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Title: The model of local mode analysis for structural acoustics of box structures
Authors: Ngai, King-wah
Degree: Ph.D.
Issue Date: 2004
Abstract: Structure-borne noise is a new noise pollution problem emerging from railway concrete box structures in Hong Kong. Its low frequency noise with intermittent effect can cause considerable nuisance to neighborhoods. The tonal noise peaks in this low frequency range should be one of the important factors in structure-borne noise analysis. In the acoustic field, the deterministic analysis of all the resonant modes of vibration is generally considered as not practical. Many acoustic experts use the statistical energy analysis as the main tool for the noise investigation whereas the application of the experimental modal analysis in the structural acoustic problem is comparatively rare. In the past, most studies mainly focused on the structure-borne noise measurement and analysis. The detail study of the cause of structure-borne noise is lack, especially for the rectangular concrete box structure. In this dissertation, an experimental and analytical approach is adopted to study a typical concrete box model. This thesis aims at confirming the importance of modal analysis in the structure-borne noise study and then at identifying the local vibration modes along the cross-section of box structure. These local modes are responsible for the structure-borne noise radiation. The findings of this study suggest that the web of viaduct cross-section is not as rigid as assumed in the conventional viaduct design and the web face is likely to be more flexible in the vertical displacement of the concrete viaduct. Two types of local vibration modes along the cross-section are identified: the centre mode and the -web mode. At the top panel of the viaduct, the centre mode has movement in the middle but not at the edges. The web mode has movement at the edges with the middle fixed. The combined centre and web mode has been found to be important in the structural acoustics of the concrete box structure. In the actual concrete viaduct, the coincidence frequency is especially low (often around 100 Hz). When the vibration mode frequency is close to or above the coincidence frequency, the radiation efficiency is high. The high radiation efficiency promotes the generation of high acoustic peaks at the local vibration mode frequency. The first two acoustic tonal peaks in the low frequency range correspond to the centre mode and the combined centre and web mode. Reduction of these two bending vibration mode magnitudes should be able to mitigate the structure-borne noise problem. The intermittent effect caused by the tonal peaks can be minimized by lack of these local vibration modes. Shifting the excitation location to the edge reduces the vibration magnitude at the centre mode. But it is not helpful to reduce vibration of the combined centre and web mode. This study established a new model for studying the local vibration of rectangular duct structures. The findings can aid the understanding of similar structure-borne noise problems. These are also beneficial for the design of concrete railway viaducts when concerning noise minimization.
Subjects: Hong Kong Polytechnic University -- Dissertations
Architectural acoustics
Noise -- Measurement
Steel, Structural -- Noise
Pages: 1 v. (various pagings) : ill. (some col.) ; 30 cm
Appears in Collections:Thesis

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