3D-Printed Acoustic Holograms Could Move Objects in Midair

3D-printed plastic blocks can now be turned into acoustic holograms that generate 3D shapes made of sound, which could function like sonic "tractor beams," according to a new study. This could lead to innovative ways to manipulate objects in midair without touching them, the researchers said.
This finding could also help scientists develop ultrasound therapies with sound fields sculpted to destroy unhealthy tissues in the body while leaving neighboring healthy cells intact, the researchers added.
Conventional holograms are a special kind of 2D photograph that, when lit up, essentially turn into windows onto 3D scenes. The pixels making up each hologram scatter light falling onto them in very specific ways, causing these light waves to interact with each other to generate an image with the illusion of depth.

The new acoustic holograms are plastic blocks with complex structures that scientists created using 3D printers . These printers form 3D structures by placing layers of material onto surfaces, much like how regular printers deposit layers of ink. When an acoustic hologram the researchers developed is placed in front of an audio speaker or a transducer, the 15,000 pixels within it can scatter sound waves to generate complex 3D fields of sound.

Sound waves apply pressure on matter, and previous research found that " acoustic tweezers " and "acoustic tractor beams" could generate complex 3D sound fields in air or liquids to push, pull and spin objects such as small animals. However, these devices usually require elaborate arrays of multiple transducers, whereas this new acoustic hologram requires only one ultrasonic transducer to generate a complex 3D acoustic field.

"Instead of using a rather complex and cumbersome set of transducers, they  use a piece of plastic that cost a few dollars from a 3D printer,".

 Acoustic Holograms
Holographic techniques are fundamental to applications such as volumetric displays  , high-density data storage and optical tweezers that require spatial control of intricate optical  or acoustic fields within a three-dimensional volume. The basis of holography is spatial storage of the phase and/or amplitude profile of the desired wavefront in a manner that allows that wavefront to be reconstructed by interference when the hologram is illuminated with a suitable coherent source.