Transmission of structure-borne sound in ships : applications to a prediction model

Abstract: This thesis examines transmission of structure-borne soundin rib-stiffened plate structures. In particular, theapplication of a prediction model for ships is analysed. Thismodel is based on a waveguide description of the structure. Atlow frequencies, the model is sensitive to the structuraldescription and the main emphasis of the work is on thisdescription in the low frequency regime. Effects of differentsource models, as well as various models for calculation ofvelocity levels, are also analysed.For the structural analysis, different methods are adaptedfor use with rib~stiffened structures. These methods are hereapplied to geometries typical of ships, but can be used withany structure with waveguide behaviour. For ship's structures,the boundary conditions that the frames constitute on thesub-panels are analysed analytically and experirnentally. Amodal decomposition technique is employed and further developedto determine the distribution of energy on different modes atmodel-scale and full-scale experiments. A hybrid finite elementtechnique is adapted and further developed to calculate thedispersion properties of typical ship's structures. Resultsfrom this calculation are compared to those obtained withvarious analytical models of the structure. At low frequenciesit is shown that wave numbers may be approximated withso-called equivalent plate theory, for which the properties ofthe ribs are "smeared" on the plating. Even at higherfrequencies the wave numbers of some of the modes can beestimated from equivalent plate theory.Energy flow in simple waveguide systems is analysed. For arow of connected plates it is shown that the energy flowthrough a particular element may be large, even at smallvibration levels. Application of damping locally will thereforegenerally not greatly reduce the energy flow. For rib-stiffenedplates, the energy flow in "stiff"modes, for which thestiffeners deflect with the base-plating, is related to that ofsub-panel modes at the same amplitude of vibration.For a structural model at low frequencies, a waveguidedescription is suggested for which the main frames of theship's hull constitute boundaries and the properties of theribs are smeared on the plating in-between. Such a stiffdescription is implemented in the prediction model and resultsare presented with and without consideration of couplingbetween sub-panel modes and stiff modes at plate junctions.Without coupling, application of smeared-out theory results intoo small transmission losses compared to those measured. Goodcorrespondence is obtained using sub-panel theory only, atfrequencies above cut-on of the fundamental sub-panel-mode Thesimple coupling models applied, also yield a fafrcorrespondence at low frequencies, but are quite sensitive tothe structural and source description.Keywords:measurement, modal decomposition, panelboundary condition, energy flow, prediction model,rib-stiffened plate, ship's hull, structure-borne sound,vibration, wave propagation, waveguide.