A paper released in Physical Review B by Boston University researchers, Xin Zhang, a professor at the College of Engineering, and Reza Ghaffarivardavagh, a Ph. D. student in the Department of Mechanical Engineering demonstrates that it is possible to silence noise using an open, ring-like structure, created to mathematically perfect specifications, for cutting out sounds while maintaining airflow.
“Today’s sound barriers are literally thick heavy walls,” says Ghaffarivardavagh. 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.
Imagine barricading a jet engine’s exhaust vent—the plane would never leave the ground. Instead, workers on the tarmac wear earplugs to protect their hearing from the deafening roar.
A shared passion of Ghaffarivardavagh and Zhang in mathematics has buoyed both of their engineering careers, made them well-suited research partners and has guided them toward a workable design for what the acoustic metamaterial would look like.
A calculation was done by them on 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. On this, they said that 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.
First, as a test case, a structure was created by them that could silence sound from a loudspeaker and based on their calculations, they modeled the physical dimensions that would most effectively silence noises.
Bringing each of the tested models to life, they used 3-D printing to materialize an open, noise-canceling structure made of plastic. While trying out in the lab, the researchers sealed one end of the loudspeaker with a PVC pipe. On the other end, the tailor-made acoustic metamaterial was fastened into the opening. With the hit of the play button, the experimental loudspeaker set-up came oh-so-quietly to life in the lab.
Standing in the room, one would never know if a loudspeaker is turned on with blasting an irritatingly high-pitched sound, based on the sense of hearing alone. But the thrumming of the loudspeaker’s subwoofers can be seen clearly if peered into the PVC pipe.
While testing the metamaterial in the lab, the metamaterial worked like a mute button incarnate ringing around the internal perimeter of the pipe’s mouth until Ghaffarivardavagh pulled it free. The next moment, the lab was filled with the screeching of the loudspeaker’s tune.
“The moment we first placed and removed the silencer…was literally night and day,” says Jacob Nikolajczyk, who in addition to being a study co-author and former undergraduate researcher in Zhang’s lab is a passionate vocal performer.
“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.”
It was found by comparing sound levels with and without the metamaterial fastened in place that an exact of 94% of the noise could be silenced, making the sounds emanating from the loudspeaker imperceptible to the human ear.
With the prototype being successfully proven to be 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,” Zhang says. Companies like Amazon are interested in using drones to deliver goods, she says, and “people are complaining about the potential noise.”
“The culprit is the upward-moving fan motion,” Ghaffarivardavagh says. “If we can put sound-silencing open structures beneath the drone fans, we can cancel out the sound radiating toward the ground.”
The acoustic metamaterials can be of great help to fans and HVAC systems that are closer to home or the office that render them silent yet still enable hot or cold air to be circulated unencumbered throughout a building.
Ghaffarivardavagh and Zhang also point to the unsightliness of the sound barriers used today to reduce noise pollution from traffic and see room for an aesthetic upgrade. “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,” they say.
According to Ghaffarivardavagh, The shape of acoustic-silencing metamaterials, based on their method, is also completely customizable. “We can design the outer shape as a cube or hexagon, anything really,” he says. “When we want to create a wall, we will go to a hexagonal shape” that can fit together like an open-air honeycomb structure. Such walls could help contain many types of noises. Even those from the intense vibrations of an MRI machine, Zhang says.
According to Stephan Anderson, a professor of radiology at BU School of Medicine and a co-author of the study, the acoustic metamaterial could potentially be scaled “to fit inside the central bore of an MRI machine,” shielding patients from the sound during the imaging process.
Zhang says the possibilities are endless since the noise mitigation method can be customized to suit nearly any environment: “The idea is that we can now mathematically design an object that can block the sounds of anything,” she says.