声波全息图s are making waves!

声波全息图s that shape sound waves into complex patterns to manipulate tiny objects, have been developed by researchers in Germany.

"We make sound holograms using 3D printing," explains Andrew Mark, one of the inventors at the Max Planck Institute for Intelligent Systems in Stuttgart.

"We produce a plastic plate, and the plate has a particular ... texture on the surface... made up of individual pixels."

Each of the pixels has a different height, so that sound waves passing through the plate from an underwater speaker are forced to travel different distances through the plastic.

As a result, waves exiting from the different pixels are slightly offset from one another, because sound travels at a different speed in the plastic than in the surrounding water.

When the offset waves from each pixel meet again above the plate, the waves' peaks and troughs no longer match up. This means that instead of a uniform sound field, they create regions of high and low intensity - a sonic landscape with hills and valleys of sound.

"The result is that the hologram acts like a very complex lens, which instead of focusing the sound into an individual point, focuses the sound into many points." Mark told the Naked Scientists. By calculating just the right heights for each pixel of the hologram, the Stuttgart team are able to create virtually any pattern of hills and valleys, and use these sound landscapes to move tiny plastic beads in water.

"Where there's high sound intensity we get crests forming on the surface of the water, and if you put particles on those crests, then the particles are trapped there," Mark says.

The team used this effect to force plastic beads to assemble into the shape of a dove.

As well as trapping, the team can engineer the sound fields to make objects surf along the water crests.

Using acoustic holograms to control complex sound fields could lead to new medical ultrasound therapies. Focussed ultrasound can be used to treat tumours, and Mark suggests that the 3D printable holograms could help provide patient-tailored treatments.

"We can imagine a scenario where a doctor takes an image of the patient's body, ...figures out what the best...distribution of the sound field inside the patient's body is, crafts a hologram by 3d printing - that is specific to that patient - and then uses that hologram to heat or ablate in way that is particular to that patient's needs."