Investigation Into the Importance of the Degrees of Freedom for the Characterisation of Structure-Borne Sound Sources

Structure-borne sound sources are ideally characterised by taking into account six degrees of freedom: three translational components and three rotational components. The complexity of such a characterisation is immense and the effort to measure all components is not realistic or even impossible. If a practical characterisation of structure-borne sound sources in building acoustics is aimed for, reliable information about the importance of the different components is essential. In search for possible simplifications of the degrees of freedom, a series of measurements was carried out on a scale model of an excentric motor connected to a plate on a small receiving room of one cubic metre. The sound pressure in the receiving room was used to verify whether a prediction based on the measured normal source components leads to a correct result. The deviations between the direct pressure measurement and the calculation give some guidance to whether the simplification can be justified or not for the particular structure-borne sound source under test. Once the relevant degrees of freedom have been determined, a simplified prediction based on separately measured source and receiver characteristics can be carried out. The measurement procedure is meant as a method to verify potential simplifications for the characterisation of structure-borne sound sources. It also quantifies in advance the accuracy of a prediction based on those simplifications.