Acoustic levitation of a large solid sphere

We demonstrate that acoustic levitation can levitate spherical objects much largerthan the acoustic wavelength in air. The acoustic levitation of an expanded polystyrene sphereof 50 mm in diameter, corresponding to 3.6 times the wavelength, is achieved by using three25 kHz ultrasonic transducers arranged in a tripod fashion. In this configuration, a standingwave is created between the transducers and the sphere. The axial acoustic radiation forcegenerated by each transducer on the sphere was modeled numerically as a function of thedistance between the sphere and the transducer. The theoretical acoustic radiation force wasverified experimentally in a setup consisting of an electronic scale and an ultrasonictransducer mounted on a motorized linear stage. The comparison between the numerical andexperimental acoustic radiation forces presents a good agreement.