Low-frequency And Multiple-bands Sound Insulation Based On Membrane-type Materials

Acoustic metamaterials possess extraordinary characteristics that cannot be observed in natural materials and traditional materials,such as negative density and negative modulus,and thus,they exhibit exceptional performances,such as acoustic cloaking,sub-wavelength imaging,anomalous refraction/reflection,one-way transmission,and focusing,etc.Especially,the acoustic metamaterials on the basis of membranes exhibit great application potentials in sound absorption and insulation.In this thesis,a hollow box with membrane-type faces is presented to realize sound insulation in a pipe.On account of different mechanisms,low-frequency and multiple-bands sound insulation can be realized using the simple structure.The thesis covers four parts as follows:In chapter 1,the origin of electromagnetic and acoustic metamaterials is introduced.Then,the structures and applications of acoustic metamaterials on the basis of membranes are presented.In chapter 2,a simple structure of a cuboid box with six faces made up of elastic membranes is presented to realize sound insulation in pipes.It is proven by the finite element simulations,owing to the coaction of the cavity,membrane-type faces,and the intervals between the box and the pipe walls,sound insulation is obtained at multiple frequency-bands on the basis of distinct mechanisms.The frequency-bands for sound insulation locate in the low-frequency range below 500 Hz and the size of the box is much smaller than the wavelength.Meanwhile,by adjusting the structural parameters and establishing an array consisting of the membrane-type boxes with different sizes,the sound insulation is enhanced,and thus,we can achieve high attenuation in more frequency-bands below 500 Hz.Additionally,because the sound insulation is achieved using membrane-type boxes with the sizes smaller than the cross-section of the pipe,gas flow and natural ventilation are not blocked by the sound insulators.In chapter 3,the one-dimensional spring mass model is used to analyze the effective mass density and effective volume modulus of acoustic metamaterials,and meanwhile,the analytical solution of the effective mass density is given by multiple scattering theory.In addition,the equivalent impedances of membrane-type or plate-type metamaterials are studied using the analytic equation,approximate model and finite element model,which can give a theoretical explanation for the special performance of the type of metamaterials.Finally,the summary of the thesis and the plans for the future work are presented in chapter 4.