Payloads of satellite are exposed on the severe acoustic environment at the process of lift-off and supersonic zone of a launcher. This acoustic environment excites the payload in high pressure and broad frequency band of random acoustical excitation, which may cause serious damage to the structures or instruments of the spacecraft inside. Space instruments are designed and verified to the acoustic environment by ground reverberant acoustic chamber in order to specify random vibration level at component interface and to verify the payloads are working in function and the structure does not have structural damage. The present load sound pressure specification assumes that the sound pressure interior fairing is uniformly distributed. In spacecraft system acoustic tests, local pressure increase occurs in the narrow gap between spacecraft primal structure and components facing toward the fairing wall. This acoustical environment load to the components differs from that the components were tested alone and the flight acoustic environment may not be actually simulated in the ground testing. It is important to clarify the mechanism of sound pressure increase in the narrow gap in order to predict the level of sound pressure increase. In this study, we focus to the investigation of the mechanism by basic experiment including acoustic testing and vibration modal survey. It is clarified that the main reason of the phenomenon is dominated by the acoustic cavity on the appropriate boundary condition rather than structure vibration. And more, we predict the frequency at which the sound pressure increase at the narrow gap and compare analysis results with experiment results by using Boundary Element Method (BEM).

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