Research On Nonlinear Acoustic Propagation Characteristics In Variable Parameter Medium

At present,with the development of the technology of vibration reduction and noise reduction of underwater submarines in various countries in the world,the radiated noise of underwater targets is gradually reduced at a rate of 1～1.5d B per year,which makes the acoustic signal of underwater targets weaker and weaker,resulting in sonar the system’s ability to detect underwater targets has declined.Therefore,it is necessary to study new underwater acoustic detection and signal enhancement methods.Based on the theory of parametric resonance in the parametric-excited vibration system,this paper uses the amplitude modulation and frequency modulation techniques of the parametric-excited fluctuation system to process the underwater target acoustic signal to increase the underwater target’s acoustic energy,thereby increasing the sonar system the detection range of the target signal.In this paper,the variable-parametric vibration system model is obtained by the acoustic-electrical analogy method,and the dynamic characteristics and resonance characteristics of the variable-parameter vibration system are analyzed to obtain the time-varying sensitivity parameters of the variable-parameter vibration system.The speed of sound is a time-varying sensitive parameter,and the modulation of the electric field on the speed of sound in a conductive medium is theoretically analyzed.In this paper,the variable-parametric vibration system model is obtained by the acoustic-electrical analogy method,and the dynamic characteristics and resonance characteristics of the variable-parametric vibration system are analyzed to obtain the time-varying sensitivity parameters of the variable-parametric vibration system.The speed of sound is a time-varying sensitive parameter,and the modulation of the electric field on the speed of sound in a conductive medium is theoretically analyzed.Consider the linear wave equation,introduce the state equation of the medium sound velocity with time,and obtain the linear acoustic wave equation of the variable parameter medium in which the sound wave propagates in the variable parameter medium and produces the nonlinear phenomenon.Then the nonlinearity of the medium is considered,and the equation of state of the sound velocity of the medium changing with time is considered,and the nonlinear sound propagation model in the variable parameter medium is obtained in which the acoustic wave produces the nonlinear phenomenon in the variable parameter medium.Then,the finite difference method is used to carry out the equation discrete processing,set appropriate sound source and reflection boundary conditions,simulate and analyze the effect of modulation depth,modulation frequency,sound source frequency and variable parameter medium layer thickness on the acoustic propagation characteristics in the medium,and simulate the actual underwater acoustic signal in the variable parameter medium characteristics.Studies have shown that: under the acoustic propagation model in variable-parameter media,when the modulation frequency is twice the frequency of the sound source,whether it is a linear medium or a nonlinear medium,the time-domain waveform of the sound wave will be amplified under the action of the modulation amplitude Phenomenon,that is,the sine wave enters from one end of the variable parameter medium layer and then becomes a little bigger and reaches a steady state;when the modulation depth is too large,the system will become unstable,and obvious nonlinearity can be observed in the time domain waveform phenomenon,the solution of the sound field is divergent at this time;and in a linear medium,a signal input with a frequency will have multiple frequency components,and a non-linear medium will also have multiple frequency components.However,the number of the two frequency components is different.At the same time in the variable parameter medium,as the thickness of the variable parameter medium layer increases,the signal amplification gain will increase,and in the variable parameter medium layer,only the frequency component of the target signal in the underwater acoustic signal is amplified without amplifying the environmental noise;therefore,it can The variable parameter medium is used to improve the signal-to-noise ratio of the received signal so as to increase the detection distance of the sonar system.