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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.

Acoustic holography (AH) is a powerful measurement tool for the investigation of complex sound radiation problems. Enabling detailed source investigations based on noncontact measurements, it is especially useful for the investigation of rotating noise sources such as automotive tires. Nevertheless, conventional AH techniques are burdened by the constraint that the source is assumed to be planar. Based on a detailed measurement series which was performed on a laboratory roller bench, the radiation of a test tire was investigated by means of AH. It was found that even though valuable information could be obtained, the most important forward and backward radiation of the tire is difficult to tackle in this way. Another approach of the radiation analysis is offered by developing an inverse BEM method. Different implementations of the method have been worked out and tested, based mainly on the sysnoise and matlab software packages. The technique is applied to experimental tire mock-ups as well as to real-life tire measurements. The practical application and inherent limitations of the technique are also discussed.