This paper reports a piezoelectric aluminum nitride (AlN) based micro-machined infrasonic hydrophone. We have conducted a systematic design study for the hydrophone sensor to meet the stringent requirements of underwater applications. The hydrophone sensor was fabricated on a cavity siliconon-insulator (SOI) substrate using an in-house CMOS-compatible AlN-on-SOI process platform. A 5 x 5 arrayed hydrophone sensor was characterized thoroughly using an industry-standard hydrophone calibration instrument. The results show that the hydrophone achieved a sound sensitivity of -182.5 dB 6 0.3 dB (ref. to 1Vrms/lPa) and an eligible acceleration sensitivity of only 196.5 dB (ref. to 1Vrms/lg), respectively, a non-linearity of 0.11%, a noise resolution of 57.5 dB referenced to 1 lPa/ Hz within an ultra-low operation bandwidth of 10Hz~100Hz, the highest noise resolution of micro-machined hydrophones reported to date, and better than traditional bulky hydrophones in terms of the same application. The size of the 5x5 arrayed hydrophone sensor is about 2mm x 2mm.