Visualizing Automobile Disk Brake Squeals and Corresponding Out-of-Plane Vibration Modes

Automobile disk brake squeal has always been one of the major customer complaints because of its extremely unpleasant, very high pitch and intense sound. Currently, diagnostics of vehicle brake squeals are conducted using a scanning laser vibrometer synchronized with squeals. This process is time consuming, especially when there is a hard-to-reach area for a laser beam to shine or when squeals have multiple frequencies for which filtering must be used so that individual out-of-plane vibration modes can be obtained. In this paper, a different method known as Helmholtz equation least squares (HELS) method based nearfield acoustical holography (NAH) is used to reconstruct all acoustic quantities, including the acoustic pressure, normal components of the surface velocity and acoustic intensity. In particular, the locations from which squeal is originated are identified and the out-of-plane vibration modes that are responsible for squeal sounds are established. The input data for this HELS based NAH are obtained by an array of microphones that are made to fit the contour of a disk brake assembly. Unlike laser scanning, this method requires no synchronization and filtering so that measurements can be done in seconds and reconstruction of acoustic pressure, normal acoustic intensity, and out-of-plane velocity distributions on brake assembly surface can be obtained all at once. Moreover, locations at which squeal is generated can be identified and correlations between squeals and the out-of-plane vibration modes responsible for squeals be established.