Bristol team makes waves with acoustic tractor beam

In what is claimed to be a world first, engineers at Bristol University have developed an acoustic tractor beam that can stably trap objects larger than the beam’s soundwaves.

The device works by creating acoustic vortices, tiny tornado-like structures with loud sound surrounding a silent core. It had previously been thought these vortices were limited to supporting objects no bigger than their wavelengths, with larger objects spun out of the field by the rotational motion imparted to them. However, by constantly altering the direction the vortices spin, the rotation of the objects in the beam can be controlled, allowing larger objects to be levitated with stability.

Published in Physical Review Letters, the study details how the team used ultrasonic waves at a pitch of 40kHz, a frequency similar to that which only bats can hear. Rapidly changing the twisting direction of the vortices allowed the researchers to expand the size of the silent core, which supported larger objects.