Sound can now be structured in three dimensions thanks to the efforts of researchers from the Max Planck Institute for Intelligent Systems and the University of Stuttgart. They have found a way of generating acoustic holograms, which could improve ultrasound diagnostics and material testing. The holograms can also be used to move and manipulate particles. Manipulating the three-dimensional structure of acoustic waves was previously only possible with what physicists call a phased array transducer. This is an ensemble of many acoustic sources positioned side by side that can individually emit sound with varying phase delays. The necessary driving electronics, however, is bulky and expensive. But, thanks to this recent advancement it is now possible to generate sound in a 3D without this complex technology. The researchers first demonstrated a hologram that generates sound pressure in the shape of Pablo Picasso's dove of peace. Microparticles suspended in a liquid follow the pattern and form the image. To achieve this, the team first computed where and how strongly the acoustic waves, or more specifically their phases, needed to shift to translate the lines of the dove into an area of increased sound pressure. In this way, they attained a map of the phase shifts. Based on this map, they then fabricated the acoustic hologram: using a 3D printer they created a relief from a plastic that transmits sound faster than the surrounding liquid. The printer applied different thicknesses of material depending on the required phase delay. Ultrasound waves transmitted through the hologram interfered behind the relief plate in such a way that the sound pressure reproduced Picasso's dove of peace. And as soon as they positioned a container filled with water and microparticles in the focal region, the particles were quickly pushed into the shape of the dove. The researchers showed that the technology can also work in 3D by forming a holographic stack with the images '1', '2' and '3'. https://www.mpg.de/10741300/hologram-acoustic-sound
