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Acoustic levitation meets Schlieren optics: 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. Styrofoam balls of approximately 2mm in diameter can be levitated in this standing wave at the pressure antinodes. This is because the air is moving rapidly back-and-forth in the low pressur
