Acoustic Levitation
Sound can make things float!
If you shake a rope up and down, you may notice there are certain frequencies where the "wave" stops progressing down the rope (Figure a). This is called a standing wave.
Sound can also form standing waves, for example when two speakers (or many more) spaced a certain distance apart emit sound waves toward each other at the same loudness and frequency (Figure b).
For loud enough sounds (louder than a jet engine!), these standing waves can suspend small objects by exerting acoustic radiation forces that counteract the objects' weights. The sound isn't audible because the speakers operate at a frequency far above what we can hear.
With enough speakers, sound can even move objects around in full 3D! This technology is sometimes called acoustic tweezers, and is currently used in precision manufacturing, simulating the effects of low-gravity environments, and creating interactive volumetric displays.
This demonstration is currently displayed in the Hallway on the 2nd Floor in Manning Hall.