A team of mechanical engineers at Boston University say that it is possible to silence noise using an open, ringlike structure, created to mathematically perfect specifications, for cutting out sounds while maintaining airflow. They have developed a new device that is specially designed to block up to 94 percent of incoming sound waves, while still letting air pass through.
Reza Ghaffarivardavagh (ENG) (front center) holds two of the open, ringlike structures over his ears while Stephan Anderson (MED) (left), Xin Zhang (ENG) (rear center), and Jacob Nikolajczyk (ENG) (right) make a racket. Photo by Cydney Scott
Although noise-mitigating barricades, called sound baffles, can help drown out the whoosh of rush hour traffic or contain the symphony of music within concert hall walls, they are a clunky approach not well suited to situations where airflow is also critical.
"Sound is made by very tiny disturbances in the air," say the researchers. "So, our goal is to silence those tiny vibrations. 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."
They calculated the dimensions and specifications that the metamaterial would need to have in order to interfere with the transmitted sound waves, preventing sound--but not air--from being radiated through the open structure. The basic premise is that the metamaterial needs to be shaped in such a way that it sends incoming sounds back to where they came from, they say.
As a test case, they decided to create a structure that could silence sound from a loudspeaker. Based on their calculations, they modeled the physical dimensions that would most effectively silence noises. Then they used 3D printing to create an open, noise-canceling structure made of plastic.
The mathematically designed, 3D-printed acoustic metamaterial is shaped in such a way that it sends incoming sounds back to where they came from, Ghaffarivardavagh and Zhang say. Inside the outer ring, a helical pattern interferes with sounds, blocking them from transmitting through the open center while preserving air's ability to flow through. CREDIT: Photo by Cydney Scott for Boston University
To test the device, the researchers sealed the loudspeaker into one end of a PVC pipe. On the other end, the tailor-made acoustic metamaterial was fastened into the opening. The speaker blasted a tone through the pipe, but from the outside it was inaudible to the human ear.
The metamaterial, ringing around the internal perimeter of the pipe's mouth, worked like a mute button incarnate until the moment when researchers reached down and pulled it free. The lab suddenly echoed with the screeching of the loudspeaker's tune.
By comparing sound levels with and without the metamaterial fastened in place, the team found that the device was able to block 94 percent of the sound.
"The moment we first placed and removed the silencer...was literally night and day," says Jacob Nikolajczyk, co-author of the study. "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."
Now that their prototype has proved so effective, the researchers have some big ideas about how their acoustic-silencing metamaterial could go to work making the real world quieter.
"Drones are a very hot topic," says Xin Zhang, a professor at the College of Engineering. "Companies like Amazon are interested in using drones to deliver goods, she says, and "people are complaining about the potential noise."
"We can design the outer shape as a cube or hexagon, anything really," says Reza Ghaffarivardavagh, co-author of the study. "Our structure is super lightweight, open, and beautiful. Each piece could be used as a tile or brick to scale up and build a sound-canceling, permeable wall."
The research was published in the journal Physical Review.