Boston University researchers have created a metamaterial noise canceller that absorbs a large amount of sound while still maintaining airflow. Shaped like a doughnut-ring like structure, the printed noise blocker effectively eliminates 94% of sound, as is apparent in an experiment using a long tube playing out high frequency sounds from a loudspeaker. The metamaterial could come in handy in heavy-noise work environments, music recording and could also make electronics quieter.
The metamaterial consists of intricate structures that, according the researchers, cause sound reflections. Thus, the printed noise blocker’s structure sends back sounds when attached to the end of the tube. Removing the blocker fills the environment with the noise once again. This is also illustrated in the video below:
As the study states, the material has a “transversely placed bilayer medium with large degrees of contrast in the layers’ acoustic properties“. As a result, it releases an asymmetrical transmission, similar to the Fano interference phenomenon. What this means is that they calculated the precise dimensions and specifications the structure would need to have so it could interfere with the transmitted sound waves, thus preventing sound-flow while permitting airflow. The 3D printed noise blocker tosses back the noise till the overall sound energy dissipates, thereby reducing it.
“The moment we first placed and removed the silencer…was literally night and day,” said Jacob Nikolajczyk, a study co author and researcher. “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”.
Metamaterial Noise Canceller
The pipe itself consists of typical PVC. The trick is not so much in the material as it is in the inner structure. The ring is a deep-subwavelength acoustic metasurface unit cell comprising with a 60% open area for air passage. The intricate structure is a mathematical marvel that turns sound reflection into a labyrinth.
There are many situations where air-permeable sound silencers are needed. The 3D printed noise blocker could serve as a smart sound barrier for fan or engine noise reduction. It could also find utility within walls of concert halls and stadiums. It may even be useful for car silencers, although admittedly, with a far different more resilient material.
Featured images courtesy of Boston University. The abstract is also available here. Credit to the authors: Reza Ghaffarivardavagh, Jacob Nikolajczyk, Stephan Anderson, and Xin Zhang.