Shaping Sound Waves in 3D

October 3, 2016

Sound Waves

Researchers from the Max Planck Institute for Intelligent Systems and the University of Stuttgart have found a way of generating acoustic holograms. The holograms can be used to move and manipulate particles, and to improve ultrasound diagnostics and material testing. A Research Group Leader at the Max Planck Institute for Intelligent Systems named Peer Fischer, works on micro- and nanorobots. He is looking for a way to move large numbers of microparticles simultaneously, so that they could assemble them into more complex structures. His research team has now found a method with acoustic holography, and this method promises a number of applications in addition to particle manipulation.

Holography exploits the information where reflected light reaches its maximum intensity. Physicists talk about the phase of the wave. The phase shifts and provides information about the three dimensional structure of the object. This gives the holograms their 3-D appearance. Physicists can now generate sound in 3-D without complex technology, whereas before manipulating the three-dimensional structure of waves was only possible with a phased array transducer. The researchers first showed a hologram that makes sound pressure in the shape of Pablo Picasso's dove of peace. 

The team calculated where and how strong the acoustic waves and their phases were. Then they needed to shift to translate the lines of the dove into an area of increased sound pressure. This way, they were able to attain a map of the phase shifts. Based on the map, they made an acoustic hologram. Using a 3-D printer, they created a relief plate from plastic that transmitted sound faster than the surrounding liquid. The printer applied different thicknesses of material depending on the required phase delay.

Ultra Sounds

As soon as the team of physicists positioned a container filled with water and microparticles in the focal region, the sound pressure of the particles quickly pushed together into the shape of Picasso’s dove of peace. Ultra sound waves were diffused through the hologram. The researchers showed that the technology could also work in 3-D by forming a holographic stack with the images '1', '2' and '3'.

Acoustic holograms create even more possibilities for manipulating particles than the researchers initially had in mind. Also, apart from exposing particles to acoustic waves, holograms could also be used with ultrasound, like in medicine and material testing. 

Rebecca Reilly