1. A method of detecting the location of a tap on an object having at least one acoustic sensor, the method comprising: storing a set of labelled training data, said labelled training data comprising digitised waveform data of waveforms captured from said sensor for taps at a plurality of different locations on said object in combination with location data indicating for each said waveform the location of the tap; processing said labelled training data to determine, for each of a plurality of tap-sensing regions of said object, mean value data and covariance data for at least two said waveforms captured from taps in the said region, said mean value data defining a mean of said at least two waveforms and said covariance data defining covariance of said at least two waveforms; capturing tap data comprising a digitised waveform of a tap at an unknown location captured by said sensor; and determining a tapped region of said object from said tap data and from said mean value data and said covariance data from said plurality of regions.