An array of piezoelectric micromachined ultrasound transducers (PMUTs) includes a substrate having first and second cavities buried therein. A first piezoelectric stack is carried by the substrate and at least partially overlays the first cavity. A second piezoelectric stack is carried by the substrate and at least partially overlays the second cavity. A thickness of the substrate between the second cavity and the second piezoelectric stack forms a membrane. Circuitry operates the second piezoelectric stack so as to vibrate the membrane to generate a pulse of ultrasound and to immediately subsequently operate the first piezoelectric stack to cause deformation of the second cavity which results in an increase in a resonant frequency of the membrane.