Surface Ripples: Analyzing Transient Vibrations on Object’s Surfaces

Abstract

The subtle vibrations on an object’s surface contain information about its physical properties and interaction with the environment. Prior works imaged surface vibration to recover the object’s material properties via modal analysis, which discards the transient vibrations propagating imme- diately after the object is disturbed. In this work, we extract information from the transient surface vibrations simultaneously measured at a sparse set of object points using the dual-shutter camera described by Sheinin et al. [38]. Like ripples on the water surfaces, transient surface waves propagate with concentric circular wavefronts on isotropic object surfaces. Such a highly structured propagation phenomenon enables us to interpret the object’s interaction with the environment, even with sparse measure- ments. Specifically, we model the geometry of an elastic wave generated at the moment an object’s surface is disturbed (e.g., a knock or a footstep) and use the model to localize the disturbance source for various materials (e.g., wood, plastic, tile). We also show that transient object vibrations contain additional cues about the impact force and the impacting ob- ject’s material properties. We demonstrate our approach in applications like localizing the strikes of a ping-pong ball on a table mid-play and recovering the footsteps’ locations by imaging the floor vibrations they create. Beyond transient vibrations on object surfaces, we also conduct exploratory experiments to measure the transient propagation of arterial pressure waves on human bodies.