Synthetic, sound-silencing structures—acoustic metamaterials can block 94% of sounds. This ring of materials will be able to make vacuum cleaners, air conditioners, fans and other devices and products much quieter.
Boston University researchers Ghaffarivardavagh and Zhang wanted to block sound but not open air.
They 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.
They modeled the physical dimensions that would most effectively silence noises. Bringing those models to life, they used 3D printing to materialize an open, noise-canceling structure made of plastic.
Trying it out in the lab, the researchers sealed the loudspeaker into one end of a PVC pipe. On the other end, the tailor-made acoustic metamaterial was fastened into the opening.
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. Inside the PVC pipe, you would see the loudspeaker’s subwoofers moving.
The team found that they could silence 94 percent of the noise which making the sounds emanating from the loudspeaker imperceptible to the human ear.
Recently, with advances in acoustic metamaterial science, the possibility of sound attenuation using subwavelength structures, while maintaining permeability to air, has been demonstrated. However, the ongoing challenge addressed herein is the fact that among such air-permeable structures to date, the open area represents only small fraction of the overall area of the material. In the presented paper in order to address this challenge, we first 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. Next, we utilize this design methodology and propose a deep-subwavelength acoustic metasurface unit cell comprising nearly 60% open area for air passage, while serving as a high-performance selective sound silencer. Finally, the proposed unit-cell performance is validated experimentally, demonstrating a reduction in the transmitted acoustic energy of up to 94%. This ultra-open metamaterial design, leveraging a Fano-like interference, enables high-performance sound silencing in a design featuring a large degree of open area, which may find utility in applications in which highly efficient, air-permeable sound silencers are required, such as smart sound barriers, fan or engine noise reduction, among others.
SOURCES- Boston University, Physical Review B
Written By Brian Wang