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This paper presents an array of five aluminum nitride (AlN) based piezoelectric micromachined ultrasound transducers (pMUTs) with different dimensions operating at 540–2360 kHz in air. Due to the damping effect of water or oil on the vibration of each individual pMUT, their frequency response tends to merge and significantly increases the bandwidth of the pMUTs array. The device is fabricated based on the deep reactive ion etching (DRIE) process on the backside of an SOI wafer to realize the circular diaphragms. Theoretical calculations, measured frequency response, simulation results, and detail of the fabrication are explained in this paper.