These tiny drones are powered by sound
actu.epfl.ch
EPFL engineers have designed acoustic cavities that convert sound waves into thrust, propelling small robots and ultralight aerial vehicles without on-board actuators or electronics.

Researchers have figured out how make shaped drone rotors that convert audible and ultrasound frequencies into stable mechanical spinning motion, leading to tiny drones that can be completely silent. While scary-sounding (no pun intended) it’s unclear how usable these would be in the real world, since a line-of-sight source of sound waves has to be provided.