A coplanar metamaterial with variable channels for ventilated noise control

Abstract

Traditional noise barriers often face limitations in balancing acoustical performance and ventilation efficiency, restricting their usage in various applications. To address this, we have proposed a novel coplanar labyrinthine acoustic metamaterial with continuously varying channel widths with a micro-hole perforated cover plate for wave entry. Using an iterative backward acoustic impedance calculation by impedance translation theorem, a theoretical model for the sound absorption by the metamaterial has been developed, and validated numerically and experimentally. The sound absorption mechanism is analyzed using acoustic particle velocity fields. The key geometrical parameters (number of channels, metamaterial diameter, cover plate thickness, and micro-hole diameter) that significantly impact the sound absorption response and the method of tuning them to attain low-frequency sound attenuation below 500 Hz while maintaining a high absorption magnitude and broader bandwidth have been determined. The material is demonstrated to be a better sound absorber than Helmholtz resonator and existing coplanar metamaterials. Additionally, the impact of the metamaterial unit cell lattice on sound transmission loss, pressure drop, and ventilation efficiency is examined. The hexagonal lattice with 9.31 % ventilation provides better noise reduction than the square lattice with 21.46 % ventilation. A 17 dB difference in sound transmission loss at 510 Hz and peak sound transmission loss of 39 dB and 35 dB respectively, for hexagonal and square lattice configurations, have been experimentally verified. The proposed metamaterial has practical applications where ventilated low-frequency noise control is desired in compact spaces.