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March 13, 2019 | By Thomas
A team of mechanical engineers from Boston University said that it was possible to reduce noise by using an open ring structure, created according to mathematically perfect specifications, to cut off any sounds. by maintaining the flow of air. They developed a new device specifically designed to block up to 94% of incoming sound waves while letting air through.
Reza Ghaffarivardavagh (ENG) (front center) keeps two ring structures open on his ears while Stephan Anderson (MED) (left), Xin Zhang (ENG) (back center) and Jacob Nikolajczyk (ENG) (right) make a racket. Photo of Cydney Scott
Although noise abatement barricades, known as acoustic baffles, may help to quiet the noise of traffic jams or contain the musical symphony in the walls of concert halls, they constitute an awkward approach that is inappropriate for situations where the circulation of air is also essential.
"The sound comes from very small disturbances in the air," explain the researchers. "So our goal is to silence those tiny vibrations, so if we want the interior of a structure to be in the open, we have to keep in mind that this is the path through which the sound travel to."
They calculated the dimensions and specifications that the metamaterial should have to interfere with the transmitted sound waves, thus preventing the sound – but not the air – from being emitted by the open structure. The basic principle is that the metamaterial must be designed to return the incoming sounds, where they come from, they say.
As a test, they decided to create a structure capable of suppressing the sound emitted by a speaker. Based on their calculations, they modeled the physical dimensions that would most effectively attenuate noise. Then they used 3D printing to create an open plastic noise canceling structure.
The metamaterials printed in 3D, designed mathematically, have a form that they return the incoming sounds, where they come from, say Ghaffarivardavagh and Zhang. Inside the outer ring, a helical pattern interferes with the sounds, preventing them from transmitting through the open center while preserving the ability of the air to flow. CREDIT: Photo of Cydney Scott for Boston University
To test the device, the researchers sealed the speaker in one end of a PVC pipe. At the other end, the custom acoustic metamaterial was fixed in the opening. The speaker sounded a sound through the pipe, but from the outside, it was inaudible to the human ear.
The metamaterial, resonating around the inner perimeter of the mouth of the pipe, worked as a mute button incarnated until the researchers bent down and released it. The lab suddenly resonated with the creaking of the melody from the speakers.
Comparing the sound levels with and without metamaterials fixed in place, the team found that the device was able to block 94% of the sound.
"The moment we first placed and removed the muffler … was literally night and day," says Jacob Nikolajczyk, co-author of the study. "It's been months that we've been seeing these kinds of results in our computer modeling – but it's one thing to see the sound pressure levels modeled on one computer, and another to hear its impact yourself- even."
Now that their prototype has proven so effective, researchers have great ideas on how their silent acoustic metamaterial could work, making the real world quieter.
"Drones are a hot topic," says Xin Zhang, professor at the faculty of engineering. "Companies like Amazon are interested in using drones to deliver goods," she says, and "people are complaining about potential noise."
"We can design the outer shape as a cube or a hexagon, no matter what," says Reza Ghaffarivardavagh, co-author of the study. "Our structure is super light, open and beautiful, and each piece could be used as a tile or brick to enlarge and build a permeable, noise-free wall."
The research was published in the journal Physical examination.
Posted in 3D Print App
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