A method of investigating vibrations and an apparatus therefor

In order to investigate and analyse vibrations of any object, light from a laser (12) is split with part being emitted to an object (15) and part being mixed with the light (16) reflected from the object to create an interference signal. That interference signal is investigated to derive therefrom a signal representing the vibrations of the object (15). If there is bulk movement of the object (15), the effect of that bulk movement may impose a popular shift on the reflected light (16). To eliminate this from the investigation of the vibrations of the object, an estimate of frequency corresponding to the effect of the bulk movement is derived from the signal, and that used to defuse a signal of reduced bandwidth encouraging the frequency due to the bulk movement and also the frequencies of vibration. Thus, the vibrations can then be derived and investigated from analysis of that signal of reduced bandwidth.