Experimental and Numerical Evaluation of Dual-Layer Sonic Crystal Scatterers with Embedded Helmholtz Resonators

Rapid urban growth has increased the demand for new transportation projects, leading to higher noise levels in cities. Noise barriers are the most common treatment for mitigating this problem, but conventional designs are often unsuitable for urban environments due to their size and esthetic limitations. Acoustic metamaterials offer a promising alternative. This research proposes a novel design of a sonic crystal unit cell that integrates multiple mechanisms to achieve broadband frequency attenuation. The design was numerically modeled to evaluate both the absorbing material used for the scatterers and the embedded Helmholtz resonators. Following the initial predictions, the concept was experimentally validated using a custom-built test tube inspired by the EN 1793-6 standard. Results show a marked improvement in insertion loss compared with an equivalent rigid configuration, with enhanced performance before and after the band gap.