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Published Articles

The Volume 2, No 3, September 1997

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Inverse and Reciprocity Methods for Machinery Noise Source Characterization and Sound Path Quantification Part 2: Transmission Paths

Jan W. Verheij


In this article and in a foregoing companion article some novel approaches to the characterization of the noise source strength of machinery and to the ranking of transmission paths are reviewed. They form an addition to the more conventional approaches of the analysis of noise control problems in terms of source-transmission path-receiver schemes. In the first article source strength descriptors have been defined in terms of equivalent fictitious elementary sources, such as acoustical monopoles and mechanical point forces. This second article presents examples, which illustrate how some of these unconventional source strength descriptors have been exploited for sound transmission path quantification. These concern applications in ships and road vehicles. In all cases it is the combination with experimental reciprocity techniques, that makes the use of the elementary substitution sources as source models very powerful for transmission path ranking. Therefore, an appendix on reciprocity relations in structural-acoustics is added to this article, including a brief bibliography on that subject.

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Active Control of Sound Radiation Using a Foam-PVDF-Plate Passive/Active Composite Device

C. Guigou, J. Pan, C.R. Fuller and C.A. Gentry


The development of a hybrid passive/active device in the form of a foam-PVDF-plate composite transducer designed for active noise control is discussed. The composite material consists of cylindrically curved sections of PVDF piezoelectric film (active component) embedded in partially reticulated polyurethane acoustic foam covered with a light weight stiff sandwich plate (passive components). For performance testing, the foam- PVDF-plate system was mounted over the surface of an oscillating rigid piston located in a baffle in an anechoic chamber. Two types of error sensors, a far-field microphone and an accelerometer located on the plate surface, were considered and compared in terms of their efficiency to control the far-field sound radiation. A feedforward LMS controller was used to minimize the signal from the error sensor. The results are presented in terms of the far-field sound radiation and, in order to understand the control mechanism, the surface normal velocity distribution of the sandwich plate. The potential of the hybrid foam-PVDF-plate passive/active device for simultaneously reducing global sound radiation at low and high frequencies is demonstrated.

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Sound Propagation in Swirling Flows

Vladimir V. Golubev and Hafiz M. Atassi


The propagation of small-amplitude disturbances in an annular duct with mean swirling flow is studied. A normal mode analysis leads to an eigenvalue problem which is not of the Sturm-Liouville type; the eigenmodes are not orthogonal and do not form a complete set. The analysis reveals the existence of nearly-sonic and nearlyconvected modes. Acoustic radiation in the duct is dominated by the nearly-sonic modes and changes significantly in presence of the mean swirl which introduces a swirl-induced Doppler shift of acoustic cut-on frequencies, refraction of propagating pressure modes due to the mean flow non-uniformity, and acousticvorticity coupling produced by Coriolis forces in a vortical swirling flow. The nearly-convected eigenvalues are vorticity-dominated, and accumulate at the borders of the convected critical layer which forms for a vortical mean swirl. Two numerical methods are used to solve the eigenvalue problem. One is a finite-difference method, the other is a pseudo-spectral collocation method. The accuracy of the finite-difference approximation is not good enough near the critical layer, but the pseudo-spectral method gives significantly more accurate results.

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