Based on the analysis results of ultrasonic signal acquired in an actual system

an empirical model named double exponential model using the linear sum of two exponentials is proposed.The full width half maximum and the rise time of double exponential model are analyzed

and the methods of model’s parameters estimation are introduced.Further the fitting accuracy and TOF( time of flight) estimation variance of double exponential model are compared to those of hybrid exponential model.It is shown that the double exponential model is suitable for the ultrasonic signal in which the ratio of the full width half maximum and the rise time is more than 2.5.