Study of metadiffusers in broadband improvement of noise barrier performance

In previous studies, installation of Schroeder diffusers on top of noise barriers has been shown to improve effectiveness of noise barriers. However, since these diffusers have thickness of the order of design wavelength, they become very bulky, costly and less practical to use. Their use for lower frequency applications is thus limited. Metadiffusers are subwavelength diffusers, capable of creating similar diffusion characteristics, but their thickness can be 10 to 20 times lower than conventional diffusers. In addition, the absorption performance can be improved by incorporating coupled Helmholtz resonators to further increase noise barrier insertion loss. The reduced thickness and incorporation of Helmholtz resonators enables better performance for a broader range of frequencies, which was not possible with a conventional sound diffuser. It was observed that both 1-D and 2-D metadiffuser designs installed on top of noise barriers improve the performance upto 5dBA in the shadow zone. A combination of number sequences such as maximum length, quadratic residue, primitive root, Luke, ternary, etc. with Helmholtz resonators tuned to different frequencies has also been reported in this study.