Low-Frequency Sound Absorption Of Metamaterials With Coiled-up Space

Low-noise quality has been one important objective in the development of modern equipment.Adhering sound absorbing materials(SAMs)onto the cabins of the equipment is an effective way for noise dissipation.Conventional SAMs,such as porous materials and micro-perforated panels,usually require large thickness to achieve perfect absorption in low-frequency range,which waste the cabin space of the equipment.Developing a material with a thin thickness to achieve good sound absorption performance at low frequency domain is currently a growing,although challenging,topic in acoustics.Recently,the rapid development of acoustic metamaterials has opened a new perspective for the solution of this challenge.Supported by the National Natural Science Foundation of China,this dissertation presents analytical,numerical and experimental investigations of acoustic metamaterials with coiled-up space for low frequency absorption.The analytical method is presented that underlies the absorption performance using the impedance theory.The sound absorption mechanism of new metamaterials in low frequency domain is investigated in detail.Further,the methods for tuning and broadening the sound absorption band are developed.The main work and findings of this dissertation include:1.By introducing the coiled-up space into convetional Helmholtzs resonators and porous materials,respectively,two types of acoustic metamaterials are proposed.It shows that the new metamaterials proposed here can obtain lower frequency absorption compared with the materials without the coiled-up space..2.All the analytical,numerical and experimental results demonstrate the metamaterials composed of Helmholtzs resonators with coiled-up cavity possessing low-frequency sound absorption.The theoretical minimum ratio of thickness to wavelength can be less than 1/80.The underlying absorption mechanism is revealed by the acoustic impedance,field pattern and reflection coefficient in complex frequency plane.It is found that the perfect absorption is firstly induced by meeting the conditions of critical coupling and well impedance-match between the metamaterial and the exterior air,then the acoustic energy is mainly dissipated by the enhanced viscous friction around the perforated region.The peak frequency of the absorption can be lowered by reducing the thickness of the proper channel or increasing the folding number of the coiled-up cavity.By combining units with various resonant frequencies in parallel,the absorption bandwidth can be broadened.3.Both theoretical and numerical results show the metaporous material with coiled-up space possessing multiple absorption peaks in the low-frequency range.The theoretical minimum ratio of thickness to wavelength can be less than 1/26.It is found that the different absorption peaks are induced by the resonances of the main and side channels,respectively.The metaporous material is impedence-matched to air at the absorption peak frequencies and then the acoustic energy is dissipated by the damping of porous materials.The synergetic tuning of material and structure parameters can lower the first peak frequency and increase the number of absorption peaks.Experimental results further demonstrate the low-frequency absorption of our metaporous materials and the validation of the tunability of the proposed methods.In summery,the goal of this dissertation is to achieve high efficient sound absorption in low frequency domain using the thin metamaterials.All the theoretical,numerical and experimental methods are used to demonstrate the low frequency absorption performance,reveal the absorption mechanisms and the tunability.Further,the means of broadening absorption bandwidth are offered.The works and findings can provide theoretical foundation for improving acoustic absorption in low frequency range using the metamaterials with coiled-up space,and offer one technical guidelines to design new materials in the actual applications of equipment noise control.