Structural members can disrupt more than a space’s aesthetics. Acoustics are often a concern, particularly in applications such as auditoriums, where the design must also facilitate communication. Working with metamaterials—substances engineered to have properties not found in nature—researchers at Duke University found a way to keep sound flowing through inanimate objects. In January, the team reported in Nature Materials that it was able to “hide” a small sphere from sound waves by rerouting them over the object by concealing it within a pyramid-shaped stack of perforated plastic plates. The result? The sphere (and plates) behaved like a flat, horizontal surface. ARCHITECT spoke with Steve Cummer, one of the research’s three authors and a professor of electrical and computer engineering at Duke, about the technology and its potential applications.
How did this research come about?
For more than 10 years, my research group has been engineering materials to bend and steer waves in ways that conventional materials can’t. We started doing that work in the realm of light and radio waves; about five years ago we started applying those same concepts to sound waves.
The prototype “cloak” looks like a pyramid—why that form factor?
The pyramid shape was a matter of convenience. It’s assembled of stacked, perforated plastic plates with an empty space underneath. Plastic was convenient; the plates could be made out another rigid solid such as metal or rigid foam. Inside that empty space, an object could be present. When you put the pyramid on top of it, even though you can see that there’s something inside, the object plus the pyramid reflect the sound waves as if they were flat. It doesn't matter what kind of sound distribution you’re hitting it with.
How does the sound reflection work?
Imagine that you make a fine square grid with one hole per grid square. The smaller each hole, the more the sound waves are slowed down. The material needs to slow down the sound waves that are traveling perpendicular to the plates but not those that are travelling parallel to the plates. When the sound waves are travelling perpendicular to the plates, the function of the holes, which are 1.7mm in diameter, is to present just the right fraction of air to solid [surface area] to slow them down. But the plastic plates have an air gap in between them, allowing sound waves travelling parallel to the plates to move at pretty close to their natural speed through air. The cloaking shell’s interior cavity [is lined with] a 1.6mm-thick solid plastic sheet to reflect the sound waves from the object placed inside, making this solution work independent of whatever is under that shell because the sound doesn't even interact with it.
Video: The scenario on the left shows the path of sound waves with no obstructions; the center scenario shows the path of sound waves with the uncloaked sphere; and the scenario on the right shows the path of sound waves with the cloaked sphere.
Is the fabrication process scalable?
Yes. What we made is structurally simple and there are no limits to the size of material samples that could be made, so it should scale well.
What applications are you targeting?
We’re coming at this research not aiming at any particular application but simply showing the kinds of interesting tricks that can be done and looking forward to how to apply this to specific problems. Invariably, that will involve a lot of application-specific design trade-offs. We rely on colleagues in other fields to ping us and say, "Can this be used for X?" A lot of those questions are coming up in noise control, as well as controlling sound waves in sound studios and around microphones. It’s broad. My goal is to get people to start thinking about things that might be doable.
In what ways could this be applied in an architectural application?
Suppose that there was a required structural feature that you wanted to be different from an acoustical standpoint. You could engineer a structure like [the pyramid] to alter its sound reflection properties in a way that could acoustically hide or reshape it. This is a little more general than what we did in implementation, in which we designed something that could take a bumpy surface and make it behave for sound waves like it’s a flat surface. You can also turn the problem around and engineer a structure that makes a flat surface reflect sound like a bumpy surface to get rid of sharp sound reflections and echoes.
Could the cloak work in other mediums, such as water?
Acoustics is sound propagation in fluid, so the big two where we hang out are air/gas and liquid/water. But making this work in water requires a much trickier design process. We’re thinking about how to do this sort of thing in water, but I am so far removed from any of those kinds of applications other than saying that yes, it generally seems applicable. We’re just trying to demonstrate the kinds of things that are feasible.
This interview has been edited and condensed for clarity and updated to reflect the version that appeared in ARCHITECT's May 2014 issue.