Materials

Acoustic metamaterial silences sound without blocking airflow

Acoustic metamaterial silences...
A new ring-shaped device can block sound from passing through the middle without blocking air flow
A new ring-shaped device can block sound from passing through the middle without blocking air flow
View 2 Images
The researchers, Reza Ghaffarivardavagh (front center), Stephan Anderson (left), Xin Zhang (rear center), and Jacob Nikolajczyk (right), "demonstrate" the new acoustic metamaterial devices
1/2
The researchers, Reza Ghaffarivardavagh (front center), Stephan Anderson (left), Xin Zhang (rear center), and Jacob Nikolajczyk (right), "demonstrate" the new acoustic metamaterial devices
A new ring-shaped device can block sound from passing through the middle without blocking air flow
2/2
A new ring-shaped device can block sound from passing through the middle without blocking air flow

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."

The researchers, Reza Ghaffarivardavagh (front center), Stephan Anderson (left), Xin Zhang (rear center), and Jacob Nikolajczyk (right), "demonstrate" the new acoustic metamaterial devices
The researchers, Reza Ghaffarivardavagh (front center), Stephan Anderson (left), Xin Zhang (rear center), and Jacob Nikolajczyk (right), "demonstrate" the new acoustic metamaterial devices

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

11 comments
toyhouse
Very interesting. It mentions white noise was reduced in a pipe and that the tech is scale-able. That leads me to wonder how/if it works with low frequency energy traveling through the air?
Brian M
Wondering if some of the reduction is about resonance rather than sound reduction? Would have been nice if the device was replaced with a non-active version, to confirm it wasn't just altering some other property of the pipe i.e. not just compared to an open pipe.
lee54
i see a number of uses: exhaust mufflers for cars, silencers for guns, air intake noise suppression.
Stomps
It would be nice to have a quiet vacuum cleaner...
guzmanchinky
Noise pollution is a huge peeve of mine (first world problem, I know). I look forward to a day when all vehicles are electric and there is a simply way to block out 100% of sound from one's ears to sleep.
windykites
guzman, try ear plugs. The best use would be on jet exhausts. Why have they not tried this yet?
guzmanchinky
windy, I have. They still let in just enough sound to keep you awake.
Expanded Viewpoint
It sort of looks like those are spiraling channels there, but the picture resolution isn't high enough to be sure. There was a car muffler I heard about from the 1960s called Auger Power, and it had some kind of a spiral baffle inside that reduced noise quite a lot, but didn't restrict the exhaust flow. I never saw one myself, just told about them by a guy I worked with. A couple of years ago my brother got some promo for a muffler with snail shell shaped baffles in it, and they are supposed to have a good tone and have low back pressure. So we built a prototype of a modified design out of some scrap stainless steel sheet metal laying in the back yard, and now need a car to test it out on. Randy
Kpar
An interesting device. I notice that little detail is given to theory of operation. I suspect that the spiral channels put some of the sound waves into opposite waveforms- a "passive" active noise reduction that cancels out the amplitude. A truly groundbreaking technology, if it proves out.
warren52nz
It's hard to believe that this device isn't frequency sensitive. I'm guessing it uses a cancellation technique where a sound of a certain wavelength is mixed with an out-of-phase "twin" that cancels it. The twin would be produced by the geometry of the device. In that case I would expect certain other frequencies to be reinforced instead of canceled as the twin being a certain wavelength would arrive in phase and reinforce the tone. However still useful for noise that has a certain constant frequency.