Ultrasonic standing waves are set up between a horn and a brass plate. Because the wavelength is short, several wavelengths fit in the space between the horn and the plate. There are locations in the standing wave pattern where the forces associated with the sound pressure and the motion of the air molecules are opposite in direction to the gravitational force. A light ball placed at any of these locations will levitate if the mass of the ball is such that the gravitational force on the ball is equal and opposite to the forces associated with the pressure and the motion of the air molecules.