The use of an equivalent forces method for the experimental quantification of structural sound transmission in ships

Structure-borne sound transmission is a complex phenomenon, the accurate measurement of which requires advanced techniques. A multi-dimensional substitution source method is presented by which structure-borne sound transmission paths can be quantified in practical situations where it is not possible to dismount the paths. The paper illustrates, by means of two examples, that reasonable results can be obtained using a suitable choice of force and response positions that are accessible in a practical situation. Additionally, techniques for quantifying confidence intervals are developed. Results are presented for the application of the method to sound transmission along realistic laboratory models of a ship drive shaft and a fluid-filled piping system. It is shown that the use of such a multi-degree-of-freedom approach to mechanical substitution sources is essential in the applications considered. For the drive shaft, the transmission through a bearing can be reproduced satisfactorily by two excitation degrees of freedom, but for the pipe even six was insufficient. Finally, numerical simulations are used to demonstrate that this is a result of transmission through the water within the pipe and that use of more excitation degrees of freedom would allow it to be taken into accoun