Measurement Uncertainties in Vibroacoustic Problems with multiple Degrees of Freedom
Master Thesis of Dreier, Christian
In contrast to sound sources in fluids and especially air there is a lack of a generally accepted characterization method of structure-borne sound sources causing considerable difficulties in many technical applications. Industrial development chains with modular development stages need precise source descriptions concerning their physical behaviour. In the automotive sector for example the vibration behaviour of an engine coupled with its subframe and the vehicle body, can be used to predict the interior sound of a vehicle. A precise look on the example of a drivetrain reveals this task not being trivial: the resulting interior sound field is a coupled inter action between the direct air borne sound radiation of the engine with an indirect structure-borne sound radiation due to vibroacoustic phenomena. Even though the topic of structure-borne sound source characterization has been questioned several years - like on transfer path analysis (TPA) - until now no measurement device solving these problems exists (see chapter 3). Furthermore, the concept of Frequency Response Functions ( FRF ) - which is used to describe the dynamic behaviour of a structural model between an applied load and its resulting vibration - is characterised in non-parametric way using a mobility matrix. However, employment of modern measurement setups is limited in terms of suffi.cient precision by sensing only nine elements of a six-by-six matrix (for single point excitation) in the frequency range of interest, the audible spectrum (see chapter 2.3). In reality up to today no precise moment um exciter exists so that inherent measurement uncertainties in the conventional TPA occur. In order to deal with this severe vibroacoustic measurement problem, the utilization of F inite-Element Method (FEM) - which has blossomed out to a competitive alternative due to increased computation ability - is able to tremendously expand the scope of analyses concerning structural-acoustical coupling (see chapters 4.4 and 5) by enabling a distinct activation of every single DOF as excitation source. Modern approaches in research and development of industrial applications usually are based on elaborated advance developments, using simulations in order to predict . . the properties of a future product as precise as possible. For example, in a fully CAD-driven physical simulation of an electric drivetrain, the machanical impedance distribution on the engine core should be auralized afterwards to virtually predict the interior vehicle sound. In contrast to the conventionally measured TDOF (see chapter 6 the complex movement of the electric engine not only excites translational velocities as conventionally considered, exhibiting the question of the audibility of rotational degrees of freedom ( RDOF) . The concern of this thesis is to deal with the multidimensional measurement problem in vibroacoustics by using numerical simulation techniques and to make the findings metrologically usable. Finally, it is proposed an approach for measuring RDOF with conventional measurement equipment to deduce the importance of rotational degrees of freedom in vibroacoustic transfer paths.