Transmission rules of ultrasonic at the contact interface between soil medium in farmland and ultrasonic excitation transducer

Abstract

To analyze the phenomenon of distortion and frequency dispersion in the ultrasonic pressure curve at the excitation transducer-soil contact interface (incident interface), a new method to study the ultrasonic propagation in farmland soil media based on DEM (discrete element method) was proposed in this paper. And its feasibility was verified through simulation and actual measurement equivalent experiments. The excitation amplitude, excitation frequency and soil compression ratio were used as influencing factors to carry out single-factor experiments. The analysis was carried out from three aspects: the time domain and the frequency domain curve of the ultrasonic, and the microscopic characteristics of the soil particles near the interface. The experimental results show that the reduction of soil compression ratio, the increase of excitation frequency and amplitude can trigger the non-linear response of the soil, which will lead to the distortion of the ultrasonic waveform. Among them, the compression rate has the most significant influence on the distortion of the sound wave. When the compression ratio is 0%, the trough and the second peak disappear. When the compression ratio reaches 12%, the amplitude of the trough and the second peak are 424% and 749% of the compression ratio of 6%, which is far greater than the influence of other factors. As the excitation frequency increases from 20 kHz to 60 kHz, the ultrasonic frequency domain range increases from 0 to 40 kHz to 0–150 kHz, the dispersion phenomenon is enhanced, which leads to a more serious waveform distortion, while other factors have less influence on the dispersion phenomenon. Under the ultrasound, it is found that the soil particles have a hysteresis phenomenon, that is, the direction of the force and the velocity of the soil particles is inconsistent. And as the soil compression ratio decreases, the excitation frequency increases, the greater the excitation amplitude, the more serious the hysteresis phenomenon, and then causes more serious ultrasonic distortion. The research of ultrasonic waves at the transducer-soil interface is helpful to reveal the propagation law of ultrasonic waves in soil, which provides a theoretical reference for the improvement of the accuracy of ultrasonic testing instruments.