Research On Acoustic Characteristic Of Composite With Gradient Change Of Impedance

The medium with gradient change of acoustic impedance is a new acoustic structure which developed from multiple layer structures.With sound absorption coating of submarine as research background,a layered sound absorption structure with gradient change of impedance is designed with low-frequency high sound absorption as the goal by theoretical derivation and finite element simulation in this paper.Also,the experimental verification of partial simulation results are performed by sample preparation and acoustic tests.Based on simple harmonic acoustic wave equation and impedance transfer formula,it is concluded by theoretical derivation that the impedance changes the acoustic absorptivity and transmissivity of medium in different gradient forms.By establishing the theoretical model of propagation of acoustic waves in anisotropic media,starting from the small-amplitude acoustic wave equation of motion in ideal fluids.The relationship between impedance distribution and time is obtained by transforming the acoustic wave equation form and solving the one-dimensional acoustic wave equation with introducing the acoustic impedance and propagation time.Discrete layering is used to associate the incident sound pressure at any interface with the transmitted sound pressure,and a way of analyzing the acoustic performance of the medium is given.That is,the corresponding position is obtained by determining the travel time of the sound wave in the heterogeneous medium,and acoustical performance coefficient of the medium is obtained.Combined with the research objectives of this paper,the impedance transfer equation is introduced to derive the acoustic coefficients corresponding to the media with typical impedance graded distribution.Based on the theoretical derivation,as a supplement,a sound absorption coating model is established based on the finite element simulation software,and the unit cell impedance change forms(linear,quadratic,exponential,catenary),and the dual cell impedance change forms are set.The acoustic performance(absorption coefficient,reflection coefficient,transmission coefficient)of the acoustically absorbing cover layer in the corresponding impedance gradient form is simulated and calculated,and the optimum impedance gradient form is selected with low-frequency high sound absorption as the goal.Also,from the perspective of experimental production and engineering,the gradient impedance is discretely layered,and two classical layering methods(division by thickness and impedance equalization)are set up to simulate and analyze the corresponding acoustic performance to select the better one.By mixing high-purity iron powder and polyurea,Fe/PU homogenous materials with different composition ratios are produced.Corresponding acoustic velocity values are tested and then the corresponding acoustic impedance values are calculated,in which way,the correlation between iron powder proportion and impedance value is obtained.As the basis,it is used to design the Fe/PU composite layered structure.By dividing the gradient impedance distribution into discrete layers and designing the equivalent gradient composite structure with different gradients of impedance,experimental samples are made.The accuracy of the simulated simulation results are verified by underwater acoustic pulse tube acoustic test.