A mobile electronic device including a casing, a signal generator housed within the casing, for generating a signal that has a frequency higher than the human audible range, at least one speaker housed within the casing and coupled with the signal generator, for producing analog output from the signal generated by said signal generator, wherein the analog output produced by the at least one speaker is (i) substantially inaudible, and (ii) causes the casing to vibrate at a frequency approximately equal to a resonant frequency of the casing.