This paper describes vibro-acoustic coupling between structural vibrations and internal sound fields of thin structures. We treat a cylindrical structure having thin plates at both ends and take notice of coupling between the plate vibrations and internal sound field, when an external force is applied to an only end plate. Coupling are theoretically and experimentally estimated based on behavior of both plates and acoustic characteristics of the internal sound field with changing the eigenfrequency of end plates and cylinder length. Consequently, when the natural frequency of the plates exists near the resonance frequency of the acoustic mode whose modal shape is similar to that of vibrations, the dominant acoustic mode, which extremely makes a contribution to the formation of the sound field, shifts with changes in the length and the phase difference between both plate vibrations is close to the same or opposite difference except the range of the length. Restrained in such a phase difference, the sound pressure level increases greatly in the vicinity of a specific cylinder length, around which the phase difference shifts rapidly and coupling is promoted. The appearance and shift of the dominant mode are also affected by the phase difference, that is, coupling depends on the vibrations of not only excited plate but also nonexcited plate.
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