1. An apparatus for differentiating touch screen users based on characterization of features derived from the touch event acoustics and mechanical impact, the apparatus comprising: a touch sensitive surface for detecting a touch event from at least one user; at least one sensor generating a vibro-acoustic waveform signal from such touch event; a converter for converting the waveform signal into at least a domain signal; a feature extractor for extracting distinguishing features from said domain signal; and a classification unit which uses the distinguishing features of said extractor to associate the features of the domain signal with a particular user.