Though less publicized than its notorious air and water counterparts, noise pollution is a growing problem.
And while modern sound-mitigating barriers may help drown out the whir of rush-hour traffic or contain the symphony of music within a concert hall, they leave little room for airflow.
So, a pair of Boston University mechanical engineers created an acoustic metamaterial designed to silence sounds at their source, without blocking the movement of air.
By scrapping the traditional idea of a sound barrier, they instead created a new material—one with unusual and unnatural properties; in this case, the ability to exert an isolated influence on sounds.
The final product: an acoustic metamaterial.
“I’ve been working on metamaterials for more than a decade,” Xin Zhang, a professor at the College of Engineering and the Photonics Center, said in a statement. “But it was [study co-author] Reza [Ghaffarivardavagh] that gradually got me more excited about the fundamental idea of marriage between acoustics and metamaterials.”
Even the most miniscule disturbance in the air formes vibrations, which, in turn, create sound waves. It’s the same principle that explains why, in the vacuum of space, no one can hear you scream.
“Our goal is to silence those tiny vibrations,” graduate student Ghaffarivardavagh explained. “If we want the inside of a structure to be open air, then we have to keep in mind that this will be the pathway through which sound travels.”
The basic premise requires metamaterial to be shaped in such a way that it sends incoming sounds back from whence they came.
In a test case, the team 3D-printed a plastic noise cancellation device, then attached it to a loudspeaker. With the press of the play button, the experimental set-up “came oh-so-quietly to life in the lab,” according to the BU press release.
“The moment we first placed and removed the silencer … was literally night and day,” study co-author Jacob Nikolajcyk said. “We had been seeing these sorts of results in our computer modeling for months—but it is one thing to see modeled sound pressure levels on a computer, and another to hear its impact yourself.”
The team found they could silence nearly all—94 percent—of the noise, making any sounds emanating from the speaker imperceptible to the human ear.
Moving forward, Zhang & Co. are eyeing their “super lightweight, open, and beautiful” structure for use in drones, fans and HVAC systems, MRI machines, and other everyday clamorous objects.
“The idea is that we can now mathematically design an object that can block the sounds of anything,” Zhang said.
“If you ask me and my colleagues, acoustic metamaterials is a relatively young direction,” she added. “It’s the future.”
The full results were published in January by the journal Physical Review.