Acoustic metamaterial blocks nearly all sound

A team from Boston University have developed avery light and openprototypesynthetic structure that can cancel out nearly all sound. The tailor-made acoustic metamaterial was shaped into an open, ring-like structure capable of blocking up to 94% of all noisewhile also maintaining airflow, and which could find uses in anything from drones to air conditioning units that would benefit from noise mitigation.

The researchers wanted to identify a design for an acoustic metamaterial that could block sound but also allow airflow, calculating the dimensions and specifications that it would need to interfere with transmitted sound waves, preventing sound but not air from being radiated through the open structure, transmitting incoming sounds back to where they came from.

As described in Physical Review B [Ghaffarivardavagh et al. Phys. Rev. B(2019) DOI: 10.1103/PhysRevB.99.024302], the structure was able to silence sound from a loudspeaker sealed onto one end of a PVC pipe, while at the other end the metamaterial was attached. When the loudspeaker was on, the system was started but the high-pitched note played could not be heard by the human ear. The metamaterial circling the internal perimeter of the pipe's mouth operated as a mute button until it was pulled free, at which point the note could be heard.Inside the outer ring, a helical pattern interferes with sounds, blocking them from transmitting through the open center while preserving the ability of air to flow through.

By comparing sound levels with and without the metamaterial in place in the plastic noise-canceling structure, which was produced by 3D printing, they showed that 94% of the noise coming from the loudspeaker were imperceptible to the human ear. The shape of acoustic-silencing metamaterials is also customizable, and the outer section could be a cube or hexagon rather than a round ring, and the method can be tailored to suit practically any environment, as they can mathematically design an object that can block the sounds from different objects.

This ultra-open metamaterial design enables high-performance sound silencing in a design featuring a large degree of open area that may find applications where highly efficient, air-permeable sound silencers are used – such as smart sound barriers, fan or engine noise reduction. For instance, fans and air conditioning systems could be improved by acoustic metamaterials that make them silent but still able to circulate hot or cold air, while such sound barriers could help reduce noise pollution from traffic.