Skip to main content
Acoustics

Acoustics

Sonic tractor beam can be made for less than $90

13 Jan 2017

A single-sided sonic tractor beam that can levitate objects without the need for complex phase-shifting electronics has been developed by researchers in the UK. Instead the device uses an acoustic lens, which can be made with a set of 3D-printed cells or a shaped surface to create the required acoustic fields. The team says that the device can be made for less than $90 with readily available components and a 3D printer, and has released instructions and a YouTube video explaining how.

In 2015, Bruce Drinkwater of the University of Bristol and colleagues developed a single-sided tractor beam that used sound waves to levitate, rotate and move objects. The device used an array of 64 off-the-shelf loudspeakers controlled by complex phase-shifting electronics. The array created 3D fields of sound – acoustic holograms – that could surround and trap objects. By controlling the output of the speakers, the acoustic holograms could be adjusted to hold, rotate and move objects – small polystyrene particles – in mid-air.

But using phased arrays to control multiple independent electrical signals is not simple, and such tractor beams are complex and expensive devices. To try to simplify the tractor beam, Drinkwater and colleagues explored whether they could create physical structures that could produce the required acoustic holograms, instead of electronics.

Passive adjustment

“To get a desired acoustic field, the key ingredient is to be able to adjust the phase distribution of the emitting surface,” explains Drinkwater. “We developed various way of passively adjusting the phase of the sound waves. We do this in three ways: a shaped emitting surface, a series of straight tubes of varying lengths and coiled tubes.”

The first of these methods uses 30 loudspeakers placed inside a bowl shape to hold them in the correct positions to direct the sound towards a focal point. The second and third designs involve 64 loudspeakers placed in unit cells – either straight or coiled tubes, of different lengths or revolutions – that act as delay lines, adjusting and focusing the sound waves.

Levitating fruit flies

When the loudspeakers are powered with a single electrical signal, each of these three passive devices generates a tractor beam that can levitate small polystyrene particles. The researchers also managed to levitate a fruit fly with the sculptured-surface device.

Drinkwater told Physics World that each device performs differently. “The sculpted-surface design is the most efficient as the sources are all directed towards the focal point,” he says. “The coil device is the least efficient, but it is the most compact as the delay tubes are coiled up into a layer that could be sub-wavelength.”

We can move the objects up and down
Bruce Drinkwater, University of Bristol

As well as creating static traps with a single electrical signal, the researchers showed that it is possible to add a second signal to allow some limited adjustment of the acoustic field. “With this system we can move the objects up and down,” explains Drinkwater.

The researchers have released instructions and all the files necessary to build all three devices with four input signals – to allow a greater range of movement of the levitated object – and a YouTube video demonstrating the process. They claim that a tractor beam can be built for less than $90 with readily available and 3D-printed components. For those who do not have access to a 3D printer, there are various online companies that will print the components.

Schools and hobbyists

Drinkwater says that simplicity is the main advantage of these tractor beams over previous incarnations. “This stems from less complex electronics,” he says. “With simplicity comes a reduction in cost of manufacture. So this opens up the possibility of making an acoustic tractor beam accessible to many more people. As well as industrial and medical applications, we are really excited about the possibility of DIY enthusiasts making these devices, or even schools.”

Steve Cummer, an electrical and computer engineer at Duke University in North Carolina, US, who has previously used 3D-printed blocks to create 3D acoustic holograms says that using physical structures is a good alternative to a phased array. “The tractor beam simply requires a sufficiently high sound amplitude to create the steady pressure required for levitation, and control over the phase distribution of the sound to create the precise sound amplitude distribution needed,” he told Physics World. “In this case, each of the blocks acts as the phase control for each emitter.”

Cummer agreed that building the device should be relatively straightforward. “The electronics are a lot simpler than the multi-transducer version they created before, in which each one required careful phase control. As long as the recipe for the blocks is followed, it should work.”

  • In 2015, Physics World visited Bruce Drinkwater’s lab and recorded audio and video pieces with researchers there. You can access this content at “ The wonderful world of ultrasound

Related events

Copyright © 2024 by IOP Publishing Ltd and individual contributors