The Influence of Cladded Resonators on Gradient-Based Sonic Crystals over the Traditional Sonic Crystals

Introduction Gradient-based sonic crystals (GBSCs) based on rigid resonators that improve the acoustic attenuation over the traditional sonic crystals (SCs) were recently studied numerically. The present work is a numerical and experimental study of the GBSCs based on rigid and cladded resonators and aims to further improve the attenuation of the GBSCs over the traditional SCs. Methods Extensive finite element (FE) and experimental studies were conducted on the GBSCs and the traditional uniform SCs using jute, a natural fibrous material as claddings over rigid resonators in the form of felts. The multiple linear regression (MLR) technique was used to find the influence of the airflow resistivity and thickness of the claddings on the attention spectrum of the GBSC. The effect of the angle of incidence of the sound waves on the attenuation by the GBSC and traditional SC was also studied experimentally. Results Though the claddings were found to improve the overall attenuation by the GBSC, they do not improve the attenuation at all frequencies. Similarly, the claddings were found to be more effective on the GBSC than the traditional Helmholtz resonator-based SC (HRSC). However, the GBSC was found to be less effective than the traditional HRSC if the angle of incidence of the sound waves changes. Conclusions For a fixed filling ratio of the GBSC resonators, the Bragg scattering and local resonance of a GBSC based on rigid resonators are more effective attenuation mechanisms in the resonance and first Bragg gap regimes than the combined effects of absorption by the fibrous cladding, Bragg scattering and local resonance of a GBSC based on cladded resonators. However, the claddings significantly improve the attenuation post the resonance and first Bragg gaps. Therefore, the claddings should only be used when the targeted frequency regime lies above the resonance and first Bragg gaps.

Automated summary

This study investigates the improvement of acoustic attenuation in gradient-based sonic crystals (GBSCs) based on rigid and cladded resonators compared to traditional sonic crystals (SCs) through numerical and experimental analysis. It is found that the claddings enhance the overall attenuation of GBSCs, although their impact is not consistent across all frequencies. The claddings have a more significant effect on GBSCs compared to traditional Helmholtz resonator-based SCs (HRSCs), but GBSCs are less effective than HRSCs when the angle of incidence of sound waves changes. The study suggests that for a fixed filling ratio of GBSC resonators, the Bragg scattering and local resonance mechanisms are more effective in the resonance and first Bragg gap regimes, while the claddings significantly improve attenuation post these regimes. Therefore, the claddings should be used when the targeted frequency regime lies above the resonance and first Bragg gaps.