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Published Articles

The Volume 4, No 3, September 1999

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An Investigation of the Differences Between On-road and Wind-tunnel Noise

Peter O?Shea, Simon Watkins, Christian Peric and Bruce Sloss


The airflow around a moving car results in wind noise which is partially transmitted into the cabin. With the trend towards quieter cars, this wind-generated noise can dominate other noise sources. Standard methods of developing car geometries (often with the aim of minimising wind noise) are based on tests in smooth-flow wind tunnels, rather than by road trials. However, it has been noted that the sound of the wind is perceptually different in the two test environments. This paper sets out to characterise these acoustical differences. Higher-order spectral statistics and wavelet analysis have been utilised and significant variations in the two environments were found. Ultimately, it is hoped that such analyses can be used to predict on-road wind noise from smooth flow wind-tunnel data (utilising knowledge of the turbulent velocity fluctuations in the atmosphere).

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Finite Wave Modeling of Extended-Tube Resonators

Y. Sathyanarayana and M.L. Munjal


Finite wave analysis of hot exhaust systems involves two modified Riemann variables and an entropy variable. These three variables move along their characteristic paths in the three characteristics approach of the method of characteristics. This paper addresses the modeling and simulation of extended-tube resonators in the time domain with the three characteristics approach. There are two known variables and seven unknown variables across an extended-tube resonator junction. Mass continuity, energy, momentum and entropy equations along with compatibility equations are simultaneously solved by means of the Newton-Raphson method to evaluate the unknown variables. The extended-tube expansion chamber is analyzed by means of the two characteristics approach as well as the three characteristics approach. The simulated results obtained by the two approaches are compared.

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Case Studies of Machine Bases as Structure-borne Sound Sources in Buildings

B.M. Gibbs and A.T. Moorhouse


A review is given of recent approaches to the characterisation of structure-borne sound sources, which can have application to the case of machines in buildings. Two primary issues are identified. The first is that the structural dynamics of the receiving structure must be known for a full description of the transmission process and the second concerns the large amount of data required for characterisation and prediction of emission. In order to reduce the data required and thus simplify the analysis and prediction, it is necessary to consider initially, representative installations in order to establish a hierarchy of contact and component excitations and thereby eliminate the least significant. This can be done in several ways: modelling idealised sources and receivers, using measured or theoretical source and receiver data, prior to assembly, and by in-situ measurement of the installed machine. In this paper, case studies are presented of machine bases rigidly connected to concrete floors or steel plates, where a combination of the above methods is used to identify the important transmission paths and the data required for approximating the total transmission. In addition, it is demonstrated that visual inspection of point force mobilities can establish which of the mechanisms, mass, stiffness or resonance are controlling mobility and thereby lead to simplifications.

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