Research On Sound Attenuation Structures Based On Membrane-type Materials

Acoustic metamaterials are used to realize special acoustic phenomena,as negative refraction,unidirectional transmission,because they possess some special characteristics,as negative mass density,negative elastic modulus,and negative Poisson's ratio,which cannot be obtained in natural materials.One type of acoustic metamterial is created on the basis of elastic membranes.This membrane-type metamaterial can break the limit of the mass law and produces large sound attenuation at a size much smaller than the wavelength,which exhibits great potentials in sound absorption and insulation.In this paper,an actively adjustable structure for perfect sound absorption is presented,which is created based on electromagnets.By actively adjusting,this structure can realize perfect sound absorption in the frequency range of 800-1500 Hz.Furthermore,we produce sound attenuation in a low frequency range by using a hollow box with membrane-type faces.This paper is divided into the following sections:In chapter 1,the origin and development of membrane-type metamaterials are introduced.Then,various active adjustable acoustic metamaterials are presented and classified.In chapter 2,the sound absorption mechanism of a combined structure of a cavity and an elastic membrane with an addition mass is presented.Furthermore,the nonlinear effects in this structure is studied,and it is found that the frequency for perfect sound absorption increases with the incident sound pressure.The theoretical and experimental research reveal that the shift of the frequency for perfect sound absorption is induced by the nonlinearity of the membrane.On account of the study of nonlinear effects,we propose an active adjustable structure for perfect sound absorption,in which by a direct-current driving voltage 14 V,we achieve actively adjustable perfect sound absorption in a wide frequency range of 800-1500 Hz.It is observed that the experimental results are in agreement with the theoretical results.In chapter 3,a hollow cavity with membrane-type faces is used to produce low-frequency sound attenuation in a pipe.The structure can produce sound attenuation over 20 d B at frequencies near 200 Hz,and two absorption peaks near 10 d B can be observed in frequency of 800-900 Hz.Moreover,this structure dose not influence the gas flow in the pipe because it is smaller than the cross-sectional area of the pipe.Additionally,we study the mechanism of sound attenuation by numerical simulation with COMSOL software.In chapter 4,the summary of the paper and the future work plan are presented.