Wave Action

Words by Chris Anderson

4 min read

As human beings, we often describe ourselves as being moved by sound. In an emotional sense, it might be due to a favourite piece of music, while physically we have probably all felt our bodies vibrating when standing close to a speaker at a concert. 

But could being transported from one place to another, perhaps in a vehicle that rides on sound waves, be something we do in the future? Scientists are currently looking at ways of using sound to create an acoustic tractor beam – holding an object in mid-air and even moving it around as they please. So far, their experiments have worked with small solid objects, liquids and tiny insects. Nothing big, but yielding potential.

Dr Asier Marzo, lead author of a paper published earlier this year by the Department of Mechanical Engineering at the University of Bristol in the UK, one of the institutions developing the technology, explains how it works: “All waves carry momentum, from those in the water to sound waves. We harness this and create a sound pattern, or vortex, that can hold an object in space. The pattern is sound twisting really quickly – 40,000 times per second – around a silent core, like a tornado, which is where the particle sits. You’ve got this central quiet region and a loud, high-intensity region. The object is held in place in the quiet part, and if it moves is pushed back by the loud sound.”

While the description seems fairly straightforward, it is only recently that the true potential of this innovation came to light. Before that, it was widely felt that acoustic tractor beams were limited, as trying to trap particles larger than the wavelength caused them to spin out of control. This was because the sound waves transferred some of their motion to the object. But now, using new techniques, the vortices fluctuate, rapidly changing the twisting direction to stabilise the tractor beam.

For Marzo and his team, the project so far has taken seven years. “Our set-up for the levitators is very simple,” he reveals. “We use lots of tiny speakers, which we call transducers. In our latest experiments, we used 192 transducers of 1cm diameter each. You don’t actually hear anything, as the working frequency is too high, and you only feel a small push when you put your hand inside the levitators.

“Compared to music, our sound fields are simpler, as they are just a pure tone at 40kHz, which results in a wavelength of 8.6mm. But the sound we generate is incredibly loud, measuring up to 165dB – fortunately, we can’t hear it. A bat would be able to hear it, though, and find it pretty uncomfortable.”

The equipment required to create an acoustic tractor beam is actually very minimal, available cheaply and portable. “We’ve even put a DIY kit together, showing people how they can make their own levitators using everyday components [details are on their website www.acousticlevitator.com],” Marzo adds. “The levitators that operate in water would be more expensive, as the wavelength and the tolerances are smaller, but overall it’s not as expensive as some technologies out there.”

As Bruce Drinkwater, the University of Bristol’s professor of ultrasonics, who is supervising the project, explains, the size of the objects the team can levitate is relative to the volume produced. “Louder sounds make the beam stronger,” he confirms. “At the moment, we’re limited by the loudspeakers we use – if they were more powerful, we could lift heavier objects. But so far, we’ve levitated insects and droplets of different liquids, and sometimes even crystals and light metals, such as aluminium.”

There are perhaps similarities with other types of technology – magnetic tractor beams, for example, are widely known. “Acoustic tractor beams are not as powerful as the magnetic versions, but they can manipulate all sorts of materials, both liquids and solids, at scales ranging from micro-metres to a couple of centimetres, and are not limited to only ferromagnetic materials,” says Marzo. “They are more versatile.” 

The team will release another paper shortly on their work lifting objects and being able to control their position. If this is a possibility, how might the technology be applied to the real world? “It could be used in a medical sense,” says Drinkwater. “If you think of targeted drug delivery, for instance, with tractor beams moving drug capsules around the body, delivering the contents to where it is needed. And in an industry setting, you could create a contact-less production line, say, for electronic components.”

For Marzo, medicine is the preferred option for the beams. “As well as delivering capsules, you could move particles out of the body, such as a kidney stone,” he says. “The beams could potentially allow you to perform an operation without making an incision, which is quite remarkable, but you would still have to be careful when applying the power – too little, the object won’t move; too much, you’ll damage the tissue. We would need a clever solution here before applying it.”

And human beings? Could the beams potentially levitate people, or, well, capture spaceships in a way beloved of science fiction? “In theory, it’s possible, but in practice, we don’t have big enough loudspeakers,” says Drinkwater. “[With people] you’ve also got a few safety concerns, such as hearing damage.”

Marza agrees that human levitation is currently not on the agenda. “I don’t think it makes sense to do that, as it’s much better to use it on a small scale,” he adds. “Acoustic manipulation is useful for small particles and samples, especially for operating in water or inside the human body, as sound travels more effectively through a liquid or flesh than it does in the air.”

So doctors may get to use acoustic tractor beams, transport companies not so much. Or at least it seems that way at present. Maybe somebody should tell Donald Trump about them. After announcing his Space Force idea, he is no doubt planning his own Death Star and could use a tractor beam to capture rogue spaceships, with the ‘Star Wars’ theme tune playing in the background. Now that is a moving sound.

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