Acoustic Helmholtz micro-resonators convert external sound waves into directional thrust, free of batteries, motors, or control electronics

https://www.eurekalert.org/news-releases/1139938

"hollow, bell-shaped cavities oscillate when excited by sound, forcing out concentrated air propelling vehicles... individual cavities tuned to specific ultrasonic frequencies controlling direction/ navigating cm-scale boats through complex water courses using external speakers... 3D nanoprinted 150-μg microfliers that can launch vertically/ spin blades up to 13K RPM for stable hovering... shape-shifting: aerial/ aquatic micro-robots, ultralight microfliers"