Development of acoustic metamaterials for efficient sound control noise mitigation
Authors:   , Nguyen Huy 
Published: December 2021

Acoustic metamaterials have been studied intensively recently since they can expose unnatural-born properties, potentially breaking the capacity limits of conventional acoustic materials. Since these interesting properties are mostly observed around metamaterials' local resonances/anti-resonance, resonance-based acoustic metamaterials are most popular in developing metamaterials. Employing resonance-based unnatural born properties such as effective negative mass density, effective negative bulk modulus, and acoustic hyper-damping on designing noise control solutions can give excellent devices showing such high performance that conventional acoustic material cannot achieve. This dissertation is an effort to employ acoustic metamaterials in designing efficient noise control. First, membrane-type acoustic metamaterials (MAM) will be employed to design a lightweight acoustic panel with high sound transmission loss (STL) in broadband at low frequencies. Negative density at around the anti-resonance of MAM gives it high capability on blocking sound. A double MAM-layer structure is proposed to double the STL performance of unit cells theoretically. Therein, simulation by using COMSOL Multiphysics is the main tool to optimize the unit cell design, panel structure, and effect of panel frame's vibration. Fabrication of the optimal design and experiments are also conducted to verify the calculation and simulation predictions. In addition to the acoustic panel, MAM is used to design a highly efficient acoustic energy harvester working at low frequencies. A magnet coin is deployed close to a magnet coil attached to the mass of MAM. The maximum oscillation of the coil due to MAM's first local resonance will induce a strong electric current inside the coil. Hence, energy can be harvested by an external resistor representing loads of harvesting devices. A complete theoretical model of the harvester is also developed in order to optimize its performance. Multiphysics simulation is conducted to verify the theoretical predictions. Besides MAM, Helmholtz has been used to design a high-performance and broadband acoustic silencer. Specifically, five slit-type Helmholtz resonators, which possess a massive viscous area, are packed together to create a single-layer silencer. In turn, two single-layer silencers are combined to form a double-layer silencer, which in theory double performance on noise blocking of the single-layer silencer. Theoretical models of slit-type Helmholtz resonators and silencers are developed completely and well validated with simulation and experimental results. Finally, Fano resonance resulting from the coupling between resonant and non-resonant channels will be explored and employed to design an ultra-broadband acoustic barrier with high ventilation. The resonant channel is generally represented a space-coiling channel, and the non-resonant channel represents ventilation or a straight and short channel. First, the formation of coupling Fano resonance will be theoretically addressed. Subsequently, acoustic hyper-damping is proposed by integrating thin acoustic foams into velocity anti-nodes in the resonant channel. In the end, an ultra-broadband acoustic barrier with high ventilation and STL is designed by employing three rows of hyper-dampened unit cells. Fabrication and experiment also are conducted to verify the simulation prediction.