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‘Acoustic metamaterial’ could be real-life mute button

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The mathematically designed, 3D-printed acoustic metamaterial is shaped in such a way that it sends incoming sounds back to where they came from (Credit: Cydney Scott for Boston University)
The mathematically designed, 3D-printed acoustic metamaterial is shaped in such a way that it sends incoming sounds back to where they came from (Credit: Cydney Scott for Boston University)

A new ‘acoustic metamaterial’ that nearly silences noise while allowing airflow through could theoretically “block the sounds of anything,” researchers have claimed.

Engineers at Boston University in Massachusetts created the metamaterial, which they shaped into a ring-like structure to successfully cancel noise being “blasted” from a speaker.

Today’s noise mitigating barriers, known as baffles, can stop noise dissipation from environments such as concert halls or busy roads – but “they are a clunky approach not well-suited to situations where airflow is also critical,” like active jet engines on an aeroplane, said the researchers.

Professor Xin Zhang and PhD student Reza Ghaffarivardavagh turned to mathematics, calculating the dimensions and specifications the metamaterial needed to interfere with transmitted sound waves while allowing air flow to radiate through the open structure. The basic premise was that the metamaterial needed to be shaped in such a way that it sent incoming sounds back to where they came from.

As a test case, the researchers silenced sound from a loudspeaker. Based on their calculations, they modelled the physical dimensions that would most effectively silence noises. They then 3D printed a ring-like physical structures, creating a noise-cancelling structure made of plastic. Inside the outer ring, a helical pattern interfered with sounds, blocking waves from transmitting through the open centre while preserving air's ability to flow through.

The engineers put the device on the end of a plastic tube sealed to a loudspeaker “blasting an irritatingly high-pitched note”.

"The moment we first placed and removed the silencer...was literally night and day," said co-author 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."

The device silenced 94% of the noise, reportedly making it imperceptible to the human ear – a highly desirable characteristic. When attached to future delivery drones, for example, silencers could allay fears of antisocial noise.

"The culprit is the upward-moving fan motion," said Ghaffarivardavagh. "If we can put sound-silencing open structures beneath the drone fans, we can cancel out the sound radiating toward the ground."

Other applications include silencing HVAC systems while allowing air to circulate, or fitting inside the central bore of MRI machines to shield patients from loud noises during scanning.

The shape of the metamaterial can be modified to prevent noise from an “endless” variety of sources, the researchers claimed, with hexagons particularly useful for structural shapes. "The idea is that we can now mathematically design an object that can block the sounds of anything," said Zhang.

The research was published in Physical Review B.


Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.
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