It is possible to silence noise using an open, ring-like structure, created to mathematically perfect specifications, for cutting out sounds while maintaining airflow, scientists say.
Although noise-mitigating barricades, called sound baffles, can help drown out the whoosh of rush hour traffic or contain the symphony of music within concert hall walls, they are a clunky approach not well suited to situations where airflow is also critical, said researchers from Boston University in the US.
In the study published Physical Review B, they let mathematics guide them towards a workable design for what the acoustic metamaterial would look like.
The researchers calculated the dimensions and specifications that the metamaterial would need to have in order to interfere with the transmitted sound waves, preventing sound—but not air—from being radiated through the open structure.
The basic premise is that the metamaterial needs to be shaped in such a way that it sends incoming sounds back to where they came from, the researchers said.
As a test case, they decided to create a structure that could silence sound from a loudspeaker.
Based on their calculations, they modelled the physical dimensions that would most effectively silence noises. Bringing those models to life, they used 3D printing to materialise an open, noise-cancelling structure made of plastic.
The researchers sealed the loudspeaker into one end of a polyvinyl chloride (PVC ) pipe.
On the other end, the tailor-made acoustic metamaterial was fastened into the opening, said Xin Zhang, a professor at Boston University.
With the hit of the play button, the experimental loudspeaker set-up came to life in the lab. Standing in the room, based on your sense of hearing alone, you'd never know that the loudspeaker was blasting an irritatingly high-pitched note.
If, however, you peered into the PVC pipe, you would see the loudspeaker's subwoofers thrumming away.
The metamaterial, ringing around the internal perimeter of the pipe's mouth, worked like a mute button incarnate until the moment when researchers reached down and pulled it free.
The lab suddenly echoed with the screeching of the loudspeaker's tune.
"The moment we first placed and removed the silencer...was literally night and day," said Jacob Nikolajczyk, a study co author and former undergraduate researcher in Zhang's lab.
"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, Nikolajczyk said.
By comparing sound levels with and without the metamaterial fastened in place, the team found that they could silence nearly all—94 per cent to be exact—of the noise, making the sounds emanating from the loudspeaker imperceptible to the human ear.
The researchers have some big ideas about how their acoustic-silencing metamaterial could go to work making the real world quieter.
Drones are a very hot topic. Companies like Amazon are interested in using drones to deliver goods, she says, and "people are complaining about the potential noise," Zhang said.