Acoustic levitation meets schlieren imaging: By reflecting a sound wave back onto itself, one can secure a standing wave if the distance between the source of the sound and the reflector is equal to an integral number of half wavelengths. In this demonstration we use 28 kHz ultrasound whose wavelength in air is 1.2 cm. The objects are small Styrofoam spheres, roughly 4 mm in diameter and 1 mg of mass.

Images employing schlieren optics are very sensitive to changes in the density of air, and these changes refract light into the camera. Note that the little spheres settle down where there are bright bands of light. The bright bands of light in the schlieren images are known to be the result of either increasing or decreasing air pressure with respect to vertical position—in other words, the pressure nodes.

For an excellent writeup by David P. Jackson and Ming-Hua Chang on the mechanics of acoustic levitation, see American Journal of Physics 89, 383 (2021); https://doi.org/10.1119/10.0002764

For more information on our schlieren optics set-up, see
http://sciencedemonstrations.fas.harv...

Although 28 kHz is beyond the range of human hearing, ear protection should be worn whenever attempting this experiment to avoid damaging vibrations in parts of the ear. The sound you hear in this video is not ultrasound but rather a subharmonic and is not dangerous to your ears.