Piezoelectric acoustic transducers consisting of a circular aluminum nitride and silicon nitride unimorph diaphragm and an encapsulated air-filled back cavity are reported. Analytical and finite element analysis models are used to design the transducer to achieve low minimum detectable pressure (MDP) within chosen size restrictions. A series of transducers with varying radii are fabricated using microelectromechanical systems (MEMS) techniques. Experimental results are reported for a transducer with a 175 μm radius on a 400 × 500 × 500 μm3 die exhibiting structural resonances at 552 kHz in air and 133 kHz in water. The low-frequency (10 Hz–50 kHz) sensitivity is 1.87 μV/Pa (−114.5 dB re 1 V/Pa) in both air and water. The sensor has an MDP of 43.7 mPa/Hz (67 dB SPL) at 100 Hz and 10.9 mPa/Hz (55 dB SPL) at 1 kHz. This work contributes a set of design rules for MEMS piezoelectric diaphragm transducers that focuses on decreasing the MDP of the sensor through size, material properties, and residual stress considerations.