For low-frequency applications, a modal approach can be useful to describe vibro-acoustical coupling. Based on combined vibrational/acoustical frequency response function measurements, either with respect to acoustical or structural excitation, modal vibro-acoustical analysis can be carried out. This paper presents a consolidation of the theory behind the vibro-acoustical modal model. The model formulation is shown to be a nonsymmetrical formulation. It is shown that this is not contradictory to the well-known vibro-acoustical reciprocity principle. The implications of the nonsymmetry for the modal model are discussed. It is pointed out which variables must be measured and what kind of scaling must be used in order to end up with a consistent modal formulation. The theory is illustrated and verified by measurements on an experimental vibro-acoustical system, consisting of a rigid cavity with one flexible wall.

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Spectral analysis of non-stationary phenomena using traditional, segment averaged, discrete Fourier transform techniques suffers from limitations related to the trade-off between temporal and spectral resolution. These limitations are inherently due to the assumption of signal stationarity over an observation interval of fixed duration. In case frequency components occur in widely different frequency bands, an alternative technique, based upon the Wavelet transform, yields results with a temporal resolution which is inversely proportional to the analysis frequency. The issue of non-stationary spectral analysis is discussed at the hand of a sound quality problem related to a door slam. It is shown how the Wavelet analysis allows to identify the critical time instances and hence the critical physical phenomena responsible for the problem.