At Duke University, North Carolina, engineers have created the world’s first 3D acoustic cloaking device. Using perforated sheets of plastic metamaterials, the device bends sound waves in a way that creates the illusion that the device and anything beneath it are not there at all.
In 2006 professor of electrical and computer engineer Steven Cummer and his team started to look at the underlying theory and maths that showed how you could design this kind of structure on paper. They began investigating what materials would bend and manipulate electromagnetic waves like light and radio waves in a way that would hide an object.
“People including myself started to think about whether you can apply the same mathematics to sound waves,” Cummer said. “Which are different to light in some ways but the same in other ways, since then the entire field has been taking steps towards building more and more complicated devices.
Professor Cummer described the tricks behind acoustic cloaking, first you need to make sure the object reflects no sound. It also needs to bend the waves around itself and the concealed object and the fill the ‘shadow region’ behind and come away as if no object were there at all. The big step that made that possible was the underlying mathematical theory that showed you what kinds of materials and material properties you needed to make a shell that would bend waves in exactly the right way.
“What we’re doing is manipulating sound waves and the theory for that says that you need to create a material that has very specific properties,” explained Cummer. “One of the things that it has to do is have a different speed of sound depending on the direction through the material that the wave is travelling, and that is not a material property that is common in natural or normal materials.”
In air for instance it doesn’t matter what direction a sound wave is travelling, it will always be the speed of sound. But in the materials used for the cloaking device, you have to be able to control the sound speed separately in two perpendicular directions. This opens the up the world of metamaterials, engineered materials that are designed to possess qualities necessary for the task in mind.
“You just need some solid, rigid structures that are carefully designed and so the basic building block of what we built is a cube,” Cummer said. “A slice through the middle of that cube is a thin plastic sheet with one hole in it and that’s like Lego building block. So what’s actually made is an assembly of many of those blocks stacked together in a pyramid configuration.”
There is some added challenge of making this type of technology work in water however, to block things like sonar. Cummer states that when designing these materials, a really important thing to consider is the contrast between the background material, which in this case is air, and the structure that you use to assemble the object, which is plastic. The contrast between waves in air and in a solid is enormous, it’s very easy to compress air but its very hard to compress plastic or metal. There’s much less contrast between water and solid material and that means that the wave energy would get into the plastic way much more than it does in air.
“This complicates the design enormously,” Cummer said. “In air we can treat the plastic like it’s infinitely rigid and any solid material and theres no wave energy that gets into it but in water it’s a totally different story. There’s no research yet, but it’s something we’re starting to definitely think about.”
There’s a great deal of acoustic design that goes on in controlling how spaces sound, Cummer is interesting in seeing whether this type of device could have applications for architectural spaces.
“We’ve all been in terrible spaces trying to listen to talks where there’s just echoes going on all around and you can’t understand anything,” he said. “Thinking about what we’ve done a little bit more generally, what we actually did was the equivalent of making a bumpy surface reflect sound as if it were a flat surface but you can also twist it another way and engineer something that was a flat surface made out of this kind of material but actually reflected sound as if it were a bumpy surface.
“You can create surfaces that look visually one way but actually interact with sound as if they are completely different structurally,” he added. “So that’s an interesting thing in toolboxes for people who are doing acoustic design.”
Cummer has spoken to designers who do a lot work on sound diffusers that are used in architectural spaces and studios who want to design an object that when sound hits it, it reflects sound in all directions so you don’t get reverb but you get this evenly diffuse, distributed echo.
The current model for this technology stands at about 30cm wide but scaling this up would not be an issue, although more testing would need to be done to make sure no mechanical vibration takes place. A further update on the research and advancements in this field are expected within the next few years.