A method of controlling a mobile device is described, the method comprising: receiving a time varying current or voltage of a signal generated on the terminals of an acoustic transducer of the mobile device, the time varying current or voltage being generated in response to a tapping and/or sliding motion on a surface of the mobile device performed by a user; comparing the time varying signal characteristics with a set of predetermined signal characteristics; and generating at least one user command in dependence of the comparison. The mobile device may reliably detect complex user commands using an acoustic transducer.