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Engineering news

Acoustic metamaterial set to track down cracks

Katia Moskvitch

When assembled, these 'bricks' create a new acoustic metamaterial
When assembled, these 'bricks' create a new acoustic metamaterial

Small, cheap 3D-printed ‘bricks’ can be used to create an acoustic metamaterial that bends, shapes and focuses sound waves.

Scientists who created the device discovered that it's possible to manipulate sound just like, say, a projector manipulates light. The concept could lead to tools that will improve spotting cracks in safety-critical engineering components by steering very precisely a beam of ultrasound.

It could also help destroy tumours without damaging adjacent tissue. “The work brings us huge possibilities towards reconfigurable ultrasound devices,” says Jensen Li, a physicist at the University of Birmingham, who was not involved in the research. 

In the audible frequency range, the development could be used for audio spotlights, for instance for advertising. 

“What we have created is – metaphorically – comparable to the box of optometrist test lenses, but for sound,” says Sriram Subramanian, a computer scientist at the University of Sussex.

Controlling the shape of the sound field we generate is not new - sound fields are already used in medical imaging and therapy as well as in consumer products, for instance, as audio spotlights and for ultrasonic haptics. It’s the approach that is different.

“Traditionally, focussing sound has required either acoustic lenses, which are inflexible and need to be recreated for each different sound pattern, or has required an array of many individually controlled sound sources providing a very flexible, but typically very expensive, solution,” says Martyn Hill, an electromechanical engineer at the University of Southampton, who was not involved in the study. 

Different bricks (coloured dots) can be combined to produce any sound field
Instead, the researchers applied a technique used in image processing and known as the wavelet decomposition. They 3D printed special fingernail-sized ‘bricks’ that, once assembled together into a sheet, reproduce any sound field possible – using a single sound source. The sheet can then be put onto the front of a loudspeaker to precisely direct the outgoing sound waves and create audio 'hotspots' that only the user would be able to hear.

The pieces, or bricks, shape the field and act as acoustic metamaterials. The internal sound path through different bricks delays the sound by a different, predetermined amount. Each brick has a ‘labyrinth’ structure, and a sound wave takes a different amount of time to travel through it, introducing different delays.

“This is how we digitise the sound,” says Bruce Drinkwater, ultrasonics expert at the University of Bristol. “Our metamaterial layer can efficiently transform the output from a loudspeaker into any shape of sound beam.” 
The layer is placed over the sound (or ultrasound) source, where it transforms the incoming waves into the desired beam. 

“By using different combinations of our materials in a 2D surface or by stacking them we can focus the sound in different depths or steer the beam as desired by a user,” says Subramanian.

For crack detection, this 2D surface could be placed as a sleeve in front of the speaker to direct sound into different regions of the surface, he adds.  

The next step is to get the bricks to change the properties of the surface dynamically, instead of physically rearranging the bricks. “At the moment, our metalayers are static, so one layer can produce a single transformation,” says Subramanian. “We are now working on dynamic bricks that will allow the transformation to be actively controlled. These dynamic bricks will open up further applications such as medical and engineering imaging where they would dynamically steer the beam around the interior of the test object - be it human or engineering structure.”

The research appears in Nature Communications.

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