Research On Acoustic Characteristics Of Pipe Muffler Based On Helmholtz Acoustic Metamaterial

Since the Industrial Revolution,the manufacturing of various machinery and equipment has brought prosperity and progress to mankind.However,it has also resulted in a significant increase in environmental noise.As a part of environmental noise,pipeline noise inevitably has a negative impact on life and work.There are mainly two ways to suppress pipeline noise,active noise reduction and passive noise reduction,among which active noise reduction is difficult to be widely used due to its complexity and stability.Relatively speaking,the passive noise reduction method is more mature and stable.A resistant structure muffler based on crosssectional mutation and cavity resonance effectively blocks the propagation of low and medium-frequency noise.However,the three noise reduction indexes of resistant mufflers:volume size,frequency bandwidth and low and medium frequency performance constrain each other and limit the conditions and usage of mufflers.In this thesis,we adopt the transfer impedance method to design a ring-shaped Helmholtz-type acoustic metamaterial muffler.Green’s function and pipe waveguide equation theory is applied to calculate the cross-sectional radiated sound field and muffler transmission loss curve.Based on the finite element analysis,the transmission loss curve of the muffler in the frequency band of 400 Hz-1000 Hz is calculated,and the sound insulation volume of the sample parts is measured by the transfer function method to verify the accuracy of the numerical and theoretical calculations,which can achieve a transmission loss of 30 d B at the peak and a noise reduction frequency band bandwidth of 527 Hz-782 Hz.The effect of flow on the noise reduction performance of the muffler is investigated by plotting the sound insulation curves of sample parts under different Mach numbers.The results show that with the increase of Mach number,the peak frequency keeps shifting to a high frequency,and at0.03 Mach number,the peak transmission loss shrinks to 21 d B,and the gas flow velocity has a greater influence on the noise reduction performance of Helmholtz-type acoustic metamaterial muffler.Secondly,in order to enhance the noise reduction performance and expand the application range of mufflers,a method has been developed based on the annular Helmholtztype muffler.This method involves reducing the application frequency of the muffler by introducing folding features into the cavity chamber using an equivalent analogy.This innovative approach aims to optimize noise reduction specifically at low and medium frequencies.It is shown that the effective frequency of the Helmholtz resonator with the folding feature is lower at the same volume,and the transmission loss reaches 35 d B at the resonance frequency with a frequency band bandwidth of 286 Hz-360 Hz.Subsequently,based on the finite element calculation results,the scattering coefficient inversion method is used to derive the equivalent medium modulus,and it is found that the equivalent bulk modulus of this structure is negative at the resonance frequency,which is consistent with the acoustic metamaterials.The structure was found to have a negative equivalent bulk modulus at the resonant frequency,which is consistent with the special physical properties exhibited by acoustic metamaterials.The response surface method was used to correlate the structural parameters of the resonator and to obtain the effect of the variation of the dimensional structural parameters on the acoustic characteristics.Finally,based on the acoustic characteristics of the parallel Helmholtz-type muffler,a 3-cavity parallel structure Helmholtz-type annular muffler is designed to improve the noise reduction bandwidth.The finite element method is used to calculate the transmission loss and ventilation efficiency of the multi-cavity structure,and the results show that the multi-cavity structure has a good ventilation effect and the transmission loss can reach 10 d B at 800 Hz-1200 Hz.The scattering coefficient inversion method is used to calculate the equivalent medium modulus,and it is found that the multi-cavity connection has a double negative phenomenon at the resonance frequency,i.e.,negative equivalent bulk modulus and equivalent mass density.Experimental measurements of the transmission loss of the sample confirm that the structure achieves a noise reduction of 5 d B at 700 Hz-1300 Hz and can achieve good broadband noise reduction.Through a series of numerical calculations and experimental verification of the Helmholtz-type muffler,the feasibility of the improved structure is fully demonstrated,which can improve the applicability and acoustic performance of the muffler and has a broad application prospect in the future engineering field.