Metamaterial "Lego" creates simple, energy efficient 3D acoustic holograms from any speaker

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This simple way of creating incredibly rich, intricate soundscapes is made from an array of 3D printed metamaterial blocks that sits in front of a speaker.  (Credit: Duke)

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Engineers from Duke University have developed a way to create acoustic "holograms" that promise to be as magical as visual holograms – all by placing an array of 3D-printed acoustic building blocks in front of a sound wave.

When we talking about "three dimensional sound waves" we're acknowledging that sound waves don't just carry sound and volume – they also convey spatial information that is missing in even the best surround sound systems. It's comparable to the difference between two and three-dimensional images: 3D images communicate depth as well as length and width, which creates a much more true-to-life viewing experience.

Just as visual holograms work by shaping an electromagnetic field to mimic light bouncing off an object, the pressure waves that make up sound can be manipulated to create a three-dimensional pattern. The research team has even been able to manipulate an incoming sound wave to resemble a letter "A" about a foot away, and have proven the technique can focus several "hot" or loud spots of sound, a foot away from the device.

A computer rendering of a sound wave that was shaped into a pattern like the letter A, one foot past the acoustic metamaterial array.(Credit: Duke)

"We show the exact same control over a sound wave as people have previously achieved with light waves," says Steve Cummer, professor of electrical and computer engineering at Duke University. "It's like an acoustic virtual reality display. It gives you a more realistic sense of the spatial pattern of the sound field."

It's all made possible by using metamaterials – synthetic materials made up of many individual, specially engineered cells that work together to produce unusual effects.

In this case, the metamaterials look like a wall of lego blocks. Each individual block is made of 3D printed plastic, and contains one of 12 different spiral shapes within it. The tightness of the spiral affects the way sound travels through it – the tighter the spiral, the more it slows down the sound waves.

One individual block can't influence a sound wave's direction; it can only influence the speed of sound wave. But by working as an array of differing blocks, the entire device can effectively alter the direction of an incoming wave. For example, if one side of the sound wave slows down but not the other, the resulting wavefront will be bent towards the slow side.

A computer rendering if the 12 different kinds of spirals contained in the metamaterial blocks that together create 3D acoustic holograms. Each of the spirals slows sound waves by a specific amount, so organizing them in a group can bend the shape of in incoming wave of sound.(Credit: Duke)

By knowing how 12 different types of acoustic building blocks will affect the sound wave, the engineers can arrange them in front of a sound wave to create any pattern on the other side they want. In other words, the sound waves can produce a specific "acoustic hologram" at a specific distance away.

"It's basically like putting a mask in front of a speaker," says Cummer. "It makes it seem like the sound is coming from a more complicated source than it is."

There are already existing technologies such as modern ultrasound imaging devices that can produce this effect. But Cummer says these approaches tend to be cumbersome and consume an enormous amount of power. "Our approach can help produce the same effect in a cheaper, smaller system," he says.

The simple, energy-efficient technique could mean that your home stereo suddenly conveys an incredibly intricate and realistic soundscape. To this end, the researchers are now talking to the sound industry about applying the technology.

The researchers say the technique could also be applied to aerial sensing, medical imaging and other ultrasound technologies.

For the metamaterial device to work, each cell must be smaller than the wave it is manipulating. For ultrasound technologies, which operate in the megahertz range, this means the individual cells would have to be 100 times smaller than in the current demonstration blocks. So the team is looking for industry partners to help them find ways to apply the idea to an ultrasound environment.

"We're currently in the exploration phase, trying to determine where this technology would be useful," says research team member Yangbo "Abel" Xie. "Any scenario where your goal is to control sound, this idea could be deployed. And it could be deployed to make something totally new, or to make something that already exists better, simpler or cheaper."

You can read a science paper about the acoustic hologram mask at Nature.com

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