Acoustic metamaterial silences sound without blocking airflow

In our increasingly noisy world, it can be hard to find some quiet time. Now, a team of mechanical engineers at Boston University has developed a new device that is specially designed to block up to 94 percent of incoming sound waves, while still letting air pass through.

Current technology can be pretty effective at blocking sound. The walls of concert halls or recording studios are stuffed with thick cladding and large cavities to muffle outside noise, and newer metasurfaces could work the same way in a fraction of the space. But either way, that's not going to allow much airflow. So the Boston team set out to design an acoustic metamaterial that could effectively block sound without blocking the passage of air.

"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."

Their design is a 3D-printed, ring-shaped device that's made to some very precise mathematical standards. The shape is specifically designed to interfere with incoming sound waves and send them bouncing back the way they came. Material in the outer ring coils around like a helix, reducing the sound that can pass through the open center.

To test the device, the researchers placed a prototype in the end of a PVC pipe, and hooked a speaker up to the other end. The speaker blasted a tone through the pipe, but from the outside it was inaudible to the human ear. When the metamaterial was removed, the tone was suddenly reverberating through the room. According to the researchers, 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."

Similar designs have been pitched before, including a window that can reduce noise by 30 decibels. Others use active noise-canceling technology, often seen in headphones, but that requires electricity and processors to crunch the numbers. Being more passive, the new device could be better suited to more places.

The researchers say this kind of device could have plenty of applications in the real world, in situations where sound needs to be blocked but air flow unimpeded. These rings could be placed around the exhaust vents of jet engines, under the fans on drones, or in the vents of heating and cooling systems in buildings. And it doesn't necessarily have to be completely round, either.

"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. The team demonstrates the metamaterial in the video below.

Source: Boston University