Boston University researchers recently created an open, ring-like cylinder designed to rigorous mathematical specifications that has a curious and unexpected property: it reflects back the sound of whatever passes through it.
To test their new “acoustic metamaterial”, College of Engineering professor Xin Zhang and engineering PhD student Reza Ghaffarivardavagh sealed a loudspeaker into one end of a PVC pipe. Just inside the other end of the pipe, the researchers inserted a 3D-printed version of their new metamaterial made out of plastic. They then set the loudspeaker to blast a high-pitched note.
The result: Nothing. Standing in the room, the loudspeaker was inaudible to human ears. This was true even though the vibrating loudspeaker was visible through the open hole of their circular plastic metamaterial.
In fact, the team found that their metamaterial silenced 94 per cent of the loudspeaker’s noise - enough to make it imperceptible to humans. When they removed the plastic ring from the pipe, the sound was back, blaring an angry high-pitched tone.
"The moment we first placed and removed the silencer...was literally night and day," says Jacob Nikolajczyk, who is both a co-author of the study and a vocal performer, according to a March 7th public release on the project by Boston University.
"We had been seeing these sorts of results in our computer modelling for months - but it is one thing to see modelled sound pressure levels on a computer, and another to hear its impact yourself."
The seemingly impossibility of containing sound while allowing air to flow through a ring can be explained by mathematics, which is a shared passion of both Ghaffarivardavagh and Zhang. Using their knowledge of how sound waves travel through air, they calculated precise dimensions and specifications for the metamaterial to interfere with sound wave transmission while still allowing air to travel through the open structure. The most important thing is for the shape of the metamaterial to reflect incoming sounds back to where they came from, the researchers said.
The innovation stands in sharp contrast to current noise dampening or noise cancellation technology. In most situations, blocking out sound requires bulky soundproofing barriers like those found in the walls of recording studios. Noise cancellation technology like the kind used in Bose headphones requires expensive, battery-powered electronics to generate counter signals to zero-out unwanted noise. By contrast, the Boston University researchers’ solution required just a plastic insert to block the sound.
"Today's sound barriers are literally thick heavy walls," said Ghaffarivardavagh. “Our structure is super lightweight, open, and beautiful.”
Importantly, the metamaterial solution allows for airflow, giving it potential for applications on blaring motors like the ones used on drones. "Drones are a very hot topic," Zhang said. Companies like Amazon are planning to implement drone-deliveries but "people are complaining about the potential noise."
"The culprit is the upward-moving fan motion," Ghaffarivardavagh said. "If we can put sound-silencing open structures beneath the drone fans, we can cancel out the sound radiating toward the ground."
There are other possible applications as well, from reducing the noise of car motors to dampening the drumming of office fans and air conditioning systems. The metamaterial doesn’t have to be circular to dampen sound either.
"We can design the outer shape as a cube or hexagon, anything really," Ghaffarivardavagh said. "When we want to create a wall, we will go to a hexagonal shape" that can fit together like an open-air honeycomb structure.”
The number of potential applications combined with the simplicity and low expense of the new technology makes it promising. "The idea is that we can now mathematically design an object that can block the sounds of anything," Zhang said.