Sound and vibrations in structures with air enclosures

Abstract: Vibrations in thin shell structures and sound fields in enclosures are investigated as well as the interaction between such structures and the enclosed sound field. Optical, non-contacting methods as TV holography, shearography and laser scanning vibrometry are used to investigate vibrations of thin shell structures with air enclosures. The sound radiation from vibrating structures is also measured with more traditional sound intensity techniques. The techniques used are briefly presented and compared in this thesis. Structural vibrations in organ pipes made of different materials are investigated as well as the sound distribution from the pipes. Operating deflection shapes of a complete violin are investigated, and the effect of enclosed air on structural vibrations is measured on a violin model. Aerial standing waves inside three cavities are measured and visualised with an optical technique. The response of the shell structures due to aerial standing waves is also measured. From the investigations it is found that the enclosed air has a significant effect on, above all, the lowest eigenmodes. It is also shown that non-contacting measuring techniques have advantages in that they do not to disturb the measured object and in that they are field methods. In numerical comparisons to the measurements it is found that the enclosed air should be included in the numerical model to predict correct results. Two different finite element models of the enclosed air are tested.