Abstract: A device for sensing a state change of a mechanical system, comprises at least one sound emitter (E), a receiver (R) as well as an electronic circuit (4) which allows to compare the received sound signal with reference values. The state of the mechanical system, such as the existence or the absence of contact among parts, is monitored by the processing of the signals corresponding to the emitted and received sound signals with the aim to allow to recognize the monitored state by the difference between the received signals and reference values. The sensing device may be used to monitor a number of variables such as contact, location, orientation, etc. of the mechanical parts by the same sensors there where traditionally different kinds of sensors were traditionally needed.