STUDY AND APPLICATIONS OF LABYRINTHINE ACOUSTIC METAMATERIALS FOR IMPROVED SOUND ABSORPTION AT LOW-FREQUENCY

Abstract

Low-frequency noise poses significant challenges across various industries due to its persistent nature and difficulty in attenuation with traditional materials. Acoustic metamaterials (AMMs) have emerged as a promising solution, offering tunable properties that enable effective sound absorption, especially at low frequencies, typically below 1000 Hz. This thesis investigates the development of novel AMMs tailored for low-frequency sound absorption, integrating theoretical modelling, numerical simulations, experimental validation, and real-life applications to optimize their performance. The proposed AMM designs include the Symmetrical Labyrinthine Acoustic Metamaterial with One Slit (SLAMOS), Symmetric Labyrinthine Acoustic Metamaterial with Two Identical Slits (SLAMIS), Symmetric Labyrinthine Acoustic Metamaterial with Two Unequal Slits (SLAMUS), Porous Labyrinthine Acoustic Metamaterial (PLAM), Porous Labyrinthine Acoustic Metamaterial with Hole and Slit Outer Panel (PLAMHS), and Coplanar Labyrinthine Acoustic Metamaterial with Variable Channels (CLAMVAC). The effective mass density and bulk modulus of each design were derived to evaluate their sound absorption coefficients (SAC) using theoretical models. Numerical analysis was conducted using COMSOL Multiphysics, employing the virtual impedance tube with the transfer function method. The sound absorption mechanisms of all proposed designs, excluding those based on porous materials, are found to involve impedance matching, Fabry-Perot-like labyrinthine resonances, and viscous dissipation within microperforation( s). For PLAM and PLAMHS, sound absorption within the porous matrix occurs as particles resonate at the labyrinthine channel entry, dissipating acoustic energy into the microporous matrix, resulting in zero acoustic pressure at the transmission end. A characteristic frequency is derived, at which the inter-scale coupling begins that leads to an enhanced sound absorption of the PLAM compared to its constituents, namely, labyrinthine and porous matrix.