Acoustic excitation and transmission of lightweight structures

University dissertation from Div. Engineering Acoustics, LTH

Abstract: In order to increase our knowledge of the sound transmission and radiation processes of lightweight wall and floor structures, theoretical models are needed. Detailed models may form a valuable tool. In lightweight floor structures, impact sound insulation is perhaps the most important property to consider. This thesis presents an overview of various solution strategies that may be useful in finding a theoretical model for impact sound insulation. Expressions for the point mobility of infinite plates driven by a rigid indenter are derived. These expressions are needed when determining the deformation close to the excitation area, which is important when studying impact noise to properly describe the interaction between the source and the floor. A detailed three-dimensional thick-plate analysis is used. The excitating pressure is found by means of a variational formulation. The point mobility is calculated by means of numerical integration. The excitation force provided by the ISO tapping machine is examined, partly in relation to the three-dimensional deformation analysis. Results found in the literature are reviewed and reconsidered. Low-frequency asymptotes are derived. A more general impact force description is derived, suited for arbitrary frequency-dependent mobilities of the floor structure. The frequency-dependency of the mobility can be due to local effects, investigated by means of thick-plate theory, and/or global effects, investigated by means of a spatial Fourier transform method. A theoretical model for a point-excited simple lightweight floor is presented. The model is used for the prediction of impact noise level. A comparison between numerical computations and measurements found in the literature is performed. A relatively good correspondence between measurements and calculations can be achieved. Lightweight walls (and floors) are often designed as a framework of studs with plates on each side. The studs can be seen as walls in the cavity, thus introducing finiteness. A prediction model for airborne sound insulation including these effects is presented. Due to variabilities, no structure can be perfectly periodic. The effects of near-periodicity are studied by means of transform technique and the expectation operator. The near-periodicity leads to an increase of the damping (if material damping is present). Resilient devices are commonly used in lightweight structures to decrease the sound transmission in a broad frequency band. Applications of such devices may be found, for example, in resiliently mounted ceilings in aeroplanes, ships and buildings. A measurement method to characterise the two-port acoustic properties of resilient devices is presented.

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