# Roller-Integrated Acoustic Wave Detection Technique for Rockfill Materials
## 2 Methodologies  
The new compaction detection technique includes two aspects: theoretical analysis and technical implementation. In terms of the theoretical model, the interaction between the soil and the vibration wheel is considered as [PERSON]'s problem, namely, the propagation problem of vibrations over the surface of semi-infinite isotropic elastic solid, and the acoustic field generated by this interaction is regarded as the infinite baffle piston radiation acoustic field. The new technique is based on the vocal mechanism as follows: when the vibration wheel interacts with the soil, the soil is subjected to the vertical contact force and the horizontal friction. The combined action of the two forces makes the corresponding particles of the soil vibrate vertically and horizontally in a compound manner and simultaneously makes a sound. Meanwhile, the fluctuation caused by the vibration of the particles causes the density and pressure of the air medium at the interface between the soil and the gas to change, that is, the state of the air medium at the solid-air interface produces change. Thus, acoustic waves are generated at the interface and radiated in a certain form through the air medium. Considering the interaction form between the vibration wheel and the soil and its boundary condition, the acoustic wave radiation field formed at the solid-gas interface can be regarded as the piston-type radiation field on the infinite plate. Namely, when the vibration wave located in the solid medium surface propagates, a specific area located in the solid medium surface can be treated as the circular piston radiator on the infinite baffle; the piston vibrates in the vertical direction at speed \(u\), and radiates acoustic waves to the semi-infinite space in front of the baffle.  
In terms of technical implementation, the detection principle is as follows: this acoustic wave detection technique includes detection and signal analysis. The detection equipment is installed on the vibratory roller and mainly contains the acoustic field microphone, the signal acquisition analyzer, the display, and the GPS receiver. When the filling layer is compacted by the roller, the time domain signal of the acoustic wave field formed near the contact surface of the vibratory wheel and the soil is received by the acoustic field microphone mounted on the outer frame of the vibration wheel, and then collected by the analyzer to form digital signals. The GPS receiver mounted on the roller simultaneously provides the spatial signal associated with the roller location. The signal acquisition analyzer performs the filtering and the spectrum analysis to the effective acoustic signal to obtain the second harmonic amplitude (SHA). According to the correlation between the SHA and the compactness or density of the rockfill material, a continuous compaction index is established after calibration. Combined with the spatial location information, a spatiotemporal compactness distribution map of the rolling region is presented on a roller-integrated display. In addition, combined with the compactness feedback module, it is easy to realize intelligent compaction by adjusting the mechanical parameters.