermore, the prospects are discussed. This method has the advantage of being simple, rapid, and efficient.     

内圆外矩同轴线内TEM波的场结构及其特性阻抗

利用数值保角变换,研究内圆外矩同轴传输线内TEM波的场结构,利用软件MATLAB、HFSS绘制其场结构图,并给出特性阻抗的计算公式.研究结论对于计算该传输线的衰减常数、了解其功率容量、考虑功率耦合及设计有关的有源器件具有一定的参考价值.     

The analysis of fibre-reinforced porous composite materials by the measurement of ultrasonic wave velocities

It is shown that simple accurate calculations can now be made of the elastic constants of fibre-reinforced materials containing matrix voids, using theoretical ideas due to Boucher. This permits the construction of calibration curves for given materials by means of which the measurement of two or more ultrasonic wave velocities may be converted to separate estimates of fibre volume fraction and matrix porosity with predictable accuracy. It is also concluded that in some cases it would be necessary to measure some additional property, such as density, but the measurement of suitable velocities could be used to quantify many defect observations made by existing conventional techniques.     

An approximate formulation for the characteristic impedance of noncoplanar fin antennas

This paper describes an approximation for the characteristic impedance of noncoplanar fin antennas. The formulation is simple and enough accurate for practical applications. The error made by the approximation is less than ±2%, it has been proved to be effective by stripline theory. The numerical results are good agreement to the experimental results given by literature. The expression is useful for the design of wideband antennas.     

The method of the boundary diffraction wave for impedance surfaces

The line integral of the boundary diffraction wave theory is extended for the diffraction process of waves by the impedance surfaces with edge discontinuities. With this aim, the exact diffraction field expression of Maliuzhinets is transformed into a line integral. The method is applied to the scattering problems of waves by a spherical reflector with edge discontinuity and the diffracted fields are evaluated asymptotically. The resultant expressions of the waves are examined numerically.     

In situ measurements of surface impedance and absorption coefficients of porous materials using two microphones and ambient noise

An in situ measurement method is proposed for obtaining the normal surface impedance and absorption coefficient of porous materials using two microphones located close to the material without a specific sound source such as a loudspeaker. Ambient environmental noise that does not excite distinct modes in the sound field is employed as the sound source. Measurements of the normal surface impedance of glass wool and rockwool have been made using this method in various sound fields. The repeatability and wide applicability of the method are demonstrated by comparing results of measurements in one room with different noise conditions and in three other environments (corridor, cafeteria and terrace). The assumed diffuse nature of the sound field on the material is validated by using absorption characteristics obtained experimentally at oblique incidence. This method allows simple and efficient in situ measurements of absorption characteristics of materials in a diffuse field.     

A simple-minded direct approach to estimating the calibration parameter for proportionate data

In non-destructive assay there exist techniques founded on physics principles and experimental design for which the quantity of interest y_True to be estimated is expected to vary in direct proportion to the true value x_True of the experimentally observed predictor quantity x. In other words, the calibration is a straight line passing through the origin, so that the assay method is fully described by a single parameter, the slope. In principle a single reference item is sufficient to estimate the slope. However, there are good reasons for including more than a single item in the calibration procedure. When multiple items are used questions arise regarding how to make best use of all the available calibration data in estimating the slope. This paper shows that the usual weighted least squares curve fitting approach can be circumvented by using only the familiar notion of a weighted arithmetic mean. In particular we draw attention to the ease with which uncertainties in both x and y can be incorporated using this simple and direct approach. Moreover the uncertainty in the calibration parameter is estimated using familiar techniques and with an appropriate magnitude for subsequent use, for instance, in setting reasonable uncertainties on assay results performed using the calibration. For completeness, weighted least squares accounting for nonzero covariances among the measurements y of y_True together with an errors in predictors approach accounting for errors in x is also presented.