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Acoustic Propulsion

Two spherical Helmholtz resonators (glass Christmas tree bulb ornaments with a resonance frequency of 386 Hz) connected by a thin rod and suspended from a thread will rotate when driven at their resonance frequency, provided that the driving sound is loud enough (in this case more than 125 dB). The cause of rotation is due to a nonzero time average of the momentum flow into and out of the opening of the resonator. Asymmetry in the opening of the resonator causes flow separation and vortex rings that produce a reaction force (and hence a torque) on the system, causing it to rotate.

I saw this demonstration mentioned in an acoustics textbook (Notes on Acoustics by Uno Ingard) and had to try it for myself. Acoustics is so cool!

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