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Researchers have built a prototype metamaterial that can successfully block out 94pc of sound while still allowing air to pass through it.
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.
However, new research shows that it’s possible to successfully block sound while maintaining airflow, with the right material.
Using an open, ring-like structure, scientists have developed an “acoustic metamaterial” that can catch and block sound while allowing air and light to travel through.
Boston University researcher Prof Xin Zhang and PhD student Reza Ghaffarivardavagh released a paper on the metamaterial in Physical Review B.
While today’s sound barriers are thick, heavy walls, they are not suitable for situations where airflow is as critical as cancelling sound. Zhang and Ghaffarivardavagh were able to calculate the dimensions and specifications the metamaterial would need to have to block sound, but not air, from being radiated through the open structure.
As a test case, the researchers created a structure that could block sound from a loudspeaker. They used 3D printing to materialise an open, noise-cancelling structure made of plastic.
They sealed the loudspeaker into one end of a PVC pipe and secured the acoustic metamaterial to the opening on the other end.
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.
