Scientists develop 'acoustic metamaterial' that blocks sound, but not air or light

The mathematically designed, 3D-printed acoustic metamaterial is a donut-shaped object.

(Web Desk) – By combining math with 3D printing, researchers from the Boston University have created a new material that seemingly defies logic: light and air have no trouble passing through it — but sound cannot.

“The idea is that we can now mathematically design an object that can block the sounds of anything,” researcher Xin Zhang said in a press release — meaning the future could be far quieter than the present.

In a paper published in the journal Physical Review B, the researchers describe the work that went into creating what they call their “acoustic metamaterial.”

They started by calculating the dimensions and specification a material would need to have to be able to reflect incoming sound waves back to their source without blocking air or light. Then they 3D printed the material into a donut shape and attached it to one end of a PVC pipe, the other end of which they attached to a loudspeaker.

When they blasted a high-pitched note from the loudspeaker, they found that the shape blocked 94 percent of the sound coming through the pipe.

“The moment we first placed and removed the silencer…was literally night and day,” researcher Jacob Nikolajczyk said in the press release.

“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.”

A Quiet Place

The researchers envision many applications for their acoustic metamaterial, which they claim isn’t limited to the donut shape demonstrated by their research.

"Our structure is super lightweight, open, and beautiful," Zhang and her fellow researcher Reza Ghaffarivardavagh said in the press release.

"Each piece could be used as a tile or brick to scale up and build a sound-canceling, permeable wall."

They also note the potential to use the material to dampen the sound of drones, HVAC systems, or even MRI machines — seemingly anything that makes noise could make less noise with the addition of this new material.

When it comes to blocking out sound, traditional methods have involved acoustic panelling that absorbs the sound. Think of a soundproof recording booth, a concert hall or even earplugs.

While today’s sound barriers are thick, heavy walls, they are not suitable for situations where airflow is as critical as cancelling sound.

Can block 94pc of sound

The metamaterial worked like a mute button incarnate until Ghaffarivardavagh removed it from the opening, which was when the lab was filled with sound from the loudspeaker.

Jacob Nikolajczyk, a co-author on the study, said the difference between having the metamaterial blocking the sound and not was like night and day.

“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,” he said.

The team found that the acoustic metamaterial could block out nearly all sounds – 94pc to be exact. This meant that the sounds left coming from the loudspeaker were imperceptible to the human ear.

 

Making the real world quieter

The prototype of this metamaterial has proven so effective that the team is already looking at the practical applications and how it could make the world a quieter place.

Zhang says drones are a very hot topic because while companies such as Amazon are looking at using them to deliver goods, “people are complaining about the potential noise”. Ghaffarivardavagh said that if they can put sound-silencing open structures beneath the drone fans, they can cancel out the sound radiating towards the ground.

Another possible application would be fans and HVAC systems that could benefit from the metamaterial, which would make them silent but still allow them to circulate hot or cold air.

As well as its unique ability to allow air and light to pass through, the acoustic metamaterial’s shape can also be customised. Ghaffarivardavagh said they can design the outer shape to whatever the situation calls for. “When we want to create a wall, we will go to a hexagonal shape.”

Zhang said this makes the possibilities for the device endless. “The idea is that we can now mathematically design an object that can block the sounds of anything,” she said.