Fourier analysis of holographic data from a noisy disc brake and its implication for modelling

Double pulsed laser holography is used to obtain images with fringe lines giving detailed information about the mode of vibration of a braking system generating high-frequency noise or squeal. If the images obtained are digitized, it is shown to be possible, using image processing techniques, to develop algorithms that determine the displacement field of the exposed part of the disc surface. Using least-squares mathematical approximations it is then feasible to generate Fourier series type representations of the out-of-plane motion of the vibrating disc. Holograms of the disc rim can also be analysed to show how the vibrating motion varies throughout the thickness of the disc (in-plane motion). The results demonstrate the existence of travelling waves as well as significantly large amplitude and quite complex in-plane vibrations. Implications for the construction of mathematical models of disc brake noise are discussed.