The density of a clutch lining is an important parameter for its quality. In this work, a nondestructive vibroacoustic test is studied for evaluating the quality of linings. First, the mechanics of the test is studied. The theoretical model shows that the high frequency components of the nondestructive impact force transmitted to the structure decrease if the contact stiffness decreases. The acoustic signals (0-20 kHz) and acousto-ulstrasonic signals (0-40 kHz) are generated by tapping on the linings. 7 linings in good quality with an average density of 1.96 g/cm3 and 7 linings in bad quality with an average density of 1.65 g/cm3 are considered. The linings are the product of a commercial company. A microphone is used to detect acoustic signals and an acoustic-emission sensor is used to detect the acousto-ultrasonic signals. The signals are captured by a data acquisition card and are analyzed in a personal computer. The spectra of the signals are smoothed by the cepstral analysis. The amplitudes of the smoothed spectra are compared in various frequency bands. The difference in the amplitudes for good and bad quality linings are compared. It is observed that the difference in high frequency components can be used as an indicator of the quality of friction linings
