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Sven Gade, Bruel and Kjaer (Denmark)
Niels Johan Wismer, Bruel and Kjaer (Denmark)
This paper introduces an improved method for estimation af Complex Modulus. The stress-strain relationship of non-resonant visco-elastic materials, generally used in damping treatment of structures, can be described by 2 properties, such as the perfectly elastic (in-phase) stress-strain modulus and the loss factor, The values of these properties need to be determined in tension or compression for material used as unconstrained damping layers and as anti-vibration mountings under foundation blocks. Using a dual channel FFT analyzer, the specimen can be excited using wide band random excitation, and the properties can be determined as a continuous function of frequency. The method can normally be used up to a frequency which is a factor of 2 to 3 below the first resonant frequency for the test-setup. For the proposed improved method the frequency range is extended to well above the first resonant frequency for the test-setup. The method is verified by use of the well known 3dB bandwidth method.
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C.C. Sung, National Taiwan University (Taiwan)
Ro Ruyen, I-Shou University
Yuh-Ming Chang, National Taiwan University (Taiwan)
Acoustically active materials (chiral materials) that lack centrosymmetry due to chirality in their microstructures can be characterized by the constitutive relations ji = ji + ij + kkij + cji and ji = ( + )ji + ( - )ij + kkij + cji. Accordingly, two longitudinally, two right circularly and two left circularly polarized elastic waves can propagate in chiral medium. Using appropriate field representations along with prescribed boundary conditions, scattering characteristics at chiral interfaces can be realized. In this paper, reflection and transmission characteristics of chiral slabs bounded by achiral media for longitudinally elastic waves with normal incident are thoroughly discussed. Results obtained can be applied for the design of broadband acoustic impedance transformer and acoustic absorbers which will be reported in the near future.
sv970538.pdf (Scanned) TOPNelik Dreiman, Tecumseh Product Company (U.S.A.)
Wide use of metals for a machine structural component such as drive trains, valve trains and where combination of high strength at both low and elevated temperatures are required, and necessity to reduce noise of the mechanism operating at such extreme conditions made it necessary to study the effect of such common factors as chemical composition, heat treatment, and method of manufacturing on acoustical characteristics of the construction metals. In the analytical and experimental work reported in this paper, sound radiation of the rectangular plates made from iron-chrome (Fe - Cr) and iron-chrome-munganese (Fe-Cr-Mn) alloys have been studied in some details. The analytical study shows that the sound radiation efficiency of a metal depends upon geometry, external static and dynamic loads, boundary conditions, surrounding medium, physical and mechanical properties of the metal. The analysis of the experimental data indicuted that the change of the chemical composition and corresponding alloys structural changes affected the attenuation rate of the sound radiated by the plutes. High rate of the sound attenuation have been recorded for the ferrous alloywith18% Cr and 12% of Mn. The variety of alloying elements added to the iron or steel to modify its physical and mechanical properties as well as heat treatment, and mechanical working of the metals should be investigated for noise reduction potential and nondestructive diagnostics of machinery and materials.
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David E. P. Lawrence, Monash University (Australia)
Charles G. Don, Monash University (Australia)
The tortuosity or structure factor of a porous medium represents the increase in resistance to normal air flow due to the air following an indirect path relative to the macroscopic acoustic pressure gradient, Along with flow resistivity and porosity, tortuosity is one of the important parameters required in many of the theoretical models used to predict the acoustic properties of porous materials. A commonly used technique for measuring the tortuosity involves saturating the material with an electrically conducting fluid, however, this is inappropriate for unconsolidated granular media such as soils. Alternatively, the tortuosity can be estimated from the high-frequency asymptote of the measured phase speed. In practice, the maximum frequency at which propagation measurements can be made will be experimentally limited. This paper outlines a simple method by which the tortuosity of a medium can be readily estimated, with the aid of a simple theoretical calculation, from experimental data that have not reached the asymptotic value, The method assumes a prior knowledge of the flow resistivity and porosity of the medium. To validate the technique, estimates of the tortuosity of plastic foams and soils are compared with values obtained through a more time consuming curve-fitting approach.
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Yue Sheng Wang, Northern Jiaotong University (China)
Zi Mao Zhang, Northern Jiaotong University (China)
Gui Lan Yu, Northern Jiaotong University (China)
It is well known that the geologic materials are generally porous media saturated with various fluids, and are usually anisotropic due to bedding, compaction and the presence of aligned microcracks. Wave propagation in such porous media is of practical importance in geophysics, hydrology, soil dynamics, earthquake engineering, etc. This paper will examine the propagation of elastic waves through layered transversely isotropic fluid saturated porous media. The analysis is based on the Biots theory. The dissipation due to fluid viscosity is considered. We first derive the characteristic equation of plane waves in an arbitrary layer, of which the closed form solution is presented. This enables us to write the general form of waves in a layer. The results show that three kinds of waves propagate in a layer: the fast and slow quasi-longitudinal waves (QP1 and QP2) and the quasi-transverse wave (QSV). Then, by consideration of continuity conditions at the interface, the transfer matrices between layers are derived. The expressions of amplitudes, phase velocities, attenuation coefficients and directions of the reflected and transmitted waves are presented.
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Konstantin Khirnykh, South Bank University
Alan Cummings, University of Hull
In this paper, a method for measuring the vibrational velocity of the solid skeleton of a flexible, porous, sound-absorbing material is described. This is based on a simple electromagnetic principle, involving a thin conductor that is structurally connected to the frame of the absorbent and placed in a magnetic field. The method appears to yield acceptably accurate data. The sample test results that are presented here demonstrate that useful comparisons can be made between acoustic data and frame velocity measurements, especially where high amplitude, non-linear, effects are present.
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