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Ultrasound can levitate large objects
Scientists have long dreamed of using acoustic levitation to float objects, but there has been one big catch: you couldn't lift an object larger than the wavelength without being picky about what you're lifting. However, it might not be a problem going forward. Researchers in Brazil and the UK have successfully levitated a polystyrene ball 3.6 times larger than the ultrasonic waves holding it up. The trick was to create a standing wave in the gap between the transducers and the object, instead of the usual pressure node between the transducer and a reflector. You can change the angle and number of transducers without messing with the effect, and it finally creates both horizontal and vertical lift -- you don't need physical support to prevent the object from drifting sideways.
