A Design Method For Sound Absorbing Structure At Low Frequency

With the advancement of science and technology and aerospace industry,the requirements for material properties have become more stringent,and the resulting vibration and noise problems have become more and more prominent at the same time.Vibration and noise can aggravate the damage of the mechanical equipment and affect the processing precision of products,and the low-frequency noise can even resonate with the human body and affect the health of the body.Therefore,it is very necessary to strengthen the research on the field of low-frequency vibration and noise control.Traditional sound absorbing materials have many defects in engineering applications,and super materials with special structure and mechanical properties have received extensive attention by people.The structural parameters and composition of the metamaterial can be changed,in order to obtain physical properties that are not possessed by existing materials in nature or superior to other composite materials.The membrane-imbedded acoustic metamaterials have excellent acoustic properties and equivalent parameters,such as negative density,negative elastic modulus and negative refraction,which have great application values in the fields of sound absorption and noise reduction.The membrane-imbedded acoustic metamaterials mainly use the locally resonant between the embedded film and the mass to dissipate the energy in the acoustic wave,achieving to use small size to control large wavelengths.Combine Bloch theory and the finite element method to explore the dispersion characteristics of the unit cells,and combine the band gaps of unit cells with different structural parameters,integrating the discrete band gaps to extend the bandgap width.Use the finite element method to obtain the vibration transmission response curve,and analyze the sound absorption performance.The mechanical properties of the structure are of great significance to the structural design of the acoustic metamaterials,which provides a new idea for controlling the acoustic wave on the propagation path.This paper uses finite element method to analyze the acoustic properties of the membrane-imbedded acoustic metamaterials.Conduct the dispersion characteristics of unit cells to investigate the effects on the band gap of the structural parameters and material composition for unit cells,and it is feasible to combine dispersion characteristics of the unit cell with different size.Design the acoustic metamaterials pipe(the pipe,the variable section pipe,the elbow pipe and the labyrinth structure),and use the transient response method.Apply the sine wave at the input end of the model,observing the wave propagation in the model and extracting the displacement component of each component in the x direction.The results prove that the acoustic metamaterials can realize sound absorption by the local resonance between the imbedded membrane and the mass.Perform the finite element frequency domain analysis on the pipe structure,draw the vibration transmission response curve to analyze the sound absorption performance of the structure.It’s found that the acoustic metamaterial structure can effectively absorb sound waves at low frequency.According to the relationship between the band gap and structural parameters of unit cell,the mass and membrane have a great influence on the band gap.Periodically adjust the size of the mass and membrane to optimize the pipi structure,to combine the band gaps of different unit cells.The optimized model can integrate the discrete bandgap ranges to achieve four times bandwidth in the low frequency range.Finally,consider the damping of the filling material,and discuss the influence of damping on the sound absorption performance of the model.Give the filling material different damping coefficients and compare the vibration transmission response curves,proving that the damping has a good absorption effect on sound waves at high frequency.It can eliminate the mutual interference between the unit cells of different sizes,and can significantly extend the bandgap width.The bandgap range is 80Hz-700 Hz,which makes the designed structure more suitable for engineering applications.