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In this paper, the acoustic character of a new MEMS device that can separate micro/nano particles on-chip by the use of bulk-mode excited membrane vibration is analyzed. Two parts of the acoustic field, i.e., the acoustic pressure caused by the membrane vibration and the damping pressure caused by the fluid medium, are studied separately. Firstly, acoustic pressure caused by silicon nitride membrane vibration in the microdevice is analyzed, and the results for device's different working modes are obtained. Secondly, the acoustic damping character of the four-clamped membrane in the device, which is caused by fluid medium, is studied, factors that affect the damping pressure are discussed. Based on these analyses, the effective acoustic pressure acting on the particles is obtained. Furthermore, the finite element method is used to study the structure-fluid coupled field of the device and the distribution of acoustic pressure in the microdevice is obtained. The process for acoustic character analysis discussed in this paper can be used to study vibrational structure in ultrasonic MEMS devices of the same kind.