Phononic crystals are an emerging class of manmade materials showing promise in advancing sound-control technologies. But, there is much to learn about how the properties of these materials can be optimized.

Dhillon et al. created an aluminum-based phononic crystal with tunable reflective properties that could serve as an acoustic shield to control white noise.

They developed their crystal structure to explore Anderson localization. The phenomenon, also known as strong localization, is observed in disordered 1D phononic crystal structures, where backscattering interference within a lattice of random solid scatterers prevents an incoming wave from propagating across a wide frequency range.

Ordered 2D phononic crystals also localize sound waves but only across a narrow frequency range corresponding to the band gap, and localization is generally weaker. With this knowledge, the researchers built their crystal layer with both ordered and disordered characteristics.

They fabricated a structure containing nine rows and nine columns of aluminum rods with asymmetric cross-section arranged in a square lattice. The rods were randomly oriented along the rows but equally oriented within the columns. The lattice was submerged in water to observe Anderson localization of ultrasound.

The rod arrangement behaved like an ordered structure if sound propagated along the columns and like a disordered structure if sound propagated along the rows.

“Our metamaterial possesses positional order and, at the same time, orientational disorder, making it possible to reduce the intensity of transmitted sound to a desired level, by controlling the level of disorder,” author Arkadii Krokhin said.

The researchers hope to enable rod rotation in real time by using external motors so the material can be tuned to control the intensity of sound as needed.

Source: “Localization of ultrasound in 2D phononic crystal with randomly oriented asymmetric scatterers,” by Jyotsna Dhillon, Andrey Bozhko, Ezekiel Walker, Arup Neogi, and Arkadii Krokhin, Journal of Applied Physics (2021). The article can be accessed at https://doi.org/10.1063/5.0041659.

This paper is part of the Acoustic Metamaterials 2021. Learn more here.