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Researchers discover how to shape sound in 3D

Sound waves are actually waves of physical pressure. We’ve used them in haptics technology to create invisible, touchable shapes in midair. Manipulating waves in 3D requires a phased array of transducers, but phased arrays are limited in spatial resolution by the physical size of each transducer. They’re also expensive. Now a team of researchers from the Max Planck Institute in Stuttgart, Germany have figured out an inexpensive way to shape sound waves in 3-space, using 3D printing.

The team first made a map of the necessary phase shifts: the places and strengths at which they wanted the acoustic waves to interact. Those interactions create localized zones of constructive interference, and therefore higher acoustic pressure.


To create complex forms, the team then turned to 3D printing. Using a plastic that propagates sound faster than water does, they made templates that work in a way not unlike the gels used in studio lighting. The templates admit sound through them at different speeds, depending on the thickness of the material, and the printer applied different thicknesses of material depending on the required phase delay. This created a plate printed with a design in relief.

Applying sound to the plate at precisely tuned frequencies created diffraction-limited acoustic pressure fields, aligned to create designs in 3D space. The researchers used them to make pictures and also to affect the movement of particles.

The acoustic holograms these researchers can create don’t themselves move around through the air — so far, they’re still limited to static phenomena by nature of the plates they use. But that doesn’t stop the scientists from using their method to move other things. They can move bits of resin around through defined trajectories. Using their plates to create a standing wave, they let loose a bit of resin onto the surface of the wave, whereupon the particle quickly surfed to an opportune place on the wave and settled into a stable orbit. Water mist, atomized in front of the transducer, coalesced into droplets trapped at the programmed points in the acoustic field.

Watch them make an image of Picasso’s peace dove in a dish of water, pilot around bits of resin, and even leave drops of water levitating in midair, all in real time:

Lead author Peer Fischer, a Research Group Leader at the Max Planck Institute for Intelligent Systems, normally works on micro- and nanobots. Holographs were not among his core interests. “However, we were looking for a way to move large numbers of microparticles simultaneously so that we could assemble them into larger more complex structures,” explains Fischer. While he and colleagues were trying to figure out how to pilot nanobots, they discovered this new way of approaching the problem.

Finer control over the shape of acoustic pressure fields already has applications in medicine, and this development stands to improve the state of the art. Beyond the use of ultrasound as a diagnostic and monitoring tool, kidney and gallbladder stones can sometimes be removed using an ultrasound procedure called lithotripsy. “There’s a great deal of interest in using our invention to easily generate ultrasound fields with complex shapes for localized medical diagnostics and treatments,” says Fischer.

I’m also hereby retconning this into being the scientifically rigorous, peer-reviewed proof of concept behind the sonic net they used in that one episode of Farscape, “Taking the Stone.” This is just part of my ongoing quest to suss out the actual science hiding in sci-fi.

taking the stone

Now read more about cool stuff we can do with sound: This ‘acoustic prism’ can split sound the way a regular prism splits light

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