PMUT array with resonance frequency tunable due to adjustable embedded cavity

An array of piezoelectric micromachined ultrasound transducers (PMUTs) comprising:

• a substrate having at least one first cavity and at least one second cavity buried therein;
• at least one first piezoelectric stack carried by the substrate and at least partially overlaying the at least one first cavity;
• at least one second piezoelectric stack carried by the substrate and at least partially overlaying the at least one second cavity;
• wherein a thickness of the substrate between the at least one second cavity and the at least one second piezoelectric stack forms a membrane;
• and
• circuitry configured to operate the at least one second piezoelectric stack so as to vibrate the membrane to generate a pulse of ultrasound and to immediately subsequently operate the at least one first piezoelectric stack to cause deformation of the at least one second cavity which results in an increase in a resonant frequency of the membrane.