A new model of medium is established,in which the elastic modulus is bigger when the sound wave is in the compressed area,while it is smaller in the expanding area.The strong nonlinear coupling effect occurs when the compressed wave and the expanding wave exist in the same area.The calculated results show that the coupling of compressed wave and expanding wave can result in the expanding of the system,which is related to the relative difference between the expanding elastic modulus and compressing elastic modulus.In addition,the results obtained from the elastic modulus of non_continuum are the same as the limit case of continuous elastic modulus.