Air-coupled ultrasonic sensing of grass-covered vibrating surfaces; qualitative comparisons with laser Doppler vibrometry

The paper addresses several sensitive issues concerning the use of air-coupled ultrasound to probe small vibrations of surfaces covered with low-lying vegetation such as grass. The operation of the ultrasonic sensor is compared to that of a laser Doppler vibrometer, in various contexts. It is shown that ambient air motion affects either system, albeit differently. As air speed increases, the acoustic sensor detects a progressively richer turbulent spectrum, which reduces its sensitivity. In turn, optical sensors are prone to tremendous signal losses when probing moving vegetation, due to randomly varying speckle patterns. The work was supported by the Office of Naval Research.