Research Triangle engineers from Duke and North Carolina State Universities have developed a simple and energy-efficient way to create 3-D acoustic holograms with the potential to revolutionize applications for home stereo systems and medical ultrasound devices.
While visual holograms that manipulate light to make it appear as though a 3-D object is sitting in empty space aren’t new, the researchers took the concept and applied acoustic technology to it.
Sound travels in waves that spread out as pressure waves that momentarily compress the molecules they are traveling through. Just like visible light, these waves can be manipulated into three-dimensional patterns.
"We show the exact same control over a sound wave as people have previously achieved with light waves," said 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."
In their work, the researchers demonstrated that they could create any three-dimensional pattern they wanted using sound waves. They were able to achieve this by using metamaterials, synthetic materials composed of many individual, engineered cells that together produce unnatural properties.
In this case, the team used metamaterials to resemble a wall of Legos. Each of the Lego-like blocks was made of plastic and constructed using a 3-D printer. Each block contains a spiral and the tightness of the spiral affects the way sound travels through it. So, the tighter the coil, the slower the sound waves travel through it.
The individual blocks don’t have the power to influence the sound wave's direction, but the entire device can. For example, if one side of the sound wave is slowed down but not the other, the resulting wave fronts will be redirected so that the sound is bent toward the slow side, according to the researchers.
"Anybody can tell the difference between a single stereo speaker and a live string quartet playing music behind them," said Yangbo "Abel" Xie, a doctoral student in Cummer's laboratory. "Part of the reason why being that the sound waves carry spatial information as well as notes and volume."
The researchers calculated how 12 different types of acoustic metamaterial building blocks would affect the sound wave, and then were able to arrange them in a wall to form any wave pattern on the other side that they wanted. That allows the sound waves to produce a specific hologram at a specific distance away.
"It's basically like putting a mask in front of a speaker," said Cummer. "It makes it seem like the sound is coming from a more complicated source than it is."
Cummer and Xie, along with Yun Jing, assistant professor of mechanical and aerospace engineering at NC State, and Tarry Shen, a doctoral student in Jing's lab, proved that their sound mask works in two different ways.
First, they assembled a metamaterial wall that manipulated an incoming sound wave into a shape like the letter "A" about a foot away. Then they showed that the technique could focus sound waves into several "hot spots" of sound.
Other technologies that can produce the same affect includeultrasound imaging devices, which use phased arrays with many individual transducers that can each produce precisely, controlled sound waves.
The researchers feel this current technology comes with negative consequences.
"If you've ever had an ultrasound done, you know there's a small wand attached to a much bigger machine a few feet away," said Cummer. "Not only can this setup be cumbersome, it consumes an enormous amount of power. Our approach can help produce the same effect in a cheaper, smaller system."
In order to get the metamaterial device to work in these applications, each cell must be smaller than the waves it is manipulating. For ultrasound technologies that operate in the megahertz range, this suggests that the individual cells would have to be 100 times smaller than in the current demonstration blocks.
The team is currently looking for industry partners to show that this would be possible, as well as contacting industries that work in the kilohertz range like aerial sensing and imaging technologies. They’re even speaking with sound companies in order to develop a single speaker that sounds more like a live orchestra.
"We're currently in the exploration phase, trying to determine where this technology would be useful," said 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."