Holographic modal analysis for the investigation of vibro-acoustic systems

In order to optimise the vibro-acoustic behaviour of panel-like structures in a more systematic way, accurate structural models are needed. However, at the frequencies of relevance to the vibro-acoustic problem, the mode shapes are very complex, requiring a high spatial resolution in the measurement procedure. The large number of required transducers and their mass loading effects limit the applicability of accelerometer testing. In recent years, optical measurement methods have been proposed. Direct electronic (ESPI) imaging, using strobed laser illumination, or more recently, pulsed laser illumination, have lately created the possibility to bring the holographic testing approach to the level of industrial applicability for modal analysis procedures. Therefore an automated ESPI system has been developed for the measurement of frequency response functions using stepped sine testing. A conventional numerical modal analysis procedure is used to obtain the modal information. The present paper discusses the various critical elements of a holographic ESPI modal testing system. Next to the optical parts, the integration with the modal analysis procedures, including the integration of geometry and response measurement, are discussed. The paper furthermore discusses test results optained on a car panel in a vibro-acoustic setup. The results show, that the quality of the frequency response functions is very good, when compared with accelearation sensor measurements. The measurement data are used to predict interior noise.