The experiment looks like science fiction — or maybe even magic. In response to a single tone from a lone speaker, tiny silicone particles floating on the surface of a glass of water arrange themselves into the shape of a dove. The image holds for as long as the tone plays, then collapses.
But this is science, not fiction. The experiment is one of several performed by physicists at the Max Planck Institute in Stuttgart, Germany, that demonstrate that they can use a 3-D-printed plate to manipulate sound waves into a complex "field" capable of creating images in water or levitating objects in midair.
"It's just like" the holograms you've seen in "Star Trek," said physicist Peer Fischer, co-author of a new study in the journal Nature. "Only we don't generate an image using light — we do it with sound."
According to Fischer, this is the first time scientists have produced this phenomenon using a single sound source.
Last year, scientists at the University of Bristol in Britain published a study in the journal Nature Communications about an acoustic system that could levitate objects. Their system works in much the same way as Fischer's: Both produce sound waves that approach the object from multiple directions and at various amplitudes and frequencies. When the sound waves interact with one another, they create a pressure gradient in air (or any other fluid) that can hold a bead aloft or cause objects to move around on the surface of water.
But the Bristol team's experiment required a whole bunch of transducers (speakerlike devices that convert electrical signals into sound), each carefully tuned to produce exactly the right kind of wave.
"If you want to generate a complicated sound field, you need a large number of sound sources," Fischer said. "And you can imagine if you get to a few hundred, that gets very difficult and costly and complicated."
So Fischer and his colleagues came up with an alternative method. Instead of arranging dozens of transducers, they use just one — but they pass the sound through a plate that distorts it into the right type of sound field. The field is invisible to the naked eye, but "if you were to take a microphone as we have and move through the space, you would hear ... a 3-D picture," Fischer said.
By arranging silicone spheres on the surface of water, scientists can make that acoustic "picture" visible. They can also alter the sound field to manipulate objects — say, levitate a droplet of water, or cause a tiny boat to zoom around.
In an analysis for Nature, Adrian Neild, an engineering professor at Monash University in Australia, called the technique "impressively simple." The Max Planck team's plates (which are technically called holograms, though the term can be applied to the resulting image as well) are 3-D printed to have just the right thickness and shape to generate the desired sound field. The resolution of 3-D printers is 100 times higher than that of simple transducers, Neild said.
The technique is way more than a neat party trick. It could improve acoustic imaging — say, sending a sound beam through steel to tell if it has a crack, or conducting an ultrasound to detect a medical condition. It could be used in research to move specimens around a petri dish without touching them, or in engineering to manipulate delicate machinery parts.