Have you ever wanted to hide an object from sound? Who hasn’t! Well, up until now, you’ve likely found yourself frustrated in this endeavour but researchers over at Duke University, North Carolina, USA, have just 3D printed the very thing for you. Lead by Steven Cummer, Professor of Electrical and Computer Engineering, they have just fired off the first ever three dimensional acoustic cloak.
Made up of metamaterials, which are smaller than the wavelength of light, the cloak is placed over an item. When sound waves reach it, they are given the impression that there is nothing there. Essentially, both the cloak itself and any object it covers become invisible to sound.
The cloak itself is made up of a series of plastic sheets, each with matching hole patterns, which have been placed on top of each other in a pyramid. This alters the trajectory of the sound waves, while simultaneously slowing them down to compensate for the sound not reacting with the surface.
To test the efficacy of the cloak, the researchers placed it over a solid sphere and then shot bursts of sound at it from a number of different angles. Using a microphone, they mapped out how the audio moved though the area before transferring these maps into videos. Comparing this with a video of sound waves interacting with an exposed solid sphere, they could see a clear difference in their motion, with sound waves passing through the cloaked sphere as if it were an empty surface.
So what, exactly, do you use an acoustic cloak for? Cummer envisions no end of usages:
We conducted our tests in the air but sound waves behave similarly underwater, so one obvious potential is sonar avoidance. There’s also the design or auditoriums and concert halls – any space where you need to control the acoustics. If you had to put a beam somewhere for structural reasons that was going to mess up the sound, perhaps you could fix the acoustics by cloaking it.
Mind, or should that be ear, bending stuff.