1. An apparatus for monitoring a structure comprising: a sensing device for producing a detected signal for determining a change in or to the structure; a processor for processing the detected signal to determine level crossing rates in the signal and, from those rates, distinguishing between noise in the signal indicative of a nuisance event and a required event; wherein the sensing device comprises: a light source; a waveguide for receiving light from the light source so that light is caused to propagate through the waveguide; and a detector for detecting the light propagating through the waveguide to determine a change in the monitored structure, and for producing the detected signal.