Acoustic sensing provides a mode by which user input is received by a computing device. As an example, a surface is acoustically monitored for interactions with one or more user-manipulated objects, such as a body part of the user or a variety of passive objects. Acoustic analysis of frequencies produced by such objects interacting with the surface enables these objects to be distinguished from each other. Multiple objects interacting with the surface at the same time may be associated with different types of user input to provide a multi-touch user interface for the computing device. The acoustic sensing techniques described herein support motion-based and tapping-based gestures using one, two, or more touch interactions with an acoustically monitored surface.