W. C. Kirkpatrick Alberts II- walberts@olemiss.edu
Roger Waxler, and James M. Sabatier
NCPA,University of Mississippi
University, MS 38677
Popular version of paper 4pPA7
Presented Thursday afternoon, October 20, 2005
ASA/NOISE-CON 2005 Meeting, Minneapolis, MN
In acoustic landmine detection, understanding of the mechanical vibration response of landmines is crucial. As such, a study of an acoustically-active landmine has been performed. Measurements of the mechanical vibration of the landmines pressure plate reveal seven modes, vibration patterns similar to those observed in the vibrations of drums, below a frequency of 1.6 kilohertz. A large frequency shift of the first symmetric vibration pattern of the pressure plate is also observed. The first symmetric pattern occurs higher in frequency than the first asymmetric pattern.
To model the vibration of the pressure plate, it is considered to act as an elastically-supported thin elastic plate. The observed shift of the first symmetric vibration pattern is caused by an air cavity beneath the pressure plate. This is confirmed by a lumped acoustic element model of the system of pressure plate and volume.
Mass loading measurements in sand reveal an interesting phenomenon: flush burial of the landmine effectively removes the effect of the volume on the pressure plate by forcing the plate to act as if it were clamped at the edge. The first symmetric vibration pattern of a clamped plate is still shifted by the volume beneath the plate, but its shift is not as drastic as in the elastically supported case. Qualitatively, this can be modeled by considering welded contact between the sand and the plate edge. This increases the stiffness at the plate edge and makes the first symmetric vibration pattern act as if the plate were clamped.
Other landmines of similar construction will have similar shifts of their first symmetric vibration patterns. Landmines with large volumes below their pressure plates will not exhibit the same shift. The study described here shows that determination of a landmines response requires that the mechanism beneath the pressure plate and the interaction with the soil must be taken into account.
This research is sponsored by the US Army Research, Development, and Engineering Command, Communications-Electronics Research, Development, and Engineering Center, Night Vision and Electronic Sensors Directorate under Contract DAAB15-02-C-0024.