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Published Articles

The Volume 3, No 2, June 1998

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Computer Modelling of Sound Propagation in Enclosed Spaces with Uneven Fitting Distributions

S.M. Dance and B.M. Shield


Current image-source based computer models assume an even floor distribution of fittings in non-diffuse enclosed spaces, such as factories. This is not fully representative of industrial spaces where gangways and open areas are present. It has been shown that the more precise the representation of the fittings the greater the prediction accuracy of the steady state sound levels. This paper presents a validation of the Complete Image Source Model (CISM) in two test spaces, a 1:10 scale model and a laboratory space configured with six different fitting distributions, which previously could not be modelled by image-source based methods. Sound propagation curves were predicted for each fitting configuration, and are compared with sound propagation curves given by a version of the Ondet and Barbry ray-tracing model, RAYCUB. The sound levels predicted by RAYCUB and CISM were found to be of a similar accuracy in all six cases, but CISM required less information to define the space and less time to produce the results.

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A new time domain approach to evaluate transmission loss in layered partitions

Roberto A. Tenenbaum, Marcelo Bruno S. Magalhaes, Moyses Zindeluk


The insulation provided by layered barriers is addressed from a theoretical point of view. An exact time-domain solution for the normal incidence plane wave propagation in layered media excited by an arbitrary input is presented, leading to a fast algorithm to compute the transmission loss in a compound wall. Numerical experiments demonstrate its agreement with the mass law for a single wall. For multiple partitions, however, the mass law is seen to be inadequate to predict correctly the transmission loss and deviates from the model presented when the number of layers increases. It is also shown that, by changing the material of the internal layers, it is possible to achieve a better insulation than that obtained by using a single wall with a greater density and the same thickness. The theoretical model presented here provides an easy way to evaluate the transmission loss and leads to a procedure for the determination of the optimal choice of materials to be used in a partition.

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On the Relationship between Active and Passive Methods of Mode Localization for Vibration Confinement

Ahmet S. Yigit


Recently, some researchers have proposed what is termed as ?active mode localization? where all or some of the mode shapes are altered by feedback in such a way that the modes are spatially localized. The resulting closedloop behavior is similar, at least qualitatively, to the phenomenon known as ?Anderson localization? or normal mode localization which is caused by irregularities (disorders) in periodic structures and which can be considered as a form of passive confinement. This study is an attempt to develop analogies between active and passive means that achieve mode localization. It is demonstrated that in certain cases a passive system can be considered as a special case of active feedback. It is anticipated that the ideas proposed in this study will be useful in developing systematic methods for integrated structure/control designs.

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