Acoustic Standing Waves and the Levitation of Small Objects

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);