Researchers find new ways to defy gravity with sound

Nearing the midpoint of the second decade of the 21st century, we finally have video phones. But where are the floating skateboards and flying cars? Perhaps what researchers are describing as “acoustic levitation ballet” points to some eventual possibilities.

Recent experiments – conducted by researchers from the University of Tokyo, Nogoya Institute of Technology, and Sony Computer Science Laboratories – take advantage of the known capability of an ultrasound standing wave to suspend small objects at its sound pressure nodes.

But there’s a long way to go before phased sound transducers could be used to avoid heavy lifting, as this “ballet” is being performed by small grain particles, a tiny electronic resistor, an iron screw, and drops of water. Fortunately for the researchers’ sanity, there is no sound emitted from the ultrasonic speakers that humans can hear, allowing any ballet music to be added as desired.

Sound wave levitation, like magnetic levitation, requires an object to remain inside a carefully constructed environment. Building on previous decades of experiments, in which the objects were moved in one dimension by controlling the frequencies or the phases of a wave, the researchers have expanded intothree-dimensional movement.

Two to four speakers face each other, creating a focal point of sound that can be moved by varying the output of various speakers. In a paper on its work, the research team noted that the two original features of its manipulation are the utilization of “the direction of the ultrasound beam” and the three-dimensional movement of a localized standing wave.

Aside from the flea-circus aesthetic possibilities, some version of this setup could be used in a microgravity environment, such as on a space station, enabling astronauts to handle objects without actually touching them. At the other extreme, one also imagines that enormously powerful speaker arrays could move very consequential objects, although being positioned inside the sound field but outside the focal point might not be such a pleasant experience.