The Application of Holographic Interferometry to the Study of Disc Brake Noise

An experimental investigation of passenger car disc brake noise has been made using the double pulsed laser holographic technique, developed to allow simultaneous recording of three orthographic visual images of a vibrating brake system. Mechanical measurements of parameters such as disc surface strain, torque, pad deflections, pressure and noise complement the visual images for both continuous and intermittent brake noise and enable phase relationships of the component parts to be determined. Visual images of the vibrational characteristics of a noisy brake show the disc to be in a bending mode with diametral modes. The pad is seen to vibrate in a variety of modes such as bending, torsion and often a combination of both and that pad end flutter is shown to make a significant contribution to brake system noise. A series of holograms of the brake emitting first an intermittent noise and then a continuous noise show that the disc waveform is not stationary and that it is moving with respect to both the calliper and the disc. Fringe movement is shown to take place when a single hologram is viewed from different positions which provides supporting evidence that the waveform is not stationary in space. Most recent tests show that the waveform does rotate about the disc at a rate equivalent to the frequency of vibration divided by the diametral mode order.