Sound-cancelling acoustic metamaterial
Boston University researchers have created a mathematically designed synthetic sound-silencing structure shaped in a way that it sends incoming sounds back to where they came from. Published in the Physical Review B paper, the researchers demonstrated that it’s possible to silence noise using a ring-like structure – created to mathematically perfect specifications for cutting out sounds while maintaining airflow.
‘Today’s sound barriers are literally thick heavy walls,’ said Ph.D. student in the Department of Mechanical Engineering and co-author, Reza Ghaffarivardavagh. ‘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 halls, they are a clunky approach not well suited to situations where airflow is also critical.’
The ultra-open metamaterial design which leverages on a Fano-like interference – a type of resonant scattering phenomenon that gives rise to asymmetric line-shape – enables sound silencing in a design featuring a large degree of open area. This has the potential for utility in applications in which highly efficient, air-permeable sound silencers are required – such as smart sound barriers, fan or engine noise reduction, and other similar applications.
Working 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 that would prevent sound, but not air, from being radiated through the open structure. The premise for the cancellation of noise is that the metamaterial needs to be shaped in a way that it sends the incoming sounds back to where they came from.
In the publication, the researchers demonstrate that a transversely placed bilayer medium with large degrees of contrast in the layers’ acoustic properties exhibits an asymmetric transmission, similar to the Fano-like interference phenomenon. The researchers then use a deep-subwavelength acoustic metasurface unit cell comprising of nearly 60% open area for air passage, while serving as selective sound silencer. The proposed unit-cell performance demonstrated a reduction in the transmitted acoustic energy of up to 94%.
As a test study, the team demonstrated a 3D-printed structure made of plastic that could silence sound from a loudspeaker, modelling the physical dimensions that would most effectively silence noises. The loudspeaker was sealed into one end of a PVC pipe – the acoustic metamaterial was fastened into the opening of the other end. Playing a high-pitched note from the loudspeaker, the metamaterial successfully cancelled out the noise.
‘The moment we first placed and removed the silencer was literally night and day,’ said co-author and undergraduate researcher Jacob Nikolajczyk. ‘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.’
According to Ghaffarivardavag, the shape of acoustic-silencing metamaterials, based on their method, is completely customisable. ‘We can design the outer shape as a cube or hexagon, anything really. When we want to create a wall, we will go to a hexagonal shape that can fit together like an open-air honeycomb structure,’ said Ghaffarivardavagh.
Since the successful research in the noise-silencing metamaterial, the researchers have considered other applications they could be used for in the future. ‘Drones are a very hot topic,’ said Professor at the College of Engineering and co-author Xin Zhang. ‘Companies like Amazon are interested in using drones to deliver goods, and people are complaining about the potential noise.’
‘The culprit is the upward-moving fan motion, if we can put sound-silencing open structures beneath the drone fans, we can cancel out the sound radiating toward the ground,’ said Ghaffarivardavagh.
Zhang said since the noise mitigation method can be customised to suit nearly any environment, the possibilities are endless. ‘The idea is that we can now mathematically design an object that can block the sound of anything.’
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