Micromachined ultrasonic transducer (MUT) arrays capable of multiple resonant modes and techniques for operating them are described, for example to achieve both high frequency and low frequency operation in a same device. In embodiments, various sizes of piezoelectric membranes are fabricated for tuning resonance frequency across the membranes. The variously sized piezoelectric membranes are gradually transitioned across a length of the substrate to mitigate destructive interference between membranes oscillating in different modes and frequencies.
